solar cell how to measure output current

Sounds like you have some circuit working already. I don't know (big) solar cells. For little photodiodes and things you can either measure the open circuit voltage (V~log (Intensity)) or the short circuit current, (I~ Intensity)

George h.

Use a voltage comparator - basically an op-amp with an open-collector output for driving a little bit more current.

The 18-21 volts could be easily discriminated (down to a few millivolts) but you'd need something like a precisian reference that doesn't fluctuate with the panel's voltage output. That could be as simple as a zener diode or just use a 3 terminal regulator to power the indicator circuit and reference against a voltage divider.

(bearing in mind that if you have a 5 volt regulator, for instance, the comparator may be destroyed if you try to plug 18 volts into one of the comparator inputs, the voltage divider takes care of that but another diode to clamp the voltage at a safe level would also be nice)

If your'e getting power from it, then measure I-V along the 'diode' curve... modulate something a little bit.

George H. (are solar panels at all close to 'ideal diodes'?)

El dilluns, 22 octubre de 2018 16:48:23 UTC+2, default va escriure:

mr Default : there must NOT be a 5 Vcc regulator : the circuit has to get voltage from the solar cell, which is the only power available on site.

Thanks. Ceba.

mr George : the solar cell is not a "working" one, but a small one (30 cm x 30 cm, max 10 W) to be used as "monitor" of sun light.

Anyway, of course I want to measure I/V along the diode curve.

My problem is I know no circuit to do that.

Thanks. Ceba.

30 cm x 30 cm is pretty big compared to things I use. But still measure Voltage OC or current SC... Or along the way Assuming you have a few millamps to power things you could make an opamp-Fet-R current sink and measure the voltage as a function of current. 10W you'd need a heat sink. How about a smaller solar cell?

George H.

The five volt regulator would work off the 18+ the solar array puts out. You can also shop around for one that uses miniscule amounts of standby current. An ordinary 7805 eats something like 3 milliamps, but they make others that go into the low microamp ranges too.

Or a zener diode to provide the reference voltage for the comparator, and operate the comparator on unregulated power.

mr George : can you clarify a bit ?

1) open circuit voltage is ? ... the measure of voltage without any load on the cell ?

2) short circuit current is ? ... the measure of intensity with only the Fluke, so R = 0 ?

Case (2) is not dangerous for the cell ??

Thanks.

Ok, mr George : I did the test. At a given moment, with a given sun, I read Voc = 4,2 volts, Isc = 107 mA.

Whay now ? Thanks for your patience ... Ceba.

Just George.. no Mr. please..it just makes me feel old.

So for a photodiode I made the following setup on my bench. Adjustable light, photodiode, measure I and V. See pics here.

Data, I(uA) V(mV)

15 275 150 349 1590 440 3700 490

The last was with the incandescent light right above the PD. I went back to repeat the lower current numbers.. and V_oc was smaller. I then remembered that the open circuit voltage depends on temperature.. so that's not going to be the best thing for you to measure.

Anyway the SC current will tell you how much sunlight is hitting your cell.

George H.

Hi, George - I am 63, so no reason to feel ... old

I can mesure the SC current, but .. is it correct to have the cell always in Short Circuit ?

If if is is providing me 10 W ... lets say 15 volts ... it means 0,6 amperes. Or 30 ohms.

Are you worried that the short circuit will damage the solar cell? I don't think that is a problem. Just put a small value resistor across it, (say 0.1 to 1 ohm.) to measure current. (0.1 if you think the max current is an amp.)

George H.

Google for MPPT (Maximum Power Point Transfer). There are ICs that do all this including DC-DC converter functionality so you can get whatever power is available out at the voltage you need.

e.g.