# Power explained

1. Feb 25, 2012

### D44

Hi

I'm just thinking over a few things and realised there's something pretty fundamental that doesn't seem to get explained often or at least very well. Maybe I'm too much of a novice, who knows...anyway...

When a power supply states a power output of say, 750W, what exactly does that mean? 750W when? Not when I'm supply something with 12V and 3A. So does it mean a maximum/combination/summation of outputs that the supply can handle before..it burns out?

For example, a psu, 750W output. This would happily supply several different components with different voltages and currents which add up to 750W. But what if there was just 1 output? Does that mean there could be an output of 375V and 2A? That doesn't seem right. But that's still 750W, right?
Also, when it comes to circuitry, things like frequency generators, how do you know what current they can handle? Because, for example, something like an induction coil/heater - dc signal into a function generator, to coil (I know there's more to it than that) - surely the high current drawn from the coil would be way too high for the function generator?

2. Feb 25, 2012

### phinds

Power supplies are either fixed or variable. In either case, the power rating usually tells you how much current you can draw out of it at its max voltage, as opposed to how much power you can draw at a lower voltage.

So a variable supply rated at 10volts and 100 watts, and cranked up to the full 10 volts output, will put out 10 amps at 10 volts if you hook up a 1 ohm resistive load.

With a fixed supply, you don't have any choice on the voltage, so you can hook up any resistive load of 1ohm or more to a 10volt supply before you start to overload it.

3. Feb 25, 2012

### D44

So in the case of a function generator etc, how would you know what the limits of the circuit were, such as max curren they could handle? Would that be the input perameters?

4. Feb 25, 2012

### Staff: Mentor

A function generator typically has its preferred load marked on it, e.g., 10Ω
So with a 10Ω load, when you turn its amplitude up to near maximum, then it's delivering maximum power.

If you are talking about the control signals fed to the function generator, then those would be available in its user manual. They will be voltages, not watts.