"Jamie" t wrote in message
...
Bart Bervoets wrote:

are fairly constant. You didn't indicate what sort of
device you are trying to power; some are much more forgiving than others.


Creative labs pc speakers in my car.


So you need a 3 ohm 5 watt resistor


And for 2 amps?
I don't really understand your calculation as i am useless in math.

Bart Bervoets
because your required voltage is so close to the supply voltage,
there isn't much in the line for a common regulator circuit to
maintain a stable regulated 9 volts


That isn't strictly true. Most manufacturers quote the dropout voltage for
their range of standard 78XX regulators, to be 2v max. Even allowing for the
vehicle engine to be off, there is an overhead of 3v, and with the engine
running, asuming that the alternator is working correctly, there will be an
overhead of 4.8v ( 13.8v - 9v ) so he should be fine with a '7809,
providing he doesn't need more than an amp, although there are higher rated
devices available. The LM317K adjustable could be used. This can source
currents in excess of 1.5A and has a max dropout voltage of 2.5v.

Arfa