Meters for transistor testing
How to carry out resistance tests. Things you must know FIRST. To begin with it is imoportant to say that the following methods are concerned with fault finding or trouble shooting. We are not concerned here with design and construction. For those processes some different tests and techniques are required, although its unlikely that anyone ever constructed an electronic circuit without being involved in at least some fault finding.
What type of meter?
Most multi-meters are suitable for testing transistors; some have special transistor testing ranges (but that doesn't mean that they are the best models to use in all situations). You can of course buy dedicated transistor testers. These will generally tell you more about a transistor than an ordinary multimeter will. But in most fault finding situations you have a transistor that was apparently working well, but now is suspect. You are not paricularly interested in the subtleties of its characteristics; you don't need to know its hfe, you need to know if it's dead! A few simple resitance or voltage checks will tell you this. Transistor testers are useful tools mainly in design and construction where you may want to know about a transistor's performance in more detail, so you don't design yourself into fault finding situation later.
Things to check on your meter
First look at the meter's resistance ranges. If you are using a digital meter you should find that one of the ranges is marked with a diode symbol. This is the range you MUST use for testing transistors and diodes; don't use any other range it won't test the device properly and the results will just confuse you!
The problem is that on resistance ranges the meter puts a small voltage across the probes tips, so that when you connect it to the component being measured, a current flows through the component. It's this current that the meter is measuring; it just displays its result as resistance (in ohms) to save you the bother of working it out from voltage and current. The point here is that the meter uses a SMALL voltage, and a different value of voltage on different ranges. In many cases the meter voltage won't be enough to overcome the junction p.d. of the component we are testing - so the junction will measure open circuit even if there is nothing wrong with it. The diode range (or whatever range the meter instruction book suggests for testing the resistance of junctions) on a meter is designed to supply just enough voltage to turn on the junction, and pass a very small current through it. This gives us usally a reliable indication of the state of the junction, without confusing results, or damaging a good junction.
Fig 3.1.24 Digital multimeter
Usually digital meters display the resistance of the junction when using the diode range, but some meters display the JUNCTION VOLTAGE on this range (a completely different number!) You should make sure you know what the meter is telling you; there is usually a "V" in the display if it refers to junction voltage but check the instruction book if not sure.
Digital meters display 1 . (one followed by a decimal point) to indicate that the resistance is too high to measure on the range you are using. Remember that since we are limited to one range when testing semiconductors this means "infinity" or open circuit for our testing purposes.
Because the range of resistances we can measure on a single range of our digital meter is quite small (usually 0 to 2K) we often need to use a moving coil meter such as an "Avo 8" to test transistors. There are a few things we should remember too before using this or similar meters.
Fig. 3.1.25 Professional analogue multimeter
When using the resistance ranges on analogue meters such as the "Avo" the Red lead is negative and the Black lead is positive. This is important when considering a junction whose resistance depends on which side of the junction is positive (when testing junction's forward resistance the positive (black) meter lead goes to the P side of the junction and negative (red) lead to the N). This is not the case on digital meters, where red is positive and goes to P for the forward resistance test, and black is negative and goes to the N side of the junction.
The Avo has three resistance ranges and no diode marking. However it is important that you only use the middle of the three ranges (usually marked x100 on modern meters). The reason for this is that the x1 range does not produce enough current range for testing junctions, and more importantly the x10K range uses a 15V battery to supply test voltages to the probes; putting 15V across a semiconductor junction whose forward voltage is only 0.6V will more than probably destroy it! (Just about the easiest way to find a faulty transistor - if they weren't faulty when you started they will be now!)
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