Solar Cells need help

[Donald S. Lambert]

Got interested in solar cells so I ordered some from Electronic Goldmine. You know, the assortment of broken ones. The instructions to solder leads to them state to use #26 stranded wire which I haven't found but did find some #24 solid wire at Lowe's (telephone wire 4 wires in a plastic cover (red, black, yellow and green) at $0.08 per foot.<p>I have googled sola cells but the information is all theory and none practical. I understand a photo cell puts out from 0.46 V to 0.52 v and short circuit amperage depends on the solar cell size. What reading I have done leads me to think that the solar cells are like a battery except that they are inexhaustible so far as a voltage/current source so long as they are in sun light or any light. And that a short circuit will not damage them like it will a battery. And they can be connected in serries or parallel like batteries can.<p>Are the above concepts right or have I missed other important concepts obout solar cells? I'll be experimenting with them and don't have a Radio Shack (or other company) experiement kit and manual to guide me. Greybie

[rshayes]

A solar cell is basically a silicon diode with a large area. Silicon diodes are also photosensitive, but usually the package is arranged to prevent light from reaching the junction area.<p>The equivalent circuit of a solar cell could be thought of as a silicon diode paralleled with a current source representing the photocurrent. The photocurrent will be proportional to the incident light intensity and the cell area. Since the voltage on a silicon diode is proportional to the log of the current passing through it, the open circuit voltage of the solar cell will be proportional to the log of the light intensity. The temperature coefficient will be about the same as that of a silicon diode, negative at about 2.2 millivolts per degree centigrade.<p>When the cell is short circuited, the voltage across the junction is zero and the current through the diode is also zero. The current through the short circuit is then linearly proportional to the photocurrent and the temperature coefficient is close to zero. <p>In both these cases, the power output is zero,since either the current is zero (open circuit) or the voltage is zero (short circuit). Somewhere between these point is a point where the solar cell delivers maximum power. This operating point is not fixed, but depends on the light intensity.<p>There will also be a large amount of junction capacitance, due to the large area of the diode and the low bias voltage. This may cause problems in feedback based circuits, such as transresistance amplifiers. I would expect thousnads of picofarads or more.

[jwax]

MOST solar cells these days are silicon based.
Some alternatives are cadmium sulfide, and CIGS- copper, indium, gallium, selenium. A few research types are gallium, arsenic, aluminum producing impressive efficiencies, but way to expensive for general use.
Greybie, the only problem I see with your analogy with chemical batteries is that a shorted solar cell may overheat if left in sunlight. That overtemperature may permanently degrade or damage it. Other than that, shine on!

[ezpcb]

Hi Greybie I think there's no too much concerns in usage of solar cell. just buy the right cell according to your voltage and current requirement, then connect them to your application. sometimes a voltate or current limiter or voltage regulator may be used. <p>I have a DIY solar cell phone battery charger, which cost me less than $15. it works nice when exposured under the sun shine of clear day. the solar cell is silicon based the size is about 15cm by 5cm, the maxim output I had ever tested is 26.8v 15.2mA, it's even enough for charging the car battery. But the efficiency seem to be low, according to the power of the sun light, only 7-8% energy of the sun light is converted.<p>I'm planning to buy many cells to cover my car, I think the cells could make my car very cool in the summer sun shine, and the energy maybe enough for my 30W car radio station. <p>mike<p>[ June 04, 2005: Message edited by: ezpcb.com ]</p>

[Donald S. Lambert]

An update,
Soldered two 4" leads to a broken photo cell. However I assumed the top (light sensitive area) was the positive pole but it turned out to be negative so I will solder the black wire to the top in the future.It took several tries to get the solder to stick. I had to use rosin solder flux to do that, stuff I bought back in 1943. I always try to have positive with red wire and negative with black wire. Greybie

[Donald S. Lambert]

Update #2
Regardless of what the information sheet from Goldmine stated it takes a fair amount of heat to do a good solder job. I soldered a few more and redid the one with the red wire on the negative so that it is red on the positive. And I dug out my old Archer VOM to make it easier to read the output voltage while the solar cell is illuminated by a bulb in a reflector. Now to mount the cells on a squareof cardboard to get higer voltage and amperage is next after I get a few more soldered to leads. Greybie