Please help me understand transistor testing

[audiocrzy]

Hello,


Can some please help me understand what I need to look for when testing a transistor that is out of circuit. I understood that a short or an open is a bad transistor but is their anything else that a tester like a Peak DCA55 ( I use one ) will tell me that makes the transistor bad. Can to much gain be bad or to much leakage be bad? Anything you can tell me or give me to help me identify if a transistor is bad will be greatly appreciated.
My main quest is if a transistor has no short or open can it still be bad?

Thanks

 

[ChrisMarantz]

The short answer to your question is - yes - a transistor can be bad even if it doesn't have a short or is open.

To have some understanding about all of the other possible faults that can effect the operation of a transistor - you need to have a more than casual understanding of how transistors work.

You also need to understand what all of the different parameters shown on your DCA55 mean. A data sheet for the particular transistor under test will give you guidelines about the specifications for that transistors - which you can use to compare the transistor under test.

While there are some transistors that have "known issues" most are fairly reliable and their failures are often caused by other failed components. There are also many differences between the different types of transistors - each type can have different kinds of common failures - all-in all, there's a lot to learn about before the picture come into focus - Chris

 

[gslikker]

Totally agree with above.

An additional note is the fact the DCA55 will not detect the "known issues" stated in some AK topics, like noise or popping sounds in amplifiers produced by transistors.
The DCA55 will not always tell you if a transistor is bad, it leaves most of the indications to be interpreted by yourself, although it might try give you some help.

Download the user guide for it and you see it will give all specifics about what it does.

If a component is bad, or deteriorated, and it behaves LIKE ANOTHER COMPONENT, the DCA55 will conclude it is the other component.

For example, if you would have a transistor with its collector open circuit, it would detect a zener diode, or it would detect a normal diode, depending on base to emitter behavior.
It gives you one of two possibilities and none of them indicates a faulty transistor, so you still will have to interprete the result, yourself.

The internet is really full of tutorials and descriptions how to measure a transistor, so the search eninge is your friend.

 

[audiocrzy]

This came to me from Peak

Gain
====
Gain of the transistor (hFE) is a good indicator of how much the transistor can amplify currents by. Some large devices can have quite low gains (say around 10) and still be perfectly within spec. Small signal devices however tend to have much higher gain, often more than 100.
If you're not sure what sort of levels of gain you're expecting then check online datasheets for the transistor concerned or compare readings with known good devices.
Note however that transistor gain can vary hugely, even for devices of the same type number. For example, a BC108 can have a gain from 100 to 800.
Generally though, low gains are what you're looking out for.

Leakage Current
===============
The leakage current is the current that flows between the collector and emitter when there is no deliberate base current drive taking place.
All modern silicon transistors will exhibit an extremely low leakage current and is generally 0.00mA as measured on the DCA55. If you see some leakage current greater than 0.00mA then make sure that you're not touching the contacts at all and try again. If you're still getting leakage current then that can indicate that the transistor is bad.
Please note that germanium transistors will normally have some level of leakage current and that is to be expected.

 

[mbz]

While the DCA55 is one of the better compact testers, it still doesn't replicate in circuit situations.
Sometimes the component needs near rails voltage before it fails. Bottom line, a component that
passess testing still may be faulty. Components that fail are definitely faulty.

Certainly check the datasheet for gain class/expected gain.

 

[Binkman]

This vid, to duplicate testing, requires a scope but does explain transistor characteristics in class amplification to better understand the little beasts in operation.
Blog - Transistor Amplifiers Explained Australia