# Easy question about Solar Cells / Electric Motor specs

Discussion in 'Electronic Basics' started by Mike V, Jul 4, 2004.

1. ### Mike VGuest

Hi,

The short of what I'm doing is trying to power a very small electric motor
with some small solar cells. I need to determine how many solar cells I'll
need to power this motor (assuming perfect sunlight, anyway). Basically
this comes down to understanding what the heck the ratings mean.

For example, one solar cell claims to "produce 3.5 volts (open circuit) @
6mA (short circuit) in noon sunlight."

Does this mean that the maximum amperage it can achieve is 6mA? When it
mentions open circuit versus short circuit, why is there a distinction?
Does it only produce voltage when the circuit is "open"? I'm having a hard
time making sense of that.

Further, I'm looking at a tiny motor that has specs like:
"1.5V gives 17.5mA free draw current (120mA stall) at 9700RPM".
What does it mean it "gives 17.5mA"? Does that mean that the motor has
sufficient resistance that, given any 1.5 volt power source, it will only
let 17.5 milliamps through? What does the "120mA stall" part mean?

I apologize for all the questions. I can easily find resources that talk
about current/voltage/resistance, but I can't seem to find too many that
talk about the specifics of these types of devices.

Thanks,
Mike

2. ### Tom BiasiGuest

Hi Mike, The cell will produce 3.5 V. with no load.
Because of its limitations it can supply a max of 6mA.
The motor will draw 17.5mA at operating speed (9700RPM).
If the shaft is held to a stop it will draw 129mA.
Hope this helps.
Tom

3. ### GarethGuest

It may help to think of the solar cell as a 3.5V voltage source in
series with a resistor. This resistor represents the internal
resistance of the cell. You know about Ohm's law since you mention
resistance, so you probably know that the voltage across a resistor is
given by:

V = I*R

where V = voltage in Volts, I = current in Amps and R = resistance in Ohms.

If the solar cell is open circuit no current flows in the resistor so
the voltage across it is zero and the voltage you measure at the output
is the voltage of the voltage source (3.5V). If you short the output
the current will be limited by the series resistance and the measured
voltage at the output will be zero since all the voltage is dropped
across the internal resistance of the cell.

You can work out what the internal resistance is since you know the
short circuit current and the open circuit voltage.

R = V/I = 3.5V/6mA = 583 Ohms

By thinking of the solar cell as a 3.5V voltage source in series with a
583 Ohm resistor you should be able to work out what current and voltage
you will get out of the cell for different load resistances.
If the motor is stalled (not turning) then the current is limited by the
resistance of the motor, in this case to 120mA with a 1.5 V supply. The
internal resistance of the motor is therefore 1.5/0.12 = 12.5 Ohms.

120mA is the maximum current that will be drawn by the motor (unless you
turn it in the opposite direction to which it is trying to turn). The
minimum current will be drawn when the motor is spinning freely at full
speed, that is 17.5mA. The actual current drawn will be somewhere
between the two depending on the load - more torque requires more current.

Gareth.

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