Regulating two voltage rails with one zener diode

[BrianJ]

Slightly off topic...

I am working on a microphone preamp that will run on +-24 volts.  It is based quite closely on the John Hardy M1 preamp if any of you are familiar.  There is a small sub circuit that needs to run on +-15 volts that monitors and cancels DC offsets in the audio circuit.  My question is can anyone confirm if I would be able regulate +-24V down to +-15V with one 30 volt zener tied across the rails and two equal dropping resistor in series with both 15v rails before the zener?  The current consumption is small and predictable and the +-24 will be well filtered and regulated so I am not worried about "zener" performance as much as board space and parts count.

Any thoughts?

[alanlan]

I don't see why it should not work provided you add plenty of decoupling to the 30V supply i.e. across the Zener.  The only issue I can see is that if there were a significant difference in the DC current draw from the + and - 15V rails, you could end up with an offset i.e. one at say +14V and the other at -16V w.r.t the existing 0V.  This could be corrected for if, as you say, the current draw was unlikely to change once known.

For the cost of an extra zener and a bit of extra decoupling, it might be worth regulating each rail separately.

[Cliff Schecht]

Yeah you have a problem there I think. You're current has to go somewhere, and with no ground reference it gets dumped into the power rails. This shows up as noise, which needs filtering, which takes a lot of room. Also, even if you know the current draw to a certain extent, you're dealing with a tolerance problem. With 5% zeners, 5% resistors, temperature drift of said parts and filtering, you're looking at a pretty bad circuit. You could instead try two surface mount zeners (link at bottom) with the proper corresponding circuitry (i.e. current limiting resistor). Honestly, with 2 zener regulators, you could even get away not filtering the zener outputs, just use 0.1 uF bypass caps on your IC's.


Mouser link: http://www.mouser.com/Search/Refine.aspx?N=4932245+4294601609&Keyword=Vishay+Semiconductor+SMD+Zener+Diodes&FS=True

[R.G.]

Alan and Cliff are correct - you're doing something that is iffy to start with, and worse yet, you're counting on this iffy thing to correct out errors in something else.

I have an opinion that is purely my own, developed over designing power supplies for a living for years. IMHO, it is unreliable and therefore bad practice to ever willingly use a zener diode as a shunt regulator for providing power to something else. Zeners are OK as references and clamps, but I've seen too many smoking holes in PCBs where zeners used to be to make me comfortable about them.

You're obviously going to some trouble to make this thing work well. Why skimp on the quality of the (probably) servo controller? If it can be run by zeners, it can probably be run by 78L15/79L15 or the full-bore 7815/7915 regulator pairs. These things are self protecting for temperature and overcurrent, and are only about $0.50 each in ones. The little ("L") ones are in TO-92 packages and the big ones are in TO-220 packages. Neither one needs a power dropping resistor, which your zeners most assuredly do.

Don't cheap out on a design you're counting on to be good.

[Cliff Schecht]

That was something else I meant to mention: You're designing a circuit that's supposed to be accurate, but a slight fluctuation in your power rails could completely throw off the accuracy of your design. Even if it's an op-amp based circuit, where the gain of the circuit is independent of the power supply, variations in V+ and V- will vary the output DC offset. Go too cheap and you'll run into problems every time, sorry.

[BrianJ]

Thanks everyone, you are absolutely right.  I will try to source some Positive and Negative regulators in a TO92 package.   I guess I was hoping the solution would be closer to elegant than iffy but it couldn't be more true that a project like this demands quality.

You are correct, RG, that is a DC servo that I am trying to regulate on board for each channel.  I actually want to put the power supply for each channel on the board with the audio circuit so I can feed each channel raw, unregulated DC individually.