So I'm trying to use a zener diode to limit the voltage across a capacitor – specifically, to 2.2V.
As you can see, I'm using a 9v supply. The zener is in series with a resistor – either 100 or 100k, can't quite remember which, didn't make a difference though. In parallel with this is a 16V 2200uF capacitor in series with another resistor, also 100-100k iirc.
There's a schottky to the left (in series with the zener) to prevent capacitor discharge, as well as a resistor in series with all of them (this was
simulate this circuit – Schematic created using CircuitLab
added later – had no effect).
As you can also see, the capacitor voltage is well above 2.2V, at least 7V and counting. The zener seems to be having absolutely no effect on the circuit. I've tried it with multiple ones – you can see the packet, it specifically says zener voltage 2.2V. It's definitely not a problem with frying the zener, since I have a very high resistance in series with it. What on earth could the issue be?
Best Answer
KVL rules here. The zener branch (R1 and D1) will be at the same voltage as the capacitor branch (D2, R2, and C1), which will be 9V.
If you want the zener to limit the cap voltage, you need to move D2 to the midpoint junction of R1 and D1.
simulate this circuit – Schematic created using CircuitLab
Don't forget that D2 will eat a few hundred millivolts, meaning your cap voltage could end up as 1.6V or so.
Once you get the basic idea of how the zener clamp works, you can start playing around with the resistor values (hint: is R2 really necessary?) and see how the performance is impacted.