# More Basic Questions From Electronman

Discussion in 'Electronic Basics' started by electronman, Feb 13, 2007.

1. ### electronmanGuest

Basics group,
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

2. ### EeyoreGuest

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

Graham

3. ### Stanislaw FlattoGuest

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

HTH

Stanislaw.

4. ### Phil AllisonGuest

"electronman"

** Since there are many ways for a transistor to go bad - more than one
test is required.

Fitting it into a circuit where it ought to work and having it do so is the
good proof it is not faulty.

Otherwise, a DMM that has a diode check function that lets you read the
actual voltage across the BE and CE junctions when forward biased is a good
start. If they are OK ( compared to a new example) then try the Hfe
est - if it is within the normal range of values for the type you have a
goer at low voltages.

Whether it is still OK at higher voltages, is noisy or behaves
intermittently when it heats up is another matter.

Fitting it into a circuit where you need it to work and having it do so is
very the best proof it is not faulty.

........ Phil

5. ### g.knottGuest

Have a look at http://homepage.ntlworld.com/g.knott/elect56.htm

6. ### John PopelishGuest

electronman wrote:
(snip)
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.

7. ### JamieGuest

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.