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hubertb avatar image
hubertb asked

Can I use a Z-Diode / Zener-Diode and a resistor to protect a solar charge controller from high voltage?

Hi there!

I recently got myself a SmartSolar MPPT 75/15 and two solar panels (30 Wattpeak, Voc 22.68 Volts, Isc 1.85 Amps), which I connected in series to charge a 12 Volt lead acid battery. As well all know, the panel voltage needs to be 5 Volt higher than the battery voltage for the charging to start.

In order to start charging earlier on the morning and to continue charging later in the evening, I am planning to connect a third solar panel, leaving me with three panels in series. On a sunny day, I measured a Voc of 24 Volts on one panel. Putting three of these panels in series gives a Voc of 3 * 24 = 72 Volts and thus, would bring me close to the maximum voltage my solar charger can handle (75 Volt).

I am wondering if I can put a Z-Diode (also known as Zener-Diode) rated at 75 Volt 1 Watt, along with a 5.6 kOhm resistor in parallel to the PV input clamps of the SmartSolar solar charger.

If the voltage would hit 75 Volts (or raise even higher than that), the Z-Diode / Zener-Diode would become conductive, effectively shorting the panels over the 5.6 kOhm resistor and letting a current of 0.013 Amps flow.

Question is: Is my above theory correct? Or am I missing something? Thanks :)

MPPT Controllers
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2 Answers
nigelfxs avatar image
nigelfxs answered ·

Hi Hubert

The broad concept of what you are suggesting is correct, but there are some practical difficulties.

If your battery is fully charged, then the Zener will need to conduct a large percentage of the Solar panel current in order to clamp the voltage to a safe level. This means that the Zener diode would need to be rated at almost the same power as your solar panels. This means a very large and expensive Zener, on a big heatsink!

Another issue is that the Zener threshold voltage is typically not very accurate, so you would have to select a part that is slightly lower voltage than your maximum MPPT input limit.

The 5.6k load resistor would make no significant difference to the maximum voltage from the solar panels - you need to load the panel with around 60% - 70% of the Imp current to drop the output voltage to a safe level.

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hubertb avatar image
hubertb answered ·

I realized I never came back to say "thank you". Sorry for that!

As for the original question: I am planning to solve the issue by upgrading to a SmartSolar MPPT 100/15 or 100/20 this year.

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