2N2222A LED Flasher question

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.
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boblob
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Post by boblob »

Hello - here's a quick poser for the EE types...

I want to add a flashing LED to a tremolo using a simple 2N2222A switching arrangement as in the attached diagram (the 330K is connected to the LFO output).

It needs a 4u7 from the base to filter LFO feedthrough (ticking), and I'm thinking - should I be worried about leakage discharging through the base-emitter from that cap and damaging the transistor. Is that ever going to happen? I could add a small current limiting resistor from the emitter to ground, but is that necessary? If so, what sort of value do I need?
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LED Flasher with LFO feedtrough damping cap.
LED Flasher with LFO feedtrough damping cap.

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jwpartain1
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Post by jwpartain1 »

Perhaps someone needs to double check my thoughts, but adding a 1k (EDIT: or smaller, if you prefer) resistor to the anode of your LED would limit the current and also raise the voltage at the anode above ground. This would ensure a definite LED off status, while the current design may only vary the intensity of the LED with your LFO.

boblob
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Post by boblob »

Thanks for the reply.

Not really bothered about having a definitive off state, though. Just need to know if I need to limit the current from the cap discharging through the base-emitter.

BTW - works fine on the breadboard, but want to double check (not being of the "if it works and doesn't smoke: game on!" school).

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Post by galmar »

I assume that the 4u7 cap is electrolytic type to concern you about leakage.

2N2222A is specified for 800mA oc continuous collector current. That means it is a quite robust design to concern you. This indicates that it can easily withstand milliamperes of base current. I don't really think that a 4u7 could possibly leak that badly.

You can as you say add a series resistor to limit current, or maybe even a bleeder resistor of some kiloohms parallel to the cap could work fine. In general it is good practice to do that in order to discharge the caps easily when power is off.

But I don't really think you should be massively worried about such things. A discharging current from the cap while power goes off is certainly of greater scale, and if your setup servives that, everything should be ok.

And remember, if no power is supplied, there is no Vbe voltage to open the base junction and enable current to flow through, so no concerns about leakage with no power I suppose. Add a bleeder to sleep relieved or even stop thinking about all these and do nothing. :)

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