Relaxation oscillators select resistors

Question

Every circuit diagram of a relaxation oscillator says I have to put a 100 ohm resistor between the capacitor and the voltage source like this:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

But I think it would be better to use an 1000 ohm resistor instead of a 100 ohm resistor. This would discharge the capacitor at a slower rate and give me a lower frequency.

What do you think?

schematic

^{simulate this circuit}

What do _**I**_ think? I think you should simulate it and see if your theory is correct :) — , May 09 '20 at 15:43
CircuitLab is also a simulator. Try it and see what happens. — JRE, May 09 '20 at 17:13
@JRE Does it simulate for anyone? Or do they have to sign up at CircuitLab in order to gain that feature here? — jonk, May 09 '20 at 18:05
Jelly, you will need a negative dynamic resistance region for oscillation. The zener doesn't have one. A UJT or PUJT would do, though. You can look [here](https://electronics.stackexchange.com/a/429550/38098) for a more thorough discussion. — jonk, May 09 '20 at 18:09
@jonk: The CircuitLab simulator works for anyone here. Just click the "simulate" link under the schematic. — JRE, May 09 '20 at 19:10
@JRE I can't test it that way because I actually ***do*** subscribe to them. So of course it works for me. (And it is too much a pain to sign off and then sign back on to test. So I'll just accept answer. I was curious, is all.) — jonk, May 09 '20 at 19:10
@jonk I don't subscribe and it demands an e-mail address (and the e-mail is authenticated, there are questions and such like). But if I edit the OP's post I can simulate without supplying an e-mail, but I have to take care to discard the edit. Not ideal. — Spehro Pefhany, May 09 '20 at 19:35
@SpehroPefhany Thanks. And yes, it sounds like a bit of a pain. — jonk, May 09 '20 at 19:38

score 0 · Answer 1 · answered May 09 '20 at 19:47

I presume your "Zener" K-A is actually something like the E-(B)-C pins of an NPN BJT.

Try this out in real life (I don't think this will simulate).

I think you'll find if you increase the resistor too much, the relaxation oscillator will be very relaxed indeed and will 'stick' after the first breakdown.

Relaxation oscillators select resistors

1 Answers1