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Hfe Beta Meaning

Zekeman

Zekeman

AK Subscriber
Subscriber
I just got a Tenma 7208155 multimeter with hfe tester. Tested some Zetex 694b's and most of their beta measurements tested around 500, a few tested around 430-450 and one tested at 360. What does this mean? Does it mean the one tesing below 400 is bad? If I have a bad transistor will it read "OL" or 0?

Thanks.
 
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Those testers are pretty awful. Read the instructions; it will tell you the operating conditions (or it should).

Beta is the ratio of collector current to base current, although to be strict one should subtract the leakage. Well, that's actually the B not the beta. Beta is the slope of the base current vs. the collector current curve. It depends greatly on how much current is passing, the temperature, and other factors.

Any reading you get is suspect. Basically you are learning if there is any beta at all, which would imply a good transistor. A bad transistor should read zero; you can prove that by not connecting a transistor. Or by connecting only two leads.
 
If I remember correctly hfe = The Hybrid Parameter expressing forward current gain in the common emitter configuration. IIRC upper case letters refer to DC parameter and lower case to AC parameters. You can find the hfe spec in the data sheet for the device.

If your tester were of good quality it could be used to match the devices to some degree I suppose but generally speaking transistor properties are quite variable and the circuits are usually designed to take this into account.

mike
 
The problem is, the tester doesn't test beta because that's an ac parameter. It would have to have an oscillator in the tester, which I'm sure it doesn't. So it's B, also known as DC Beta that is tested. But at what current and voltage?
 
It's not important. No one is looking to confirm data sheet specifications with a $50 handheld meter.

The meter is good for three things. One, to confirm the transistor is good. Two, to identify the location of the C, E, and B. And three, to give a decent relative reading of one device's gain as compared to another, which is useful for matching transistors for diff-pairs. The actual numbers measured are only of minimal consequence.
 
If you look at any small signal transistor spec sheet, you will see that h_fe, even
with the specified value of collector current, is a highly variable quantity from
device to device. A range of 50 to 200 for a given transistor type is not uncommon.
Transistor circuits must be designed with that variability in mind. That's part
of the art of electronic design. I've been studying this since the age of 12, and
37 years later I still have a lot to learn. Better waste no time!

P.S.: when the picture in my avatar was taken, I was still into tubes only!
 
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So the components that measure in the 300 or 400 range as I nted above will be OK as long as they have a mate?
 
They are okay, and matching isn't important (in general). If you really want to match, I don't think that tester is adequate. It's easy to show how two wildly differing transistors could give the same readings.
 
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