Simple series Zener diode to drop voltage - okay?
Simple series Zener diode to drop voltage - okay?
(OP)
I think I already know the answer (because it seems to be trivial), but I really want to make sure I'm not missing anything.
I'm installing a system which is rated for a supply voltage range of 9 to 30 volts DC. The available power supply is a nominal "28 volts", but the power supply specification actually allows the supply to wander up as high as above 30.5 volts. So there's an issue since 30.5 > 30.0.
I'm considering using a simple Zener diode in series with the + supply line to knock off about 6 volts. This should drop the voltage down into the sweet spot (~22 volts) and avoid the over-voltage issue.
The current is fairly low (~25mA) so the power dissipation is not a significant road block.
I've seen this simple technique used before, but it was always presented as a 'hack' (to use the modern word).
So, the question: Is there any reaon not to use a simple series Zener diode to drop a few volts? And side-effects or details that I need to watch out for?
Thanks.
I'm installing a system which is rated for a supply voltage range of 9 to 30 volts DC. The available power supply is a nominal "28 volts", but the power supply specification actually allows the supply to wander up as high as above 30.5 volts. So there's an issue since 30.5 > 30.0.
I'm considering using a simple Zener diode in series with the + supply line to knock off about 6 volts. This should drop the voltage down into the sweet spot (~22 volts) and avoid the over-voltage issue.
The current is fairly low (~25mA) so the power dissipation is not a significant road block.
I've seen this simple technique used before, but it was always presented as a 'hack' (to use the modern word).
So, the question: Is there any reaon not to use a simple series Zener diode to drop a few volts? And side-effects or details that I need to watch out for?
Thanks.





RE: Simple series Zener diode to drop voltage - okay?
However, as long as you exceed the minimum voltage and current for avalance breakdown, you should be fine with a zener.
RE: Simple series Zener diode to drop voltage - okay?
The current is quite low, about 25mA. Thus a 6 volt Zener would be under 0.25 watt, so I could probably use a 1-watt version (to be confirmed by worst case analysis later).
The 'normal' solution seems to be to use a $1k Mil-Spec DC-DC converter (28 in , perhaps 15 out) and so on. Seems to be a bit much...
RE: Simple series Zener diode to drop voltage - okay?
Not that it is likely to happen, but if one of them gets shorted, the other one will still drop around 5 volts.
You are right when you say that this is trivial. Actually, I don't think you will have any problem at all connecting your PSU directly to the device. Half-a volt above spec shouldn't kill your low-powered application.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Simple series Zener diode to drop voltage - okay?
6.2V is a standard zener voltage.
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If we learn from our mistakes I'm getting a great education!
RE: Simple series Zener diode to drop voltage - okay?
On the other hand, 5.1 isn't in the E12 series either - but 5.6 is. Funny world, this electronic world.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Simple series Zener diode to drop voltage - okay?
RE: Simple series Zener diode to drop voltage - okay?
I mentioned 'about 6 volts' because it seemed to be enough to get away from the over voltage condition, and yet not be so high that the power dissipation on the Zener becomes significant.
RE: Simple series Zener diode to drop voltage - okay?
And one in series is fine. Do make sure you give it a lot of thermal head room. I have seen so many caramelized boards around zeners.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Simple series Zener diode to drop voltage - okay?
RE: Simple series Zener diode to drop voltage - okay?
So I'm still leaning towards the series 6.2v Zener diode to knock-off about six volts. The thing that had me a bit worried was that this technique was always presented as a 'hack' (a cheat). So I wanted to double-check in case I'm missing something.
RE: Simple series Zener diode to drop voltage - okay?
Will the load's minimum current be above the Zener's leakage current?
RE: Simple series Zener diode to drop voltage - okay?
RE: Simple series Zener diode to drop voltage - okay?
Also, the MTBF should improve if we keep the power supply further away from the upper limit; probably more than enough to compensate for the reduction in MTBF from having an 'extra' part, the Zener diode.
Typically, a 3-terminal regulator application note calls for capacitors on the input and output pins. Although capacitors are cheap, it suddenly becomes a 'design project' and a paper storm ensues.
RE: Simple series Zener diode to drop voltage - okay?
The device was spec'd at 24V, and I measured the supply at 24.7 volts. They wouldn't cover it under warranty. Traces burned off the board.
Dan