Capacitor sizes

[Trevor Smith]

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

[Trevor Smith]

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

[Trevor Smith]

Blame Google Groups for it posting twice

[meow2222]

This is not a good idea, as you'd need a capacitor the size of atlantis plus the V_out would ramp down not stay steady. If youre deterimned to do this you could just wire the lighter to an always on fuse, disconnecting the old feed. But this wont really work either, since system v drops heavily during cranking.

NT

[Dave Plowman (News)]

Wouldn't it be easier to either re-wire the cigar lighter to the ignition circuit or provide a second outlet so wired?

[Andy Wade]

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.

[nightjar]

That reminds me of the power supply I made for my first high level language computer - a UK101 - a kit that involved soldering hundreds of connections by hand. I decided I wanted to make the supply really smooth, so I put a

10,000 microfarad capacitor across the outlet. It was big, but it would run the TTL based computer for several seconds after I disconnected the power.

Colin Bignell

[Trevor Smith]

Thanks for your replies, I hadn't realised it would have to be that big so I think I will take Dave's advice and hook it into the ignition circuit instead. Regards Trevor Smith

[Guy King]

The message from [e-mail] contains these words:

If he stuck a diode in the feed so it wasn't trying to backfeed the rest of the car and provided the drain on the socket wasn't too high he'd get away with it.

[The Natural Philosopher]

Indeed. One reason that aircraft used to use (and may still do) rotary converters so that huge electrical drains like lowering undercarriages or flaps do not result in instrument blackout. A big heavy rotor spinning at many thousand RPM stores a lot more energy than the same sized capacitor.

Which leads to a potential third suggestion. And auxiliary 12v (or less) battery with a low Vf diode in series with it (shottky) that powers the phone..and is recharged at a slightly lower voltage from the main battery..

In fact thats a very nice case for a 3 cell lithium battery. With the diode it will never get overcharged.

[NoSpam]

If you dig-out your old school physics books you'll find that CV=IT so for particular values of load current (I), hold-up time (t) and acceptable voltage droop (V) you can work out the size of C ... but much more practical to feed the phone kit from somewhere else.

Dave

[Staffbull]

Wire it direct to a permanent live with its own fuse, much easier than having to hide a "coke can" somewhere!!

[dingbat]

I'd use 3 capacitors in parallel. 22uF (which is probably adequate), then a 1uF and a 100nF all in parallel, because I'd be worried about filtering high frequency electrical noise from the starter. Feed them from a diode.

If 22uF isnt enough, then either fix the engine to start more quickly, or parallel up another one. As we don't know just how much the bluetooth gadget draws for current, it's not practical to calculate this beforehand.

[Clive George]

Does that actually work? My rather simple understanding of capacitors was that if you stick them in parallel, they just behave like one larger one. Or is the resistance of the wiring somewhere providing a useful effect?

cheers, clive

[dcbwhaley]

No ammount of rewiring the sockets will get round the fact that voltage on the main bus drops to a few volts during cranking

This is the scheme I used for the radio and GPS in my last but one car. The NICAD supplies the voltage when the supply is less than 9volts and even when the main battery is removed. The Nicad trickle charges through the 330R resistor

SUPPLY LOAD ----------|-------|>|-------------- / | \ 330Resistor | / | \ | |-------|>|----+ | ------- --- 9 VOLT ------- NICAD --- ------ --- | | GROUND

[Dave Plowman (News)]

So what? It's unlikely the phone needs more than 10 volts.

[Matt]

Well it would do that.

Forget the capacitor idea it's ridiculous. Rewire the socket or add another one suitably fused directly back to the battery. Or you could just not use the phone while driving or alternatively chuck the whole lot away and use a phone box.

[The Natural Philosopher]

More that the coiled up-ness of the big capacitor puts substantial inductance in series with it. Also electrolytic capacitors have a measurable internal resistance.

[Trevor Smith]

Trouble having to remember to switch it off when I leave the car, memory not as good as it used to be. No I will wire it into a ignition feed as I only really need it when driving not parked up with the engine off. Thanks again for the feedback. Trevor Smith

[Trevor Smith]

Trouble having to remember to switch it off when I leave the car, memory not as good as it used to be. No I will wire it into a ignition feed as I only really need it when driving not parked up with the engine off. Thanks again for the feedback. Trevor Smith