# The Naked Scientists Forum

### Author Topic: What is causing voltage drop?  (Read 1563 times)

#### thedoc

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##### What is causing voltage drop?
« on: 04/06/2015 11:50:01 »

I was playing with solar cells from calculators. Out of one cell I get 2.45 dc volts. When I hook 4 of  them together in a line I get 11.36 volts DC so I hooked that to a led flashlight that takes 4.20 dc volts to work and the light is very dim. I checked the volts when hooked up and only getting 2.47 why is that?

What do you think?
« Last Edit: 04/06/2015 11:50:01 by _system »

#### chiralSPO

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##### Re: What is causing voltage drop?
« Reply #1 on: 04/06/2015 14:32:39 »
It's hard to guess without seeing it. Could be a bad connection, or greater resistance than you're expecting. Also, even if you have enough voltage to activate the LED does not mean there is enough current to give a bright light.

It is also possible you have it connected the wrong way. LEDs are diodes, so it matters which direction the current flows through (it will have much higher resistance one way vs the other).

How are you measuring the voltage of the complete circuit (between which two points?)

Those are my best guesses...
« Last Edit: 04/06/2015 14:35:07 by chiralSPO »

#### alancalverd

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##### Re: What is causing voltage drop?
« Reply #2 on: 04/06/2015 20:00:06 »
A solar cell has a significan internal impedance - consider it to be a battery in series with a resistance. A flashlight LED has a fairly low impedance, so Ohm's law applies.....

#### syhprum

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##### Re: What is causing voltage drop?
« Reply #3 on: 05/06/2015 00:49:07 »
Solar cells are more like constant current devices than constant voltage devices so the voltage you get out depends on the resistance of the load.

#### evan_au

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##### Re: What is causing voltage drop?
« Reply #4 on: 05/06/2015 12:52:21 »
When a LED is conducting electricity, there is a fairly fixed voltage across it, depending on the material of which the LED is constructed.

Like all diodes, the current can vary over a wide range without the voltage changing very much. This voltage is related to the bandgap of the semiconductor material, so if it takes 2.6eV to push an electron through the semiconductor junction, then there will be a voltage above 2.6V across the diode. Photons are emitted when electrons drop down across the bandgap and recombine with holes, emitting photons of less than 2.6eV.

At 15-40% efficiency, LEDs are better at turning electrical current into visible light than incandescent lamps.
The LED torch probably has some circuitry to convert the battery voltage into the optimum voltage for the LED, and this will waste a bit more power.
At around 5-10% efficiency, solar cells are much less efficient at turning light into electricity.
So overall, perhaps 1-3% of the solar radiation is getting turned into visible light.

Small red & green LEDs typically operate over currents of 1-10mA, while the modern "white" LEDs used in torches and household lighting operate at far higher currents. You could set your multimeter to the "mA" range, and see how much current the solar cells are delivering into the torch; it could be less than 1mA, as solar-powered calculators don't require much power (especially if they are designed to work indoors).

If you want light (and don't care too much about missed educational opportunities), a small mirror will be much more effective at redirecting light from one place to another.

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##### Re: What is causing voltage drop?
« Reply #4 on: 05/06/2015 12:52:21 »