In my car I have a bluetooth phone kit that plugs into an auxiliary
cigar lighter socket that is controlled by the ignition switch, which
means it turns off when you turn the key to the start position. I would
like to put an electrolytic capacitor across the terminals of the
socket to give the phone kit enough juice to keep it powered up during
the time it takes to start the engine but I'm not sure how they are
rated, ie: a high number uF a lower number. I know the current that the
kit draws will be needed to get the right capacitor for the right time
but I haven't checked this yet, and I wouldn't the formula to work it
out any way so I was going to go for the biggest.
Any advice would be appreciated
Regards
Trevor smith

Blame Google Groups for it posting twice

Trevor Smith wrote:

I know the current that the kit draws will be needed to get the right
capacitor for the right time but I haven't checked this yet, and I
wouldn't the formula to work it out any way so I was going to go for
the biggest. Any advice would be appreciated

I echo the comments from NT and Dave, but if the current is low enough
it might work. You need to know the current (I), the time for which the
power will be off (t) and what drop in voltage might be acceptable
(delta_V). Then the required capacitance is I * t / delta_V (with units
in amps, seconds and volts, the answer will be in farads).

So for example if I = 100 mA (a wild guess), t is say 3 seconds and
assume delta_V of one volt then you'd need 0.1 * 3 farads, i.e. 300,000
uF, which is rather a large capacitor - large enough, possibly, for the
initial charging current to blow a fuse, unless you take precautions.

--
Andy