Zener Diodes

Re: Zener Diodes

Simple answer: you don't.

Solar panels have internal bypass diodes like this: http://www.solar-electric.com/8ampbypdiod.html The purpose of which is to prevent shaded cells from being overwhelmed by reverse bias Voltage from non-shaded cells.
And may have internal or external blocking diodes like this: http://www.solar-electric.com/blocdiod8amp.html The purpose of which is to prevent battery current from leaking back through the panels at night causing the battery to discharge. Usually these are found of small, stand-alone battery maintainers as large panels will be connected with a charge controller which should have enough internal semi-conductor circuitry to prevent the problem.

Zener diodes are just about the opposite of blocking and bypass diodes, in that whereas they allow forward current most of the time they have a specific Voltage point above which they allow reverse current. They are most typically used in simple Voltage regulating circuits: an in-line resistor to the (+) side of the diode, the (-) being connected to ground. If Voltage exceeds Zener it is shunted to ground. This has to be able to handle the current potential of course.

As solar panels are a current source not a Voltage source this type of regulation is not well-suited to solar electric.

Re: Zener Diodes

The problem is you're trying to take the output of a solar panel which varies in both Voltage and current and turn it into a power source that doesn't vary in either Voltage or current.

I think you're going to have to investigate the possibilities inherent in IC Voltage regulators like an LM723, along with a few extra support components.

Circuit design is not my field of expertise. Besides, I'm so out-of-date it would probably be based on a 5U4.

Re: Zener Diodes

Except that solar panels are current sources, not Voltage sources.
A little bit of light hits them and the Voc goes right up. A Voltage regulator is going to try and regulate Voc, which applies a load to the terminals causing the Voc to drop like a rock until enough light falls on the panel that it can produce some current. Meanwhile the regulating circuit has switched off again and the Voc has shot up again and the regulator tries to clamp it down again and ... eventually the panel will manage something along the lines of Vmp * Imp. But as far as the panel is concerned, maintaining Imp is what's important. Put more load on (i.e. try to sink excess Voltage) and it will allow V to drop so long as it can maintain I. You could end up with a panel producing full I at some small portion of V and a regulator going nuts trying to manage this into a fix V*I output within the right range.

In short, this makes it a bloody awful power source. Perfect for charging batteries though, since you want max I at the low V point of the discharged battery and then have it taper off as V increases.

A simple circuit isn't going to do it without risk to the device being powered. The closer the panel's spec are to the loads, the easier it will be. You're after <8 Watts of power from a 24 Watt panel. In overly simplistic terms that means 16 Watts has to go somewhere. Try for something like the PWM used in simple charge controllers substituting a capacitor for the battery part. Just enough cap to smooth things out and keep the thing running and just enough pulsing to keep the cap charged. When the panel is disconnected it produces nothing, so there's no "excess" power that needs to be shunted anywhere.

Thirty years ago I would have known what I was talking about here. That's no longer the case, I'm sorry to say.