Need some help with a zener voltage knock-down circuit

[midwayfair]

I had a Selmer Buzztone build come back for repairs recently. I had a zener diode regulator in there to knock the voltage down to about 3V:

9V -> 100R - 1N4001 - 1N5817 ->(~3.1v) Circuit
                                |
                             3.3v
                               G

I had the 100R at a 1/4W, but that actually burned up (completely fried, like to the point where it took out a capacitor sitting next to it and disintegrated its leads). I replaced it and noticed it was getting a little hot, so I increased the wattage to a 1W (or 1/2W) and it got a tiny bit warm still but not enough to fear it would burn up.

The circuit doesn't really look like it should draw enough current to fry a 100R at a 1/4W while running on 3V:



I assume that my Zener circuit is the one that's really responsible for the larger current draw.

Anyway, I decided that I'd knock down the voltage a bit with an LED in series with the 1N4001 before it hits the Zener. Adding this has made the 100R barely warm to the touch, so it's saving the resistor, but now I'm worried about the LED, which is the reason I didn't include it in the first place. The LED's current is around 26mA right now, so while it's fine running basically forever on a 1Spot, I'm worried that if someone connects a 12V or higher to it, it's just going to blow the LED instead and the pedal will be right back in for repair.

I can't increase the 100R without the circuit acting weird. I could stick a 470R or something to ground right after the 100R would be better than the LED. This would knock the voltage down to about 7.4V before the LED and leave me 3V for the buzztone circuit, and would survive 12V, but it won't save the pedal if they connect an 18V.

The other thing is that I could instead try to fit a 5V regulator in before the Zener, to knock the voltage down most of the way. I think the LM7805 can survive 18V for a brief period. (It's the ceiling of the operating range according to the datasheet). Is that my best option? Is there anything I can do that I haven't thought of?

[Scruffie]

A 78L33

Where did you get the 18V number for the 7805? They have a terminal voltage of 30V.

[midwayfair]

A 78L33

Where did you get the 18V number for the 7805? They have a terminal voltage of 30V.

Do they? Maybe I misread it. That's not for the version with the heat sink, is it? Well, that settles it for me, then.

And yeah, if I had the foresight to order 3V regulators, I would have used those in the first place ... I don't do the digital stuff that requires them to keep them around, though ...

[TFZ]

You just need to add one transistor. This avoids the problem of having to accomodate the resistor for both the zener current and your circuit. Make the resistor 1-2k, the capacitor is for filtering the zener diode noise, 100n will do. For the transistor use any standard high-beta type, BC550C or similar. If you want to have 3V, you need to add one V_BE drop to the zener diode, so use a 3,6V or 3,9V. Or add a silicon or schottky diode in series, depending on where you want to land exactly.

The other advantage of this circuit is it is more stable and less load-dependent.

[Drew Hallenbeck]

Another simple solution to keep the 100R resistor from overheating would be to replace it with two 200R resistors connected in parallel. You'd still end up with the overall value of 100R but each resistor would only be carrying half the current.
Of course this doesn't solve the problem of having a higher than intended current draw but hey, it's a simple fix.

[midwayfair]

You just need to add one transistor. This avoids the problem of having to accomodate the resistor for both the zener current and your circuit. Make the resistor 1-2k, the capacitor is for filtering the zener diode noise, 100n will do. For the transistor use any standard high-beta type, BC550C or similar. If you want to have 3V, you need to add one V_BE drop to the zener diode, so use a 3,6V or 3,9V. Or add a silicon or schottky diode in series, depending on where you want to land exactly.

The other advantage of this circuit is it is more stable and less load-dependent.

This is what I use in my mic preamps to regulate to 48V.

This works well, it's nice and quiet. I put a 1N4001 in series with the Zener and it's sitting at 3.2V, which is good enough. Thanks for the help.