Germanium Transistors - testing Hfe and leakage

[Stickman393]

A'ite:

Am I correct here in presuming that the cheapie amazon/aliexpress TC-1 multi-function tester is essentially useless for testing germanium transistors?

Some it seems to do OK on, some it's just like...way off vs the RG Keen method. An AC128 on the TC-1 reads 320uA leakage and 142 gain, but the RG keen method yeilds 298uA leakage and 182 gain (corresponding to measurements of 1: 0.738Vdc and 2: 2.12Vdc).

I'm using a trimmer to dial in exactly 2.472k ohms as measured by my Fluke 87V. I'm inclined to believe that this setup is going to be more accurate, especially considering that some of these transistors seem to take some time to stabilize once under test: a quick two second test doesn't seem like it would be able to accurately account for this.

This is especially true for the AC128's and OC75's I've tested: these take a solid 5 minutes to stabilize. The USSR era MP41's seem to stabilize after about 30 seconds or so.

So...hey, sounding board, yeah? Whatcha'll think?

 

[Brett]

Last I looked, the TC-1 (and others in the same vein) do not factor leakage into gain calculations. The RG Keen method does. Your leakage numbers are reasonably close to each other, but this could account for at least some of the discrepancy you're seeing.

Your results with the RG Keen method will be more accurate.I still believe that the inexpensive testers like the TC-1 still deserve a place in the toolbox because they're useful for quick preliminary testing and rough sorting.

 

[Brett]

but the RG keen method yeilds 298uA leakage and 182 gain (corresponding to measurements of 1: 0.738Vdc and 2: 2.12Vdc).

It's also worth noting that your calculations are incorrect here.

It's (#2 - #1) * 100 = gain

That'd be (2.12 - 0.738) * 100 = 1.382 * 100 = 138.2 for gain, not 182.

Much closer to what the TC-1 is telling you.

 

[Stickman393]

It's also worth noting that your calculations are incorrect here.

It's (#2 - #1) * 100 = gain

That'd be (2.12 - 0.738) * 100 = 1.382 * 100 = 138.2 for gain, not 182.

Much closer to what the TC-1 is telling you.

Mmm, that's not the way I understood the article. From this section here in the section referring to leakage testing, before the 2.2M resistor is connected:

A 2472 ohm resistor is 2.472 volts per milliamp, so a milliamp of leakage will cause 2.472 volts to display.

What Keen refers to here is ohms law: if the meter reads 2.472Vdc, and the resistive load is 2472 ohms, then the current passing through the load is 1mA. With the equation V/R=A, as long as we know two of the three values we can calculate the third.

In my specific case, actual leakage current will be 0.738 (V) / 2472 (R) = roughly 0.000317A=0.317mA

Subtract that from the measured gain value: 2.12 - 0.317 = 1.803 x 100 = 180, roughly (though the numbers I provided here were rounded a bit from what I was working with).

That's what surprised me about how far off the TC-1 is. It has a tendency of veering way off the Keen methodology.

That said: I don't know the math behind his HFE equation, and why 2.2Meg and 2.472K with a 9v power supply will yield a direct gain measurement.

Although...huh...I really should measure my power supply voltage: that's gotta have an impact. 9.12Vdc. That could be throwing off my measurements...how to compensate for that though...

 

[Brett]

The previous comment was based on this excerpt from: http://www.geofex.com/article_folders/ffselect.htm

"Let's say the device really leaks 93uA, and has a gain of 110 - a prime specimen. What happens when we test? We chuck the thing in the socket, and read (93uA)*(2472) = .229V. Then we press the switch, and read 1.330V. To get the real gain, we subtract 0.229V from 1.330V and get 1.101V. The true gain is just 100 times the reading."

I'm basing my calculations on the voltages you provided and the excerpt above.

I don't know the math behind his HFE equation, and why 2.2Meg and 2.472K with a 9v power supply will yield a direct gain measurement.

I'd like to better understand the math myself. An equation would go a long way in understanding how to compensate for resistor inaccuracies.

9.12Vdc. That could be throwing off my measurements...how to compensate for that though

I'd expect some level of deviation if the supply voltage is different from the ideal testing conditions. Once again, having an equation for the calculations would prove useful.

 

[Stickman393]

"Let's say the device really leaks 93uA, and has a gain of 110 - a prime specimen. What happens when we test? We chuck the thing in the socket, and read (93uA)*(2472) = .229V. Then we press the switch, and read 1.330V. To get the real gain, we subtract 0.229V from 1.330V and get 1.101V. The true gain is just 100 times the reading."

Ah ha! That makes a bit more sense. For some reason I had gotten it in my head that it was the actual leakage in mA rather than the measured voltage that had to be subtracted.

Woof. Appreciate the heads up there. That saves me some head scratching.

 

[Big Monk]

It’s not useless but it is generally more accurate the lower the leakage is.

I’ve had it compare favorably to the RGK method up to 100microamps of leakage.