How to tell difference between Transistor and PUT?

Most of my components have either been purchased in bulk from ebay closeouts or stolen from boards of old through mount stuff that doesn’t work. Often these things won’t have any numbers left. How can one tell the difference between a standard transistor - lets say a 2n2222, and a PUT like the 2n6027?

Thanks in advance.

I think Grumpy_Mike had a circuit that you can plug a transistor into, it does some low current/voltage testing and reports back what the pins are and what the gain of the device is.

Not sure what more you could ask for when starting from scratch.

Find one of his posts where he gives a link, like to his decoupling article, and browse around there. I thought it was pretty clever.

Often Digital Multi Meters (DMM) have an HFE test circuit on them and would give you gain but you wouldn't be able to tell if it was no good or a fet. Package size would give a rough guess of wattage ie to92 is usually 1/2 a watt and to220 can be 15 watts while to3 can be 50 watts. There are heat sinks for transistors (even to92) and there is no such thing as too big a heat sink (when it comes to removing heat)

Here's the link to grumpy_mike's transistor tester.

I haven't come across a PUT for about 40 years, for those who don't know a PUT stands for Programmable Uni-junction Transistor. Great for making timers and one active component oscillators. My first ever published circuit in 1970 practical Electronics was two cross coupled uni-junction oscillators I used as a door bell in my student flat. It sounded like "a soprano in pain".

If you have a resistance meter you can do a quick check, with a uni-junction gate to any other pin will look like a diode. And the other two will look like a short. Where as in a transistor base to any other pin will look like two one diode in series. And the other two pins will look like an open circuit in both directions.

in a transistor base to any other pin will look like two diodes in series.

huh? Vbe is one diode drop, isn't it (and probably Vbc too...)

Yes sorry too late at night. This is what I was trying to say:-

Mike, How does your tester do with FETs? Ever try it?


No I have never tried it with FETs.

I haven't come across a PUT for about 40 years, ....

That was my first thought when I read the title of the thread. I think I can find one around here if I look hard enough.


Wow... I have a few... and I've had them about 40 years. These little guys were fun... but they look EXACTLY like a TO-92 transistor.

Funny thing is... I don't recall ever seeing a PUT tester.

I had never even heard of one, but the MAKE: Electronics book by Charles Platt calls for the part # I listed in original post.

Judging from the responses from you guys in the know, I think I'll just skip that project and move on to the next one ( It was really just a project showing how you could manipulate a PUT with threshhold voltage)

I still see the 2n6027 sold at digikey, so I wouldn't necessarily just skip the experiment entirely. Still, you're unlikely to find a PUT in a random collection of harvested parts...

Ok, thanks. I do need to place a rather large digikey order (I usually find most of the things i want are obsolete, get frustrated, and give up ;D )

I would like to do the project for the knowledge gained, but from what I'm gathering from you guys, It's pretty unlikely that I would ever end up using a PUT in the digital age?

It's pretty unlikely that I would ever end up using a PUT

It seems that way. You'd think that a currently available part, a common example of its part type, would have some projects out there on the web, but I couldn't find any. It looks like it might have application in power inverters. PUTs make for simple relaxation oscillators, but then so do neon bulbs, and they're not any more common these days...