zener diode kludge?

[nocentelli]

I'm going to breadboard this envelope filter tonight, but i have no 3v9 zeners. I have other values, maybe 5v1. I've seen plenty of OTA filters of this type, but i've not come across this particular variation with what looks like a bias voltage applied to pin 1+16, so i'm not sure if it could be kludged with a straight voltage divider, say 6k8 instead of the zener?

[CodeMonk]

I have no idea really, but I'm gonna take a stab at it anyway.
The zener is for an ABSOLUTE voltage regardless of the input voltage, whereas the output of a voltage divider is dependent on the voltage going in.

I may be wrong here, but that's my guess.

[Rixen]

two forward biased LED's.. or..
six forward biased silicon diodes- I recently had to substitute for a 3.3V zener, and a chain of 1N4004 diodes actually performed better..

[nocentelli]

Thanks, i'll try a couple of leds

[nocentelli]

So, like this?

[slacker]

Yeah like that.

I think codemonk is correct, the zener keeps the voltage more or less constant even if the supply voltage changes. Useful for battery powered effects as the voltage won't change as the battery goes flat for example.

[duck_arse]

stick in the 5V1 you said you had. put to K 10k//22k to ground, take 3V9 from the junction.

Mr Ohm is our friend, always.

[edit: ]my resistor values might load the zener V, I dunno.

[nocentelli]

Thanks, just to be clear, do you mean put a 22k in parallel with the (5v1) zener to ground (-v on the schem)?

[duck_arse]

yerp, V-. I missed the V/2 to ground trickery on the diagram. the resistors just string across the zener, whatever it is connected to.

probably wouuldn't hurt to add an electro across the zener, for smoothing. unless the whole thing falls over, then I'd leave it off.

[edit: ] so, that's (V+) - 10k - [new electro and] zener and new 10k and new 22k - (V-)

[nocentelli]

Ah! So the zener provides 5.1v, and a new 10k/22k divider slices that voltage to around 3.9v?

[nocentelli]

If that's the idea 10k/33k would be closer to 3.9v. Thanks, I'll try that as well.

[earthtonesaudio]

I have no idea really, but I'm gonna take a stab at it anyway.
The zener is for an ABSOLUTE voltage regardless of the input voltage, whereas the output of a voltage divider is dependent on the voltage going in.

I may be wrong here, but that's my guess.

Technically not absolute, but they have a much sharper knee than regular diodes. Next to a zener, a Ge diode looks almost like a resistor.

If that's the idea 10k/33k would be closer to 3.9v. Thanks, I'll try that as well.

Note that the 10k+33k are in parallel with the zener, which will (slightly) affect its voltage. But it should be very slight and might not matter.

Another option would be to put 2 Si diodes in series (replacing the 10k between the 33k and the zener in the above image) which would give about 3.7v.

[bluebunny]

@leo: I just noticed you're in Leeds.  I could have posted you a 3V9 Zener by now!  PM me if you want one (and assuming I do indeed have one!).

[nocentelli]

Wow, very kind. I've put it on my next order along with a whole lot of other bits. It's off the breadboard for a while to make space for a simpler build i know i can have boxed-up by sunday.