Well, one fairly easy way. Use this after you've verified inputs seem to be OK, no chips are overheating, etc. This requires the pinout of the IC, AND a known good (reference) IC. Bend the output pins of the reference IC out slightly. Piggyback the reference IC onto the suspect part, and scope the outputs, comparing the outputs of the suspect and reference ICs. If the suspect and reference have different outputs, start tracing. The suspect could be bad, or something could be loading the outputs. Notes: 1. Obviously, this requires a LOT of 'reference' ICs (one of each type used in the circuit). 2. For complex (active) signals, the checking is even easier if you have a dual trace scope. Hook one probe to a pin on the reference, the other to the same pin on the suspect. Invert the B input. Set the scope to display A+B. You'll get a straight line if the outputs are the same. 3. There was an IC tester available at one time that used this principle. Don't know if it's still available. Bill ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Kevin Cornwell" writes: > Ok... Pulled out a Nichicon 4.7uf 315v 105 C capacitor (H9112) and it > tested bad. Any recommendations on where to find a replacement? How > critical is the voltage for match up purposes? I cannot find this > particular unit. In fact the closest I could find was a 4.7uf 400v. Help > please... :) That would be fine. The 105 degree C would be desirable though. In general, an electrolytic cap can be replaced with one that is modestly higher without any problems as long as it fits. I wouldn't go 10 times higher but up to at least 2X is OK. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Substitution really is the best but any DMM with a capacitance function or a capacitance meter will provide a reasonable fail-no conclusion test. I.e., if it fails on one of these, it is probably bad. If it passes, the cap could still be shorting at full voltage. ************************************************************************ * CAPACITORS: TESTING WITH A MULTIMETER AND SAFE DISCHARGING * * * * **** Version 1.10 **** * * * * Copyright (C) 1996 * * Samuel M. Goldwasser * * Comments or suggestions to: s...@stdavids.picker.com * * * * --- All Rights Reserved --- * * * ************************************************************************ Introduction: ------------ This note describes techniques for the testing of capacitors using a multimeter without a capacitance test mode. Information on safe discharging of high value or high voltage capacitors and a discharge circuit with visual indication of charge and polarity is also included. Testing capacitors with a multimeter: ------------------------------------ Warning: make sure the capacitor is discharged! Some DMMs have modes for capacitor testing. These work fairly well to determine approximate uF rating. However, for most applications, they do not test at anywhere near the normal working voltage or test for leakage. A VOM or DMM without capacitance ranges can make certain types of tests, however. For small caps (like .01 uf or less), about all you can really test is for shorts or leakage. (However, on an analog multimeter on the high ohms scale you may see a momentary deflection when you touch the probes to the capacitor or reverse them. A DMM may not provide any indication at all.) Any capacitor that measures a few ohms or less is bad. Most should test infinite even on the highest resistance range. For electrolytics in the uF range or above, you should be able to see the cap charge when you use a high ohms scale with the proper polarity - the resistance will increase until it goes to (nearly) infinity. If the capacitor is shorted, then it will never charge. If it is open, the resistance will be infinite immediately and won't change. If the polarity of the probes is reversed, it will not charge properly either - determine the polarity of your meter and mark it - they are not all the same. Red is usually **negative** with VOMs, for example. Confirm with a marked diode - a low reading across a good diode (VOM on ohms or DMM on diode test) indicates that the positive lead is on the anode (triangle) and negative lead is on the cathode (bar). If the resistance never goes very high, the capacitor is leaky. The best way to really test a capacitor is to substitute a known good one. A VOM or DMM will not test the cap under normal operating conditions or at its full rated voltage. However, it is a quick way of finding major faults. A simple way of determining the capacitance fairly accurately is to build a 555 oscillator. Substitute the cap in the circuit and then calculate the C value from the frequency. With a few resistor values, this will work over quite a wide range. Alternatively, using a DC power supply and series resistor, capacitance can be calculated by measuring the rise time to 63% of the power supply voltage from T=RC or C=T/R. Safe discharging of capacitors in TVs, video monitors, and microwave ovens: -------------------------------------------------------------------------- It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge. This doesn't mean that every one of the 250 capacitors in your TV need to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found after powering off (or before any testing - some capacitors (like the high voltage of the CRT in a TV or video monitor) will retain a dangerous or at least painful charge for days or longer!) A working TV or monitor may discharge its caps fairly completely when it is shut off as there is a significant load on both the low and high voltage power supplies. However, a TV or monitor that appears dead may hold a charge on both the LV and HV supplies for quite a while - hours in the case of the LV, days or more in the case of the HV as there may be no load on these supplies. The main filter capacitors in the low voltage power supply should have bleeder resistors to drain their charge relatively quickly - but resistors can fail. Don't depend on them. There is no discharge path for the high voltage stored on the capacitance of the CRT other than the CRT beam current and reverse leakage through the high voltage rectifiers - which is quite small. In the case of old TV sets using vacuum tube HV rectifiers, the leakage was essentially zero. They would hold their charge almost indefinitely. The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage). Then check with a voltmeter to be double sure. Better yet, monitor while discharging (monitoring is not needed for the CRT - discharge is nearly instantaneous even with multi-M ohm resistor). Obviously, make sure that you are well insulated! * For the main capacitors in a switching power supply, TV, or monitor, which might be 400 uF at 350 V, a 2 K ohm 25 W resistor would be suitable. RC=.8 second. 5RC=4 seconds. A lower wattage resistor (compared to that calculated from V^^2 / R) can be used since the total energy stored in the capacitor is not that great. * For the CRT, use a high wattage (not for power but to hold off the high voltage which could jump across a tiny 1/4 watt job) resistor of a 1 to 10 M ohms discharged to the chassis ground connected to the outside of the CRT - NOT SIGNAL GROUND ON THE MAIN BOARD as you may damage sensitive circuitry. The time constant is very short - a ms or so. However, repeat a few times to be sure. (Using a shorting clip lead may not be a bad idea as well while working on the equipment - there have been too many stories of painful experiences from charge developing for whatever reasons ready to bite when the HV lead is reconnected.) Note that if you are touching the little board on the neck of the CRT, you may want to discharge the HV even if you are not disconnecting the fat red wire - the focus and screen (G2) voltages on that board are derived from the CRT HV. * For the high voltage capacitor in a microwave oven, use a 100 K ohm 25 W (or larger resistor with a clip lead to the metal chassis. The reason to use a large (high wattage) resistor is again not so much power dissipation as voltage holdoff. You don't want the HV zapping across the terminals of the resistor. Clip the ground wire to an unpainted spot on the chassis. Use the discharge probe on each side of the capacitor in turn for a second or two. Since the time constant RC is about .1 second, this should drain the charge quickly and safely. Then, confirm with a WELL INSULATED screwdriver across the capacitor terminals. If there is a big spark, you will know that somehow, your original attempt was less than entirely successful. At least there will be no danger. DO NOT use a DMM for this unless you have a proper high voltage probe. If your discharging did not work, you may blow everything - including yourself. The discharge tool and circuit described in the next two sections can be used to provide a visual indication of polarity and charge for TV, monitor, SMPS, power supply filter capacitors and small electronic flash energy storage capacitors, and microwave oven high voltage capacitors. Reasons to use a resistor and not a screwdriver to discharge capacitors: 1. It will not destroy screwdrivers and capacitor terminals. 2. It will not damage the capacitor (due to the current pulse). 3. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles. Capacitor discharge tool: ------------------------ A suitable discharge tool for each of these applications can be made as quite easily. The capacitor discharge indicator circuit described below can be built into this tool to provide a visual display of polarity and charge (not really needed for CRTs as the discharge time constant is virtually instantaneous even with a muli-M ohm resistor. * Solder one end of the appropriate size resistor (for your application) along with the indicator circuit (if desired) to a well insulated clip lead about 2-3 feet long. For safety reasons, these connections must be properly soldered - not just wrapped. * Solder the other end of the resistor (and discharge circuit) to a well insulated contact point such as a 2 inch length of bare #14 copper wire mounted on the end of a 2 foot piece ... There is a method which requires only a Digital Multimeter, a stopwatch and some knowlege. It is based on the relationship between charge, current, time and voltage. Current is defined as the quantity of charge in motion per unit of time according to the formula Q = I * t and... the voltage on a capacitor depends on its capacitance and the quantity of stored charge.... Q = CE equating these two gives CE = It which can be solved for C = I*t/E where the current is constant. The digital ohmmeter comes in here. Most digital ohmmeters work on the principle of forcing a constant current through the unknown resistance and measuring the voltage drop across it. Frequently (but not always) the constant current is a multiple of 10 (10 uA, 100 uA, 1 mA, 10 mA) This results in a voltage drop which can be converted to resistance by simply moving the decimal point. To use this method you must know the current output on each resistance range and verify that it remains constant over all resistance values of the range setting. This can be done by connecting the digital ohmmeter in series with a variable resistor and another ammeter and observing the current as you vary the resistance. You must also know the value of the voltage across the resistor at the limit of the range setting. For example, many meters on the 2K ohm range will produce 2.00 volts across a 2K ohm resistor (1 mA of constant current). This can be checked with an additional voltmeter using the same circuit described above. Now for the procedure: (It helps to have 4 hands to do this, but....) 1. Completely discharge the capacitor by briefly shorting its leads together. 2. Set the ohmmeter to a moderately high resistance range depending on the expected value of the capacitor. Experience and trial and error will probably be the best indicator of the initial range selector. 3. Connect the negative ohmmeter lead to one lead of the capacitor. (If it is an electrolytic, connect it to the negative terminal.) 4. Simultaneously connect the positive ohmmeter lead to the other cap. terminal and start the stopwatch. 5. Measure the time required for the ohmmeter reading to rise from zero until it goes overrange. Stop the watch the instant it reaches overrange. (If the time is too short to accurately measure, select a lower range. Best readings will occur with measurement times in the 10-15 second range.) 6. Calculate the capacitor value from the formula C = I * t/E where: Capacitor value = Ohmmeter range current * Charge time ____________________________________ Overrange voltage Typically, this method will give measured values of capacitance within 5% of that given by a capacitance meter, depending on the precision of the methods used. It works best with electrolytics, as small value capacitors will charge too rapidly for accurate timing. A fairly simple device to do the job could be built with a constant current source, an op-amp comparator and a digital timer circuit, but that is beyond the limitations of this format. Let me know how it works for you. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Neil Preston, CET Instructor, Electronics Technology Amateur Radio Station WB0DQW Herndon Area Voc-Tech School Internet: npres...@vax1.umkc.edu Raytown, Mo. "Never confuse motion with action." - Benjamin Franklin Something I learned the hard way... I was replacing a bunch of electrolytic capacitors in a video monitor. I removed the originals all at once, taking careful notes about each location, its reference designation, and the value and voltage of each capacitor. All the original caps went into a zip-lock bag. Then I got a fresh batch of new capacitors, tested each one first, and installed them in one go. I carefully checked off each capacitor on my list as I went. (Another tip: I colorize the top of each new electrolytic capacitor with a red magic marker so I can readily see if I've missed something later.) This was about 30 capacitors. It's very easy to put one in backwards, as you well know. I was very, very careful. I apply power, and... BOOM! One cap literally got blown completely out of the monitor! It sailed past my ear and landed about three feet away! It was a small one, a 10 uF, 50 volt job, but it learned how to fly that day. Well, poop. I quickly removed power and found the location of the fried capacitor. I checked my notes again. Something in my gut told me to re-double-check. I went back through the bag of original capacitors and checked off each cap's value and voltage rating. Now, here's the golden moment: I found my mistake. I had the right value of capacitance, and I'd installed it correctly. My mistake was the voltage rating. I had installed a 50-volt capacitor, which is what the original capacitor was rated for. But... The original capacitor had some white rubbery gunk on it; that had been applied on the circuit board at the factory to keep parts and wiring from moving around much. It's basically a rubberized version of hot glue. This white gunk had hardened all over one side of this cap, and it had masked something critical: one digit. That's all, just one digit. I removed the gunk. The original capacitor was rated for 250 volts, not 50! No wonder the replacement cap exploded. :) So, keep this in mind the next time you're replacing a bunch of capacitors or other electronic parts: make sure you can read ALL the numbers and letters on them!!! Matt J. McCullar ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Common sense dictates that you change one capaitor at a time. You should also verify polarity and value of the capacitor you're replacing and the new one too. Remember that monitor manufacturers sometimes have incorrectly marked the polarity on the chassis too (Hantarex I'm pointing directly at you). I never remove ALL the caps at one time. I do it one at a time and verify voltage and polarity of the old cap first before installing the new one....sometimes the markings on the circuit board are wrong. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- TESTING ESR OF ELECTROLYTIC CAPACITORS I recently found an easy and cheap way to test ESR (Equivalent Series Resistance) of electrolytic capacitors, in circuit, that might save some people a lot of time. It requires only an oscillosope and a simple signal generator. I had an oscilloscope that I was trying to repair (Intensity control had little effect. Horiz sweep was only halfway across screen at higher freqs. One power supply rail was too low and others were too high.) and I had already checked every electrolytic capacitor in several/many different ways (all in-circuit), and even compared each of the readings to those from an identical unit: Powered off: Looked at signature from component tester (single-curve tracer) across each cap, and from each end of cap to ground, did resistance check with DMM, did capacitance check with DMM, checked resistance from each end of cap to ground. Powered on: Put scope across each cap, and scope from each end of cap to ground, used DMM and measured DC and AC voltages across each cap and from each end of cap to ground. I did find some bad caps (and some other bad components) and replaced them. But the problems were still there! I had been wanting to order an ESR meter, but hadn't done it yet, and needed to get this scope repaired immediately. I went to Sam Goldwasser's excellent repair-FAQ site, at http://www.repairfaq.org/ and found a GREAT method for testing ESR of capacitors in-circuit that requires only a signal generator and an oscilloscope (and some cables), and had found and fixed the problem within about ten minutes!! Here's what I did (This technique is basically directly from Sam's repair faq site): I used a signal generator and an oscilloscope to set up what I now call an "ESR Scope": At the output of the generator, I connected a BNC "tee" adapter. I ran one 50 Ohm BNC cable from the tee to a good (Tek 2465A) scope (with a 50-ohm BNC terminator on the scope input). On the other side of the tee, I connected another BNC cable that had alligator clips on its other end (It might have been a 75 ohm cable; shouldn't matter too much?), which I clipped onto the banana plugs of a set of cheap DMM-type probes. (Terminator note: I used a Tektronix 50 Ohm "pass-through" terminator, on the scope end of the BNC cable. But, you should also be able to use, instead, another BNC "tee" on the scope input, with an "endcap" terminator on one side and the cable coming in on the other side of the tee. A standard 10BaseT Ethernet 50 Ohm coax terminator (and 50 Ohm Ethernet BNC coax cables) should work fine. And they're available at Radio Shack, and probably Staples, et al.) I set up the signal generator to produce square waves at about 100 kHz, with about 100 mv peak-to-peak amplitude as seen on the attached scope, and no DC offset (A simple 555 timer circuit would do the job, too!). Then, I turned the scope's v/div to 5 mv/div, with time/div at 1 microsec, with AC coupling of the input. Shorting the probes together gave me a display on the scope that was about one division high. It was basically a square wave, with large narrow peaks at each leading edge. But I only looked at the horizontal part's p-p amplitude. That's the whole setup! No resistors. No nothing. Just cables (and a terminator). I did also try it with a decade resistor box in series with the probes, just to see what it would look like. I could clearly see each one-ohm increase, on the scope display, with the probes shorted together as well as with the probes across a good electrolytic capacitor. When I applied the probes across a GOOD capacitor in-circuit, there was little, if any, change in the scope display, compared to when the probes were shorted (since, depending on the frequency, a capacitor should look more-or-less like a short circuit, to AC). But, when I tried it across a BAD capacitor, usually the display would be almost-totally off the screen. And, there were some caps that looked marginal, making the display go from about one div p-p up to about 3 to 5 divs (which probably corresponded with somewhere between 5 ohms and 20 ohms of ESR, if I recall correctly.) Anyway, within just a few minutes I had found one more bad electrolytic filter cap in the power supply, two smaller bad electrolytics in the P.S., a bad one on the horizontal sweep switch's board, four bad ones near the middle of the main board, and a couple more that I can't remember right now. I made a note of each one. When I was all done checking, the first thing I did was replace the filter cap in the power supply, and then power it on and check the power supply rails' voltages. BINGO!!!!! YESSS!!! They were all normal again! Not only that, but the horizontal sweep problem and the Intensity control problem were both GONE!! Yippee! That filter cap had checked out as perfectly OK, using every one of the other methods that I described above (all were "in-circuit", though), and compared OK to the other identical scope's same cap, in all of those cases. But with this "ESR Scope" method, it was totally obvious, immediately. And the same cap on the other scope tested good, with this method (So, the earlier comparisons WERE bad cap vs. good cap, but showed nothing!). [I also noted that after the bad cap was removed, it tested bad in the same way that it had while it was in-circuit, with a basically identical scope display. And all of the other ones that I replaced also tested bad, when OUT of the circuit, even with the other methods.] This " ESR Scope " method isn't a perfect panacea, of course: There were some cases where, without an identical unit to compare to, the displays would have been difficult for me to interpet, and possibly misleading. (However, it *always* worked with every *electrolytic* that I tried it on, IIRC, from 10 uF 10v to at least 1000 uF 100v, with no need for an identical unit to compare to.) But, then again, I haven't played around with it enough, yet, either. I assume that adjusting the frequency for different capacitances might be helpful, especially if non-electrolytics were to be tested. I also seem to remember that a DC offset in the signal is usually used, when testing ESR. I'll try that, later. And maybe increasing the amplitude of the square wave would be useful, sometimes, too. But, usually, I think I'll want it to be low-amplitude, probably less than +/- 0.4v, so the signal doesn't turn on any semiconductor junctions. Well, that's it! I hope that this can save some of you some time, sometime... Tom Gootee tomg@fullnet.com http://www.fullnet.com/u/tomg (Good used Electronic Test Equipment) ------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------- ESR is mainly used for higher value caps (>1uF). These SMT ones are almost certainly much lower. Assuming they are ceramics, then I think the possible failure mechanisms are either leaky/shorted, or plain open circuit due to cracking. An ordinary resistance check (out of circuit) and/or capacitance check are more useful measurements. I spent hours once debugging a faulty vertical output stage, which turned out to be an intermittent partial short in a 100pF SMT cap. Or maybe it was _under_ the cap, it went away after I'd removed the cap to test it. The cap measured good but then the fault tended to take an hour to appear anyway. ;-) Mike. An ohmmeter works if you know the circuits resistance and the cap has a low enough resistance, when bad to lower the overall value. Many I have found defective have 1K or less but not all. Jeff In general, there is no easy or effective way of checking SM caps in circuit other than checking proper operation of the circuit. About the only way to get the value is with the schematic, which is usually impossible to get for most monitors. The three biggest failure modes for those type of capacitors: 1. The glue they use under them at time of manufacture becomes conductive making for really odd circuit problems. 2. They physically crack and become open or intermittently open. 3. During manufacturing the solder flow process leaves little hairs of solder along the underside of the capacitor which may cause a short circuit. David ---------------------------------------------------------------------------------------------------------------------------------------------------------------- This module goes bad usually because of leaking SMD electrolytics. You can try cleaning up the board and replacing the electrolytics but it is cheaper and easier to replace the module. Available from most parts jobbers, MCM, etc for anywhere from $7.50 to $16.50 each. Capacitor kits are also available for about the same price......go figure! Also if the tiny traces are damaged because of the leaking electrolytics, the caps alone won't fix it..........just replace the board. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- I have a Genie ESR meter which I have used daily, in anger, for several years. I prefer it to the Wizard, which a couple of colleagues use, for its quick response, and unambiguous digital readout. It also auto zeros, and auto scales. Oh yeah, and it's also a third the price ... It can be used to test surface mount caps as well as ordinary ' leaded ' types, but only electrolytics, of course. ESR is not a factor with other types of capacitor. About the only thing which I would say is that surface mount electrolytics tend not to give the same sort of readings as leaded types of the same value. A surface mount cap may give an ESR reading of twice that of a similar value and rated leaded type. Also, an ESR meter will not tell you anything about the value of an unknown - you need a capacitance meter for that. I'm surprised that you are coming across electrolytics - either SM or leaded, which are ' unknown ' as even the smallest SM types are still plenty big enough to have their value and rating printed on. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Hmm. Not a lot really. I have to say that in general, SM caps of the usual ' ceramic brick ' type are pretty reliable. I have had them short, and I have had them leaky, but it's rare. Their value is pretty stable in general. Probably, the commonest problem with them, is open circuit, due either to body cracking, or end cap detachment, or solder joint cracking. This appears to be caused, in most cases, by bad board handling / excess flexing. ----------------------------------------------------------------------------------------------------------------------------------------------------------------To test the capacitors you will need an ESR meter. It is very well possible that one or more electrolytic capacitors are bad. Coincedently I repaired a Compaq TFT screen yesterday (randomly flashing lines); while tracking down the cause of the problem I did notice that many of the SMD caps did have a poor ESR. In this case the problem was caused by a bad electrolytic cap near the power input. --------------------------------------------------------------------------------------------------------------------------------------------------------------- An alternative for using an ESR meter is to use a oscilloscope to watch the signal over the capacitor when the device is powered on. If the capacitor is good you should see no or only a very small signal. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Don wrote: > Jeg har en SMD elektrolyt kondensator. Den hedder: > 10F16 > E1 > Den har en fed hvid streg på venstre side. Men er det så plus polen eller > den negative pol? Stregen er plus. Ikke som man skulle forvente som almindelige leadede komponener hvor stregen på lytten er minus. Se evt. dette link for mærkning af SMD (Små Mærkelige Dimser) komponenter: ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Flere forskellige metoder. 1) Kapacitetstest. 2) ESR test 3) Fysisk inspektion 1 klares med et passende multimeter. Testen fortæller kapaciteten, men selvom den stemmer med påtrykt værdi kan en lyt godt være defekt alligevel. Hvis kondensatorer (altså ikke lytter) ser fysisk ok ud og måler korrekt er de normalt iorden. 2 er til lytter og den måler den ækvivalente seriemodstand. God test til at afgøre om lytter er defekte. 3 er visuel. Bulede/lækkede/revnede/eksploderede lytter er i regel defekte :-) ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Det har du ret i, faktisk skal der 3 check til: 1) ESR-meter, 2) Ohm-meter og 3)kapasitets-meter. Først da kan man afgøre om en lyt er helt ok. Indrømmet, indtil nu kun checket med ESR. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Tja, er kondensatoren/lytten kortsluttet kan du måle det med ohmmeteret. Er den afbrudt er det straks sværere. En normal kapacitetstester kan til nød anvendes, men man finder hurtigt lytter som er for store til at det kan lade sig gøre. Jeg bruger min ESR tester til lytter (0.47 til uendelig uF kapacitet) og alm. kapacitetstester til boosterkondensatorer (1.5nF til 11nF eller højere). ---------------------------------------------------------------------------------------------------------------------------------------------------------------- They probably use a special 'in-circuit' meter for testing the capacitors. They might check the ICs with a volt meter, and if the reading seems wrong, they then check the supporting components that are near that IC for faults. A bad resistor will probably have a scorch mark or some residue around it. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- First of all you need a basic understanding of what a scope is and what it can be used for. An oscilloscope is a device with one or more inputs which are voltage sensitive. It is a kind of Voltmeter. If you connect the probe to some point in a circuit you can see how high the voltage is on the screen, higher voltage gives a higher dot or line. It is also a time measuring device, and you can set the speed of the moving dot with the timebase section to show what happens at a speed you choose. There are controls on the scope to adjust input sensitivity, just like on a voltmeter, and there are controls to adjust the zero position, the vertical position on screen. These controls are often labelled channel 1 and 2, or X1 and X2. A voltmeter can only tell what the voltage is in a certain moment, in an oscilloscope we can make the dot move from left to right over the screen with many different speeds and this makes it possible to study voltages which change over time, waveforms which change very fast and would not be possible to study with a voltmeter. You, the user, decide how fast the dot shall travel over the screen. This is done with the timebase section on the scope. There is also a trigger unit, which makes the sweep start at the same point on the waveform every time, which is necessary to get a stable picture of the waveform over time. The trigger unit can also be set to free running sweep, which is good to use to begin with, so you get a line on the screen and you can see what is happening. You use an oscilloscope by first setting the input probe and input channel to a suitable voltage range. If you do not know what range is suitable you start with a high voltage, connect the probe, and reduce the voltage setting until you see a signal. Just like you do with a voltmeter. Set the trigger to freerunning to begin with, so you get a line on the screen and you can see what you are doing. Set the timebase to something sensible for the frequency you want to study. If you assume the signal to be changing at 10 kHz you set the timebase to the inverse of 10 000, that is 0.1 microseconds, or 100 nanoseconds. You need to be able to convert frequencies to period times to handle a scope. Use a pocket calculator and use the invert function 1/n. 1 MHz is one microsecond period, for example. 1/1000000=>0.000001 When you measure a time period on the screen you can convert it to a frequency in the same way. To get a stable picture use the trigger unit to stop the waveform from moving and duplicating itself all over the screen. As you get a picture on screen you can adjust timebase and voltage range so you can see the details you are interested in. Measure voltages by multiplying the waveform screen height in screen divisions with the settings of the input voltage selector and the probe setting if it is adjustable. Measure times by measuring the waveform horizontally in screen divisions and multiply with the timebase settings. If there are fine adjustment knobs for voltage and time which are not graduated you need to put them in the calibrated position to make measurements valid. There is usually an input selector close to the input which you can set to AC, DC or Ground. In AC position the input signal is going through a capacitor which removes the DC voltage from the signal. In Ground position the input is grounded, which is useful to compare a DC input voltage with or to set the vertical position of the line on the screen so you know where zero voltage is on the screen. Your first practical experiment with a scope could be like this: Turn it on and set the trigger to free running sweep. Set the timebase to 1 microsecond. Put the input selector to Ground. Adjust the vertical position knob until you see a horizontal line on the screen. Put it in the middle of the screen vertically. Change the input selector to DC. Set the voltage selector to 5Volt per div. Put the probe and ground connection on a small 9 Volt battery. See how the line on the screen jumps up to another position vertically on screen. The height difference is 9Volt from the grounded vertical position. Adjust voltage selector, probe setting and try to understand how you can get 9Volt from the settings and the height difference you see when you connect and disconnect the battery to the probe tip. Find out how you would know that the voltage is 9 Volt even if you did not know what voltage the battery had from the beginning. You have now made your first DC voltage measurement with the scope. Roger J. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- > P.P.S > Are there any common ways to damage a scope I should watch out for > I know not to turn the intensity up too high, but any more... Do not apply excess voltage to the inputs. I think something like 100 volts is the most the scope is good for and that may apply only to when the attenuator is set to the high range (5 to 20 volts per division). ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Crystal are use to keep the frequency of the clock from drifting. If the signal from this clock stops, or is weak, or the pulses begin to vary, the electronic equipments might show intermittent faults or might stop altogether. The microprocessor pins that hold the crystal are usually called OSC IN and OSC OUT as shown in Figure 1 and the frequency is marked on the crystal. Typical examples of crystal oscillator frequency are 3.58MHZ, 4MHZ, 8MHZ, 24MHZ etc. Testing Crystal Crystals are quite fragile components because of their construction. Unlike a resistor or capacitor, if you drop one on the ground from a decent height, its 50-50 bet whether it will work again. Testing the crystal is not a breeze either. You cannot just take out your trusty multimeter and plug the crystal in it. In fact, there are three right ways to test a crystal. Using Oscilloscope: A crystal produces a sine wave when excited. It is appropriate then, to see a waveform representative of a sine wave on the clock pins. If the clock is not functioning properly, replace the crystal. In most cases this should solve the problem since microprocessors are usually very reliable. Check the crystal with power on. Frequency Counter Frequency Counter can be use to check the frequency of the crystal. The reading must be taken when the equipment power is switch "on". Place the probe of frequency counter to the crystal pin and read the measurement. Be sure that your frequency counter meter has the range that is higher than the crystal frequency you are measuring. Crystal CheckerWith this method, usually the crystal is placed in the feedback network of a transistor oscillator. If it oscillates and the LED is lighten up, this mean that the crystal is working. If the crystal doesn't work, the LED stays off. Instead of using LED, some other crystal checker uses a panel meter to indicate if the crystal is working or not. Kada .eli. da ispita. napon baterije stavi preklopnik na podru.je za JEDNOSMERNI NAPON(V-), 20(V). Na displeju .e ti stajati napon baterije u voltima, na primer 1,56V za dobru AA bateriju ili 1,32V za punjivu AA NiCd ili NIMH bateriju. Ako je napon dosta manji npr. 1,42V za Alkalnu bateriju onda je ona skoro prazna i mora se uskoro zameniti, odnosno napuniti ako se radi o RECHARGERABLE(punjivoj NiCd ili NiMH) bateriji. Obrati pa.nju .ta na njima pi.e. Nemoj nikad puniti obi.nu bateriju od 1,5V. Punjive .e. prepoznati po tome .to daju manji napon 1,2V .to na njima i pi.e. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- >If the capacitor under test reads within 10%-20% of the marked capacitance >value with the above meter, How good of an indication is this, that the >capacitor is good? Not very good. For large electrolytic capacitors, foil perforation can raise the internal resistance, and the tester won't even notice. >What is a more accurate or complete method of testing these. First, you need an AC ripple current source. Then, you tune to the frequency of interest (120 Hz for rectifier power supply filter capacitors is usual) and apply both the AC current and a DC voltage bias. Measure the phase shift between the current and the voltage (for a perfect capacitor, this is 90 degrees) and measure the induced voltage (for a perfect capacitor, this is I*2*pi*f*C). Take the tangent of the difference of the phase shift and 90 degrees. (this is 'tan(delta)' and appears on the spec sheet for the capacitor...) Then remove the AC, and crank the DC bias up to the voltage surge rating; measure leakage current. Ramp the DC bias down to the working voltage rating; measure leakage current. Raise temperature and repeat the capacitance, phase shift, and working-voltage measurements at the max temperature the capacitor is rated for. John Whitmore p.s. yes, it DOES sound rather elaborate, but that's the test that the manufacturers use. ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Testing transistors with DMM or VOM Edited by Sam Goldwasser (original author is unknown) This note describes procedures for testing of bipolar (NPN or PNP) transistors for catastrophic failures like shorts and opens. In most cases, this will identify bad Silicon transistors. Gain, frequency response, etc. are not addressed here. While the tests can be applied to Germanium devices, these are more likely to change characteristics, it would seem, without totally failing. Note: Analog and Digital meters behave quite differently when testing nonlinear devices like transistors or diodes. It is recommended that you read through this document in its entirety. Most digital meters show infinite resistance for all 6 combinations of junction measurements since their effective resistance test voltage is less than a junction diode drop (if you accidentally get your skin involved it will show something between 200K and 2M Ohms). The best way to test transistors with a DMM is to make use of the "diode test" function which will be described after the analog test. For both methods, if you read a short circuit (0 Ohms or voltage drop of 0) or the transistor fails any of the readings, it is bad and must be replaced. This discussion is for OUT OF CIRCUIT transistors *ONLY*. One exception to this ocurrs with some power transistors which have built in diodes (damper diodes reversed connected across C-E) and resistors (B-E, around 50 ohms) which will confuse these readings. If you are testing a transistor of this type - horizontal output transistors are the most common example - you will need to compare with a known good transistor or check the specifications to be sure. There are some other cases as well. So, if you get readings that do not make sense, try to confirm with a known good transistors of the same type or with a spec sheet. Before testing an unknown device, it is best to confirm and label lead polarity (of voltage provided in resistance or diode test mode) of your meter whether it be an analog VOM or digital DMM using a known good diode (e.g., 1N4007 rectifier or 1N4148 signal diode) as discussed below. This will also show you what to expect for a reading of a forward biased junction. If you expect any Germanium devices, you should do this with a Ge diode as well (e.g., 1N34). The assumption made here is that a transistor can be tested for shorts, opens, or leakage, as though it is just a pair of connected diodes. Obviously, simple diodes can be tested as well using the this technique. However, LEDs (forward drop too high more most meters) and Zeners (reverse breakdown - zener voltage - too large for most meters) cannot be fully tested in this manner. Testing with a (Analog) VOM: For NPN transistors, lead "A" is black and lead "B" is red; for PNP transistors, lead "A" is red and lead "B" is black (NOTE: this is the standard polarity for resistance but many multi-meters have the colors reversed since this makes the internal circuitry easier to design; if the readings don't jive this way, switch the leads and try it again). Start with lead "A" of your multi-meter on the base and lead "B" on the emitter. You should get a reasonable low resistance reading. Depending on scale, this could be anywhere from 100 ohms to several K. The actual value is not critical as long as it is similar to the reading you got with your 'known good diode test', above. All Silicon devices will produce somewhat similar readings and all Germanium devices will result in similar but lower resistance readings. Now move lead "B" to the collector. You should get nearly the same reading. Now try the other 4 combinations and you should get a reading of infinite Ohms (open circuit). If any of these resistances is wrong, replace the transistor. Only 2 of the 6 possible combinations should show a low resistance; none of the resistances should be near 0 Ohms (shorted). As noted above, some types of devices include built in diodes or resistors which can confuse these measurements. Testing with a (Digital) DMM: Set your meter to the diode test. Connect the red meter lead to the base of the transistor. Connect the black meter lead to the emitter. A good NPN transistor will read a JUNCTION DROP voltage of between .45v and .9v. A good PNP transistor will read OPEN. Leave the red meter lead on the base and move the black lead to the collector. The reading should be the same as the previous test. Reverse the meter leads in your hands and repeat the test. This time, connect the black meter lead to the base of the transistor. Connect the red meter lead to the emitter. A good PNP transistor will read a JUNCTION DROP voltage of between .45v and .9v. A good NPN transistor will read OPEN. Leave the black meter lead on the base and move the red lead to the collector. The reading should be the same as the previous test. Place one meter lead on the collector, the other on the emitter. The meter should read OPEN. Reverse your meter leads. The meter should read OPEN. This is the same for both NPN and PNP transistors. As noted, some transistors will have built in diodes or resistors which can confuse these readings. How to DeSolder Surface Mount Components by Mark Neff I've used several different techniques for removing flat packs, all with varying degrees of success. The most obvious is to use a temperature controlled iron, solder-wick as much solder as you can off the pins, and then heat each pin individually, applying slight force from behind with a very fine dental-type pick until the pin pops free. In most cases you can do a couple of pins at a time, and if you're very careful, you won't lift a trace. It's not very pretty, and the chip is mangled when you're done, but it works in a pinch. Another technique is to take a length of thin guitar string or piano wire and slide it behind and through the pins on one side of the chip. Anchor the string with an alligator clip or hemostats to the board, and then gently pull the free end away from the chip as close to the surface of the board as possible, while heating the pins where the force of the is applied. As the pins begin to come free, move the iron along with the string. It takes a little practice, and it won't work well if there are a lot of tall components around the chip, but in most cases it works great, and because the string is pulling each pin outward, instead of upward, the chances of lifting a trace is very slim. The pins around the chip are left relatively unscathed- I've pulled 84 pin chips from scrap units this way and reused them in emergency situations with no problems. Getting more expensive, our shop has a special flat-pack desoldering tool that I found doesn't work very well. It has a variety of "tips", shaped like the outlines of popular IC packages. The tip fits over the IC and applies heat to all the pins by contact simultaneously. When the solder is hot enough, the technician pushes a button on the handle that applies suction to the center of the chip. The suction causes the IC to "stick" to the desoldering tip and then the tech lifts the iron and (hopefully) the chip away. The problem is that the tips are so large that even heating is nearly impossible, and I've had a few cases where traces (several, not just one) got lifted when I thought all the pins were free. Lifting the iron with the chip in it just gives no "feel" for when the pins are free. It also takes a long time to warm up, and requires a lot of tinning to get the tip to work at all. The latest gadget we are now using is a hot air desoldering station. This is simply the best, fastest, easiest way to remove a flat pack. It has a selection of tips with nozzles that direct hot air around the perimeter of standard chip types. The velocity and temperature of the air are adjustable. After about a 5-minute warm up, the technician holds the tip over but not touching the pins, and in about 5 or 10 seconds, with a slight lift from a dental pick, the chip comes easily free. Jaws drop when I demonstrate this thing. All pins are heated at the same time, and the solder fully flows, so board damage is nearly impossible. The chip is in perfect shape (with tinned leads!) and can be easily reused. The only thing that can be a problem is the possibility of loosening and blowing away nearby small resistors and capacitors if the velocity of the air is set too high. Turning the velocity down on densely packed boards will minimize it, though. I simply can't say enough about this machine, and I highly recommend it if your shop does a lot of flat packs. The high cost will justify itself in the time saved with this method. But if funds are tight or the quantity of flat packs is minimal, try the guitar string. I've done many that way with great success. . .