I've been experimenting with a 2N2222 transistor (in the TO-18 metal case) and using it as an avalanche transistor -- by not connecting the base and having the emitter and collector connected in reverse (so maybe I'm actually using the reverse mode, I'm not really sure)... basically it avalanches and continues being on until current drops to zero, then resets itself undamaged.

I (mistakenly) thought that all X2222 variants would behave the same, such as the BT2222A (an SMD package) that I was about to order.

Now, experimenting with PN2222 (TO-92) and it's not avalanching the same way (if it really is doing so). if it's avalanching, it tops off to a certain voltage (around 9.15V on a 16V supply, with voltage drops on a few other components, of course), then very, very momentarily avalanches just to go slightly under 9.15V, then stops the avalanche. then it repeats the cycle... or at least I think this is what it's doing.

Why is this, and what can I do to make PN2222 behave more like 2N2222?

  • \$\begingroup\$ That doesn't sound like an avalanche effect. To get that you need actively reverse bias a PN junction. According to the 2N2222 datasheet that should happen around -5V for the base-emitter voltage and 30+ for the other. So what you are seeing is more likely noise being amplified. For the rest you should perhaps explain better how you actually want this to work like. \$\endgroup\$
    – HKOB
    Feb 19, 2015 at 13:29
  • \$\begingroup\$ yeah... the 2N2222 is an old component... probably just noise... but it's too regular though... \$\endgroup\$
    – Dehbop
    Feb 19, 2015 at 13:43
  • \$\begingroup\$ To be honest, maybe I don't know what I'm talking about. I have done a bit of googling, and it could be that it is avalanching since you connected it in reverse. I would suggest you try to connect the base like in other typical avalanche transistor circuits though, just with collector-emitter swapped like you were trying (not the typical way), unless you have a higher voltage available (then try the typical way). \$\endgroup\$
    – HKOB
    Feb 19, 2015 at 14:15
  • \$\begingroup\$ I don't understand the avalanches and then resets itself. If you are above the avalanche voltage then current should be on all the time. Maybe a circuit and /or 'scope shot. \$\endgroup\$ Feb 19, 2015 at 15:55
  • 1
    \$\begingroup\$ Read about Jim Williams pulse generator. It uses a 2N2369 in avalanche mode. I used a similar circuit with 2N3904 transistor (but with a different power supply, that generates 200V and it works). The transistor is not connected backwards (emitter is connected via a 50 Ohm resistor to ground and the base to ground via a 10 kOhm resistor) \$\endgroup\$
    – Nick_F
    Sep 24, 2016 at 11:56

1 Answer 1


If you're making an avalanche relaxation oscillator, most transistors are not really designed or characterized for this so it should not be an enormous surprise that one maker's part will behave differently from another, or even other parts with the same part number from the same source.


simulate this circuit – Schematic created using CircuitLab


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