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More Basic Questions From Electronman

Basics group, Thanks again for all your helpful responses last week. Also, I purchased an old copy of Forest Mims' "Getting Started in Electronics" for $5.00 and am finding it clear, concise, and fairly easy to understand. Here are my latest newbie questions.

TRANSISTORS

In experimenting with some of the simple circuits in Mims' book, I burned out several 5mm round LEDs ... they're cheap ... and I learned a lot from my mistakes. But I haven't quite figured out how to determine when I've blown out a transistor. Here are some things I've learned about transistors by using my digital multimeter.

Transistor HFE Resistance between Base/Emitter

2N2907 312 80,400 ohms 2N2222 170 81,000 ohms 2N3906 156 80,000 ohms 2N3904 170 80,200 ohms

So I'm deducing that all (?) small transistors have a base/emitter or base/collector resistance of about 80,000 ohms. I've learned that HFE is a measure of forward gain (what unit of measurement is attached to HFE?) But this experimentation hasn't helped me to determine if I've blown out a transistor. What method can I use to determine if a transistor is bad?

Reply to
electronman
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No. There's a diode junction there. That's mainly what you're measuring.

Graham

Reply to
Eeyore

It depends how you blew it. If by destroing the junction between two legs and having open cirquit you have infinite resistance in both directions. If by heating the junction and welding it you have "zero" resistance in both directions. In a _normal_(whatever it means?!) transistor you should measure different resistances on the same junction depending on direction of the diode and your leads.

HTH

Stanislaw.

Reply to
Stanislaw Flatto

Have a look at

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Reply to
g.knott

electronman wrote: (snip) > ...Here are some things I've

Any diode junction does not have a constant resistance (volts per ampere), but one dependent on the current passing through it. Each resistance scale on your meter uses a different test current, so, if you can switch resistance scales, you should be able to see this effect. Each scale should show a different resistance, with the lower ohm ranges (that use a higher test current) showing a lower junction resistance.

It is a ratio of two currents, (collector current)/(base current), so the units cancel. It is just a number.

The diode test scale on your meter applies a constant current across the leads, and displays the voltage drop across them, usually with the decimal point showing millivolts. A good silicon diode junction (base to emitter or base to collector) should show something like 500 to 600 mV. If you see both those junctions, and the voltage goes off scale indicating an open circuit emitter to collector in both directions, proceed to the Hfe measurement. It should show a much higher gain with the emitter and collector correctly oriented, and a much lower gain with them interchanged. If you get all this, the transistor is probably fine. Failures usually short one of the junctions, and kill the gain.

Reply to
John Popelish

Learn the basic's about diodes. learn how to test them using your meter. After that, picture a transistor as 2 diodes connected to each other on one end with the polarity back to back. Now at this point, the leads that are twisted together would be the "BASE". the other leads could be the Collector/Emitter. study that with your meter and you then will understand how to do a simple quick test of a transistor.

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Reply to
Jamie

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