Mike said:

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.

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.

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?

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.

--