Maker Pro
Maker Pro

impose current ceiling

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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That's still a pretty small heat sink.

Go to the resource on heatsinks and calculate the size heatsink you need.
 

flippineck

Sep 8, 2013
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Thanks will do. I was ok at physics at school but my maths sucked. Dug it up & bookmarked it, probably take me a while to get to grips with it.
 

flippineck

Sep 8, 2013
358
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Been a while but still keep coming back to this. The whole mosfet-based idea works well but I'm still wading brain deep in the heat transfer maths and physics. Ultimately this is definitely the way to go I think for my main project.

Deviating from the main project for a while though I bought a smallish 'toy' solar panel kit - 2 solar panels and a charge controller fitted with a lithium battery of some kind. Plan was just to power some gatepost lamps via a 12V dawn to dusk sensor.

Works fantastic on the sunniest of days (couple of hours of light from two 3W LED bulbs) but usual dull days only give me a small amount of power.. maybe the LEDs last 10 minutes before the batterys dead.

Looking at the specs of the 2 panels I have in front of me that came with and are designed for the kit, they have a nominal output of 17.5V / 5W and an open circuit output of 22V under standard test conditions. Nominal current 0.29A, short circuit current 0.32A.

I was reading around about VDRs and as far as I can figure, zinc oxide devices have a very sharp 'knee' at the clamping point on the IV curve, are available with clamping voltages around 18V or so, are extremely high resistance below the clamping voltage, and are quite good at dissipating large currents.

I also have three 100W solar panels rigged up on the wall outside. These have very close voltage specs to the smaller panels that came with the toy kit, within small fractions of volts, but obviously much larger current ratings.

So for a quick and dirty solution would this work.. connect a zinc oxide VDR with a clamp voltage around 18V in series with a 24V auto filament bulb, and then place the combination parallel across the input from the large solar panels (3 connected all in parallel).. then take the 'protected' feed into the toy solar charge controller via say, a 1A fuse for good luck?

The hope would be that on the usual dull days, the VDR would not conduct, the lamp would not glow, and the full (although meagre due to the clouds) output from the large panels would be available to the charge controller. On really sunny days the voltage would rise up to the clamp voltage, and the 24V lamp would shunt the majority of the high current away from the controller.

Yes, again a waste of solar panel area and good energy just thrown away, but overall, gatepost lamps that always stay on after dark for a reasonable time..

Would I be likely to find a VDR that could handle the stress and / or be heatsinked? Would I be better with a reverse-biased stud mounted high wattage zener on a big heatsink?

I don't think it's likely I'll get anywhere near the full 300W from the big panels for 99.9% of the year but it could happen I guess.. so maybe a fuse is in order on the feed incoming from the panels, to protect the zener or VDR.. I could live with changing a fuse once every blue moon

Ok where are the holes in my cunning plan, why would this not work.. I'm sure there are plenty!
 
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