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Topic: Get specs from unknown solar panel/cell (Read 1 time) previous topic - next topic


Thanks Ran for that extensive answer.
I did some measuring a minute ago using a really bright spotlight on the panel and got a voltage of 2.4-2.6 V and a current of 50-110 mA. However I should mention that I've kept the a protective plastic film on the panel so it might not get full exposure. I haven't tried with a resistor yet since I don't have a 50 Ohm resistor neither a 30 Ohm resistor (But if memory serves me right I should get about 50 from two 100 Ohm resistors in parallel, and 33.3 from three 100 Ohm resistors in parallel). Well I'll look into that later.

Thanks for the tip on the diodes aswell I'll be sure to pick some of them up in my next order.

However there is one thing I'm a bit puzzled about. That is when I connected the positive probe to the positive terminal on the panel and the negative probe on the negative terminal i got a negative voltage.
So my question is:
a. Did i connect my multimeter in the wrong way?
b. Could the labeling of the panel be wrong?
c. Some other reason as to why a solar panel gives out a negative voltage?


I just took one of my solar lights apart.. the circuit is amazingly easy.. just uses an LDR to turn the LED on and off.

But yeah, diodes are highly suggested, so you don't get discharge when you're not using it.

Generally, if you connect it backwards it will show up as a "negative" voltage, when really it's just connected backwards. But from the solar panel I got, I took the whole circuit, battery and what not, just hotglued the battery and circuit to the back, with the LDR sticking out.

It's outputting about .8 volts in my bedroom with the "real sunlight lightbulbs".. which is nowhere near the sun, obviously. But other thing you can keep in mind if you're playing around with these things, is you can get higher capacity batteries, like 1000maH, 1.2v for $4 each online.

But it sounds like you got the resistor idea down.

It sounds like the panel may be labeled wrong, but that seems kinda doubtful if they're mass produced. I haven't heard of any solar panels outputting a negative voltage (but I haven't searched for one either)


I've been looking for the LDR on this device since I bought it this summer but I really can't find one. Although my experience with LDRs don't really go much further than the CdS I have in my component box. However the circuit is as you say pretty darn simple. But there is a big black blob on the circuit which I'm a bit curious about. I doubt it's a LDR since I can put my thumb on it and shine a light on it without anything happening. Could it perhaps be a diode?
The thing is the LED in the circuit lights up when it's dark, or if the solar panel is disconnected. Which leads me to assume that if the battery is getting charged by the panel either the battery does not emit any current or the led is turned off by the circuit.

On another note regarding a regulator. Could one say that the battery acts like a regulator?


The lights I have use the output from the solar panel to determine when they need to switch on. A neat solution and 1 less component :)


Ran Talbott

Could one say that the battery acts like a regulator?

One could,  but one would be mistaken  :)

As a rough guide,  you can continue to feed about .1C ("C" refers to the battery's Capacity in mAH) into a Nicad or NiMH battery,  even after it's fully charged,  with "no harm" (it might do a tiny amount of damage to its long-term life,  but it's so small that the professionals say us ordinary folks can ignore it.  It might be different if you were designing a deep space probe,  but we're not).  So,  if you have a 100mA solar cell,  and a 1000mAH (or larger) battery,  you don't need a regulator:  if the solar cell tries to overcharge the battery,  the battery will just dissipate the extra energy as heat.

Having a charge controller becomes important when you have a solar panel that supplies a lot more energy than that:  e.g.,  if you design a system that is intended to power the electronics and make sure the battery is fully charged even on a cloudy day,  it probably has the ability to damage the battery when it's sunny.  Then it's important to include some circuit that can stop the charging when the battery is full.

For your purposes,  with these small panels,  you can probably get away with leaving out the charge control.


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