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I am making some pretty crude devices that are directly powered from solar panels. The instability is not an issue, but I want to avoid over-voltage.

I am using two 100W panels in series and want to make sure on a very (very!!) sunny day it does not exceed 30V.

If I use a 30V Zener diode alone would that be OK? Looking at the power curve am I right to assume there will be no power if a Zener shorts it? Does the panel try to fix the current so the voltage drops?

Current-Voltage Curve

This is opposite to a constant voltage supply that would make it blow up? Or have I missed the point?

JYelton
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Shorthair
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  • what is the open circuit voltage and the short circuit current? – Andy aka Mar 11 '14 at 13:35
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    Solar cells have nicely predictable maximum output voltages. Just make sure to not put so many cells in series that they can exceed 30 V with maximum possible insolation. – Olin Lathrop Mar 11 '14 at 13:41

1 Answers1

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The zener doesn't "short" the power source (panel), it simply limits the terminal voltage to a fixed value. The zener will be dissipating power equal to the zener voltage multiplied by whatever current the panel can deliver at that voltage (minus whatever current the load is drawing, of course), so it needs to have a suitable power rating.

You can "amplify" the power handling capability of a zener diode with a power transistor, which you can then bolt to a heatsink. Such a setup would be able to absorb the full 100W of the panel if necessary.

schematic

simulate this circuit – Schematic created using CircuitLab

This circuit doesn't do anything at all as long as the input voltage is less than the zener voltage. However, once the zener starts conducting, the transistor will also conduct about 50× the zener current, and will dissipate about 50× the power. If the panel can deliver, say, 3A @ 30V, the transistor will dissipate about 88W and the zener will dissplate about 2W.

Dave Tweed
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    Dave's answer is exactly correct, and the approach may be OK if the Zener is almost never on (which may be the case). When on, The Zener will be dissipating 30V*Ipanel. If you used a circuit with a pass element (a linear regulator) you would dissipate only (Vin-Vout)*Ipanel. The pass element could be a low RDSon FET so that the normal losses are small. Best solution is to build or buy a MPPT controller to hook to the panel to make full use of its energy as well as regulating the output voltage. – John D Mar 11 '14 at 14:46
  • Ah yes, I started realizing that's how it works after I asked the question, thankyou for helping! My load is a 40W amplifier but probably barely uses power when silent, so if on a sunny day and the amp is silent the zenner will probably take quite a lot of that power. – Shorthair Mar 12 '14 at 12:37
  • How about adding one more Zener in parallel. – AKR Mar 13 '14 at 06:56
  • For anyone seeing this 6+ years later, two zener diodes in parallel don't work because due to manufacturing imperfections one will have a slightly lower on voltage than the other. This means that one will turn on first, conducting all the current and the other will never turn on at all. – Blaine Dec 01 '20 at 22:27