Testing transistors with a meter

Site Administrator Join date : 2010-11-22

A known problem with Germanium Transistors, IS WHISKERS.

They make a lot of noise or the transistor no longer functions.
Here a good video one what I consider a possible solution if you have a High Voltage Source.

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I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!

Moderator Join date : 2010-11-22

I will add to Cliff's info that the old 260 is one of the best VOM's to test transistors with because it actually puts a real voltage and current on the elements.
None of the digital ones I tried do this and won't show the leakage that can be indicated with the 260 series meters.

Of note is the fact that testing bias diode stacks, HV TV silicon rectifiers, boost and focus rectifiers with a VOM will not give an accurate reading of their condition. It will indicate if they are shorted, however.
Excellent write up, Cliff!

Site Administrator Join date : 2010-11-22

There are several things you can do to test transistors.

1. you can find out if the transistor is good or bad (open leads) (shorts).

2. you can find out the leakage

3. you can find out the DC gain

4. you can find if the transistor is NPN or PNP

5. you can find out if the transistor is of Germanium or Silicon construction.

6. you can find the best method to test an RF, AF, or AF Power type without damage.

7. At what Temperature the testing needs to be done at.

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First of all there are two main types of meters to use for these tests.
The analog and the digital.

I am going to concentrate on the analog variety.
Such as the Simpson 260 and the Triplett 630 these have a 20,000 ohms per volt sensitivity.

Note that there are meters that have a ~~1,000 ohms per volt~~ but for transistors they are unsatisfactory. Those originally were used for high voltage tube sets. Where the newer units had the Ohms/per/volt increased to not burden a newer circuit(s) with developing designs.

You first need to understand that the resistance ranges use different voltages which must be taken into account.

A lot of defects in transistors cannot be tested for by DC tests, (static)
Expensive transistor testers use several ways that are not built into a VOM. (dynamic)

Gain checks are usually not worth doing on RF or IF transistors, again because they work at higher frequencies that the VOM cannot imitate.

On the other end of the spectrum AF transistors operate at frequencies very close to the DC voltage. In this case the only range of resistance to be used is the low resistance range such as R X 1. The reason is that the voltage is only 1.5 volts at the test prod leads. For some transistors such as Audio drivers and Audio power transistors using the R X 1 range is satisfactory because they have much lower internal resistance and therefore the R X 1 scale will suffice.
The R X 1 range can usually provide up to 100 mA of current which we are interested in, rather than the voltage (it is usually 1.5 Vdc). Power transistors can draw up to that current in this test.

In testing transistors, remember there are 2 different Sets of elements inside of the normal transistor. They are the Base and Emitter and the Base and Collector.
We will use a VOM to test transistors. First thing to consider is that most VOMs have a sensitivity meter of 20,000 ohms / per/ volt DC. Don’t use a 1,000 ohms / per/ volt DC meters for these tests.

Testing of I.F. and R.F. transistors will usually be done with the R X 100 range. With most V.O.M.s the higher ranges will have from 7 volts to 30 volts on the probes.

So, the first test will be to check for internal open connections within the transistor. First apply one probe to the Base lead and the second to the Emitter lead. You should note the resistance on the meter scale as to a high or low resistance. Now reverse the leads and see what the reading is. If the leads on the transistor measure high one way, and the other way other is lower, then that indicates that that diode junction is good. Do the same test but now on the Base to Collector leads. You should have similar results. If you get no results then either way shows the transistor is headed for the trash.

Now you can determine whether the transistor is an NPN or a PNP. The emitter is the 1st letter of the three normally. The 2nd letter is the base and the collector is the 3^rd.
This indicates the polarity of the device, N = negative, P = positive.

Out of circuit tests
These tests are done out of circuit at room temperature. Testing them after pulling out of an active circuit my give erroneous readings because of higher operational temperatures.

For small Diode and transistors such as, RF. IF. Or Converter stages, which are low power, then the recommendation is to use the R X 100 ohms range, (if there is no R X 100 then use R X 1000).

The front to back should be very high in one direction of the meter scale. If the Meter reading is at the high end of the scale it doesn’t mean the diode sections (Emitter to Base and Collector to base) are open. If a low reading cannot be obtained after reversing the leads then you can surmise that one of the elements is open.
Small transistors that are, RF. IF. Or audio driver stages usually show a very high resistance in the reverse direction
Power Transistors can be tested on R X 10 or R X 100 and no Higher as the higher ranges would make them appear shorted in the reverse direction and a forward direction of less than 500 ohms in germanium and less than 1000 ohms in silicon.
On the other hand, Audio power types usually have over 5,000 ohms reverse, and less than 100 ohms forward direction.
The leakage test involves just testing the emitter and collector leads only. Check the resistance value in one direction and then the other. The readings should be similar. If the meter reads zero or moves after the connections are made the Transistor is usually bad.
On power Transistors the good reading should be between 10 to 500 ohms with the range on R X 1.

Gain tests on R.F. and I.F. transistors are not going to be productive because D.C. gain has little if any true correlation because of circuit RF dynamics.
on lower frequency transistors however are done using a resistor, (The value of resistor should be 1,000 ohms) and a switch in series with the collector to base. And the leads from the ohmmeter leads connected from the Emitter to Collector. Since the power is applied from the meter then the resistor will act as a path to the base as the bias to turn the transistor on, so the lower the resistance reading the higher the gain. If you get no reading reverse the test meter leads. Just to make sure the polarity is correct. A small audio transistor should have no more of a power rating of 75 Milliwatts and no more than 1.5 volts at the probe.

Identification of unknown NPN and PNP transistors:

to test for the polarity of transistors put one probe lead on the collector and the other probe (I prefer to hook up tiny alligator clips rather than probes, it’s easier that way). Be sure you identify which probe has voltage that is positive and which is negative. Some meters are reverse and some have a reverse switch for just such an application. Triplett Model 360 is one such case. Use the same range R X 100 but power transistors should use R X 10.

For NPN the low reading will be on the negative lead going to the collector with positive lead on the base.

For PNP the low reading will be on the positive lead going to the collector with negative lead on the base

The low resistance reading should be as follows: Silicon below 1000 ohms and below 500 for Germanium.

Remember that Germanium has a lower voltage drop of between .2 to.3 volts ( 2 to 3 millivolts) and a silicon transistor is around .6 volts (6 millivolts.)

So that's the crux of simple testing of transistors.

Hope this Helps

_________________
I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!

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