# Constant power Supply

Discussion in 'General Electronics Chat' started by antennaboy, Mar 27, 2012.

1. ### antennaboy Thread Starter Active Member

Jan 31, 2008
48
0
Hello forum,
I am familiar with constant voltage supplies (Batteries) and constant current supplies: they provide the load with a constant voltage and constant current regardless of the load impedance, respectively...

Do constant power supply exist, i.e. regardless of the the load impedance, the power delivered to it is constant and set by the power supply?

thanks
antennaboy

2. ### MrChips Moderator

Oct 2, 2009
19,027
6,109
No, this is not possible.

Power P = I x V.

There is an infinite number of solutions to this equation. You have to specify either V or I in order to constrain the power.

3. ### antennaboy Thread Starter Active Member

Jan 31, 2008
48
0
I see, thanks MrChips.

I am experiment with a solar cell. When the right amount of sun hits the cell, it develops across its terminals a voltage of about 0.5 V.

I have a small electronic device that I want to transfer power to. The device needs a certain amount of watts to properly operate.

If I connected the solar cell to the device and put an ammeter in series, I would have both the voltage (0.5 V from the cell) and the current (from the ammeter). I would then know the power P=IV going into the device. Is that correct?

How could I transfer the max amount of power from the solar cell to the device? Would I need to match the impedance of the device (which I don't know, who knows what is the total equivalent impedance of that device? it has a little circuit board) to the resistance of the solar cell (max power transfer theorem). But I see that hard happen.....

thanks
antennaboy

4. ### pilko Senior Member

Dec 8, 2008
213
20
If the load resistance does not change, then either constant voltage or constant current will give you constant power.

P = I^2*R
P = V^2/R

pilko