Looking for a transistor tester

BuddytheReow

Breadboard Baker
Finally got my small bear order in and need to measure pinout, hre , and leakage. I've got npn and pnp for both silicon and geraniums. Is there a cheapo one from Amazon you guys can recommend? Thanks
 

fig

Village Idiot
This is Mr. Keen's DIY test method...

If you have a batch of germanium transistors, how do you tell which are going to sound good and which will not? To a first order, you can just gain select them in a DMM that has a transistor checking range. However, all modern DMM's assume that the transistor being testing has no leakage at all. They just put a metered amount of base current in and look for how much collector current comes out. With germanium's inherently higher leakage, this just makes a leaky device look like a higher gain device.

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This is one way to sift the leakage from true gain. You hook up a couple of resistors and a DMM to the device, and the resistors set up conditions you can control to see what is what. If you really want to do this, get a 2.2M resistor and a 2.4K ; better, get one each 2.2M and 2.49K metal film 1% resistors. This will set you back about US$0.30 if you get them from Mouser, and slightly more or less than that from other sources. If you're going to do much of this, get a transistor socket to, so you can easily test a large number of devices.

If you are satisfied with an indication of gain but are willing to settle for lower accuracy, you can carbon film at 5%, but recognize that the accuracy will be less. If you can, get several 2.4K resistors and measure them. You may find one that's closer to 2.472 ohms, which would be ideal. I'm being picky about the ohms because if you get exactly 2.2M and 2472 ohms, and use a 9.0V battery, you'll find that the voltage across the resistor will be numerically equal to the indicated gain! That's why the somewhat odd resistor values, and the discussion on the values. It makes the final numbers on your DMM come out about right - multiply the voltage by 100, and that's the gain.

To do the test, stick the transistor in the socket, and read the DC voltage across the 2.4K resistor. The resistor will convert any leakage current from the transistor into a voltage that you can then read on your meter. A 2472 ohm resistor is 2.472 volts per milliamp, so a milliamp of leakage will cause 2.472 volts to display. That is incredibly too much leakage, so any transistor that does that is not going to be useful for a FF. In fact, although it will differ a bit, any transistor that shows more than a few micro amps of leakage is suspect. Because of the resistor scaling, the indicated value on your meter is "false leakage gain" and will have to be subtracted from the total reading that you do next.

To test the total gain, press the switch that connects the 2.2M resistor to the base. This causes a touch more than 4 microamps of base current to flow in the base. The transistor multiplies this by its internal gain, and the sum of the leakage (which doesn't change with base current) and the amplified base current. If the transistor has a gain of 100 and no leakage, the voltage across the 2.4K resistor is then (4uA)*(100)*(2472) = 0.9888V - which is almost exactly 1/ 100 of the actual gain. Pretty neat, huh?

But we know that germanium really does have leakage - that's why were doing this little dance in the first place. So, let's say that the device leaks 100uA to start with. We stick the device into the socket, and read the voltage before we press the switch. It reads (100E-6)*(2472) = 247mV. So the leakage is making the meter believe that there's a "gain" of almost 25 with no current into the base at all.

How much leakage is too much? 100uA is common, 200 happens pretty often. More than 300uA means the device is suspicious, and more than 500uA I would say is bad.

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.

Hey! How come it's 110.1, and not 110? Well, that's from this being an imperfect world, and from this tester being built with some approximations. The exact base current is 4.046...uA, assuming that the transistor's base conducts that much with a forward voltage of 0.1V (reasonable with germanium at these currents) and that the battery is *exactly* 9.0000V, and that the resistors are 2.20000M, and... well, you get the picture. 0.5% accuracy is doggone fine for work with such blunt tools, and much better than you actually need to make a fine sounding FF. Besides - if you're clever, you'll flip the switch and watch the voltage while you put your finger on the transistor. Simple finger heat will make the gain rise rapidly. What's the real gain? All of them are - at the temperature and conditions of the moment.
 

bowanderror

Well-known member
I use the same one. Decent enough for Ge transistors, here is how I interpret the measurements:

Germanium Transistors:
  1. hFE - DC Gain - hFE = Gain parameter measured under DC conditions
  2. Vbe - Voltage b/w Base & Emitter - (mV) - 0.65-0.7V for Si, 0.15 to 0.3V for Ge.
  3. Ic - Collector Current used for test - (mA) - Most pedal transistors run Ic <1mA. Some (i.e. Harmonic Percolator) run below 100uA. Some testers measure hFE at 5mA or higher, which is not particularly useful to pedal builders.
  4. Iceo - Leakage Current at the Emitter - (mA) - Collector-to-Emitter current w/Base lead not connected to anything (an extreme case as in most circuits, the Base has a DC path to somewhere) - Anything <100uA is good
  5. Ices - Collector-to-Emitter current with the Base lead shorted to the Emitter - (uA) - Leakage Current in-circuit is somewhere in between Iceo & Ices.
So not always the best for leakage, but it will give you a good idea of the comparative values between individual transistors.
 

Big Monk

Well-known member
I use the same one. Decent enough for Ge transistors, here is how I interpret the measurements:

Germanium Transistors:
  1. hFE - DC Gain - hFE = Gain parameter measured under DC conditions
  2. Vbe - Voltage b/w Base & Emitter - (mV) - 0.65-0.7V for Si, 0.15 to 0.3V for Ge.
  3. Ic - Collector Current used for test - (mA) - Most pedal transistors run Ic <1mA. Some (i.e. Harmonic Percolator) run below 100uA. Some testers measure hFE at 5mA or higher, which is not particularly useful to pedal builders.
  4. Iceo - Leakage Current at the Emitter - (mA) - Collector-to-Emitter current w/Base lead not connected to anything (an extreme case as in most circuits, the Base has a DC path to somewhere) - Anything <100uA is good
  5. Ices - Collector-to-Emitter current with the Base lead shorted to the Emitter - (uA) - Leakage Current in-circuit is somewhere in between Iceo & Ices.
So not always the best for leakage, but it will give you a good idea of the comparative values between individual transistors.

I compared the TC1 to the R.G. Method enough times to stop the R.G. Method altogether.

It’s close enough and easier.
 

SillyOctpuss

Well-known member

I honestly have the worst luck with these. I've had two of the model above and neither of them lasted more than four weeks. Both of them turned off during a test and just didn't turn on again.

I also had a TC7? I think that was the model number and the screen died on it. You could still just about see the text so it must have been the backlight.

I was going to buy a Peak Atlas but don't really have the cash at the minute and I need to test some transistors for a build this weekend. I thought I'd try one more and this guy turned up about 20 mins ago

20211009_183751_resized_1.jpg

I have a few issues so it's going back to Amazon tomorrow. Problem A - the battery cover won't fit over a 9v battery. As you can see below the battery sits higher than the shelf the flat cover slides over.

20211009_183941_resized_1.jpg

Problem B - The blue adaptor to connect the components was in the battery cover when it arrived. It just slides into some holes below it. Whenever you put a part in to test you have to hold the other side or it just lifts up when you lock a part in place

20211009_184318_resized_1.jpg


I know these are cheap but this one feels especialls pants

On the plus side it does seem to be pretty accurate.

20211009_184341_resized_1.jpg
 
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Coda

Well-known member
I honestly have the worst luck with these. I've had two of the model above and neither of them lasted more than four weeks. Both of them turned off during a test and just didn't turn on again.

I also had a TC7? I think that was the model number and the screen died on it. You could still just about see the text so it must have been the backlight.

I was going to buy a Peak Atlas but don't really have the cash at the minute and I need to test some transistors for a build this weekend. I thought I'd try one more and this guy turned up about 20 mins ago

View attachment 16844

I have a few issues so it's going back to Amazon tomorrow. Problem A - the battery cover won't fit over a 9v battery. As you can see below the battery sits higher than the shelf the flat cover slides over.

View attachment 16845

Problem B - The blue adaptor to connect the components was in the battery cover when it arrived. It just slides into some holes below it. Whenever you put a part in to test you have to hold the other side or it just lifts up when you lock a part in place

View attachment 16846


I know these are cheap but this one feels especialls pants

On the plus side it does seem to be pretty accurate.

View attachment 16847
Does it measure leakage?
 

SillyOctpuss

Well-known member
I don't know. I've only tested some Bc549 and ba282 diodes. I'm about to put my kid to bed then I'll check and let you know.

It should do, the other two models I had measured leakage and they all use the same basic software.
 

finebyfine

Well-known member
Does it measure leakage?

I have this one, and if it does I don't know how to. It's also bad at recognizing JFETS. It sucks that there isn't something $35 as good as the DCA75 but it was a worthwhile purchase for me personally. I use the one pictured to check caps and resistors though and it's great for that and easier than a multimeter for me.
 

SillyOctpuss

Well-known member
Wow so this one wont measure leakage like the TC1 and T7 will. I just tried to test some ASX12D PNP transistors and it only gives the same data as the screenshot I posted above. Pinout, HFE and Vf. Sorry @Coda but if you want to measure leakage you'll need one of the others. Can you cancel your delivery? I'm requesting a return label for mine now and will send it back tomorrow.
 
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BuddytheReow

Breadboard Baker
Based on what I’m reading here and elsewhere there isn’t a Swiss Army knife type of tester under $35. I may just get a basic one and use the Geofex method of measuring leakage and just call it a day
 
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manfesto

Well-known member
I have a Mega328 (you can buy them prebuilt or as a kit, with or without a case, depending on how much you want to spend). It does hfe, Iceo and Ices, and it’s been *so* handy for testing batches of germaniums I’ve been getting from eBay to suss the good ones from the duds.

@Chuck D. Bones was the one who recommended it, which is basically the highest praise one can give a product ‘round these parts right?

This is the exact listing I bought but anything “GM328” is gonna be the same thing
 
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Big Monk

Well-known member
Based on what I’m reading here and elsewhere there isn’t a Swiss Army knife type of tester under $35. I may just get a basic one and use the Geofex method of measuring leakage and just call it a day

The TC1 is about as good as it gets in that price range. I’ve yet to find something it doesn’t do.

It compares very favorably to the R.G. Method and is light years simpler and there is no math involved.
 
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