# Physicist in training with a dumb question...

Discussion in 'General Electronics Chat' started by charlesread, Jun 19, 2008.

Jun 19, 2008
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Hey everybody!

Maybe I overthink things...

I'm trying to teach myself electronics, yeah it's a bit bumpy, but LOTS of fun! But I have a few questions that I would be embarrassed to ask a professor, so maybe you can help me.

I am playing around with solar cells, and am trying to understand some basics, I see that some cheap cells provide say 0.5 V and 0.8 A in full sunlight, so is that to say that the cell can deliver 0.4 W to a load?

Kind of related, when a load has a power rating like 100W on a light bulb, does that mean that in order to operate normally the light bulb NEEDS 100W? If so, does that mean that ANY source that would produce 100W would light the bulb? Like some crazy solar cell that put out 0.5V and 200A?

And I'm getting confused when I see power supplies give specifications in voltage and current... I thought it was voltage that 'created' current (by Ohm's law), so why does the power supply (or whatever, solar cell.. anything) give me a current specification??? What does this mean?

Any help is greatly appreciated!!!!

2. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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Your solar cell example is correct: 400 milliwatts maximum in full sunlight.

Load power consumption ratings are generally indexed to rated voltage. For example, a 120V 10 Watt incandescent light will consume much less power at 12V than at 120V.

The voltage rating of a power supply is the voltage it will supply. the current rating is the maximum current it can supply without having the voltage drop below rating. A load will only draw as much current as it needs.

So... A "crazy solar cell" rated for max current of 500 A at 0.5V would not be able to light up a 120V 5 Watt bulb at all.

3. ### recca02 Senior Member

Apr 2, 2007
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However, A "crazy solar cell" rated for max current of 500 A at 0.5V would be able to light up a 0.5 V 250 Watt "crazy bulb".

4. ### thingmaker3 Retired Moderator

May 16, 2005
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Now that is indeed a bright thought!

5. ### recca02 Senior Member

Apr 2, 2007
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I just had a light bulb moment.

Usually AFAIK the power that can be delivered by a solar cell is limited. It hardly has an efficiency of some 10%(IIRC, not sure) of the total solar power incident on it. So the crazy cell must be huge in area.

6. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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As a rough rule of thumb, solar energy is often estimated at around 1kW/mSq at the earths surface. Solar panels behave like current sources and there is an optimum load condition for maximum power output but I can't remember what it is.

Jun 19, 2008
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Thanks everybody!!!

blocco, so you're talking about the luminosity of the sun as measured from the surface of the earth? Interesting! I did not know that! So that is to say that the maximum power I could pull off of a square meter of solar cell is 1 kW? (Assuming 100% solar cell efficiency?

8. ### Wendy Moderator

Mar 24, 2008
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A common technique to boost solar cell efficiencies is to reflect more sunlight to it's surface, either with a lense or mirrors. You have to be careful of the heating effects though.

9. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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Looks like the best solar cells have an efficiency of around 40% which is still quite impressive at 400W from a square meter but I estimate that in a typical installation after regulation, battery charging and inverter losses the actual usable power could be around half that.

10. ### thingmaker3 Retired Moderator

May 16, 2005
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State of the art may be 40%, but state of the market is closer to 12%. Note also the per-square-meter figures are for something pointed directly at the sun.

11. ### Audioguru New Member

Dec 20, 2007
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If a solar cell can output 500A then it can light up a piece of wire.

12. ### thingmaker3 Retired Moderator

May 16, 2005
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Yes indeed. There would be a nice "pop," or even "bang" sound accompanying the wire if it were large gage.

Jun 19, 2008
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So if I am figuring out how many solar cells I would need to drive a load, I should hook the load up in just a straight up circuit, measure the current and voltage being delivered, and then choose solar cells that can provide that right? Like if I'm going to recharge a battery I need to make sure my combo of solar cells matches the battery voltage, or if I'm running a motor I need to match the current going to the motor off of a normal power supply?

14. ### hgmjr Moderator

Jan 28, 2005
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There are regulating units that are designed to use with solar panels that maintain its output constant.

hgmjr

15. ### thingmaker3 Retired Moderator

May 16, 2005
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If you live in Egypt or Arizona, and have a tracking system to change the angle of the panels throughout the day, such a plan might work. If you live in my area, you'll need a lot of extra panels to make up for the diminished sunlight.

Also, will you be driving that load 16 hours per day? 24? If so, you'll need the Watt-hours produced by your panels to equal or exceed the Watt-hours used by the load.

Expect reasonable losses from any battery charger or inverter.

16. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Last edited by a moderator: Jun 22, 2008