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What size Capacitor do I need ?

championc

Apr 2, 2016
3
Hi all,

Sorry but I'm useless when it comes to these electronics formulas to calculate stuff. I have a 12v CarPC connected to an ignition switched circuit. I want to give the PC about 5 mins of power after the ignition is turned off and so I think the simplest solution would be a 12v capacitor. I want to potentially keep the PC powered if I turn off the PC at a petrol station or at quick stops. Additionally, the reason for connecting to the ignition is that if the CarPC did not shut down correctly, it previously continued to drain the battery when directly connected.

So basically, a 12v device running a load of about 2 amps, what size in Farad's of a 12v Capacitor would give me about 5 mins of power after the ignition is turned off ?

Arouse1973

Dec 18, 2013
5,177
Assuming a drop of say 0.5 Volts so the capacitor starts at 12 Volts and drops to 11.5 Volts then you will need approx. 1200F. So say a 100F at 2.7 Volts you will need 6 in series for the voltage rating (to be on the safe side) and 12 in parallel. So that's 72 capacitors. At about £3.00 each that's £216.00 (GBP). Or for about £5.00 you could buy a small battery
Thanks

Colin Mitchell

Aug 31, 2014
1,416
I would buy the capacitors at £216.00.

championc

Apr 2, 2016
3
Here's my dilema - I was thinking about a re-chargable battery which would be perfect except for the fact that I have the PC set to auto boot on connecting power. So in the mornings, I can just put the key in the ignition and start the car, and the PC starts up. If I had a battery which didn't fully discharge, then there would still be power and so, the PC would not auto-start, hence the capacitor idea.

A small 12v re-chargable battery would work if I could find some simple way to discharge it completely each time the 5 mins after key switch off had expired.

Maybe there's another approach to this that I'm missing. My main aim is to kill the power to the PC 5 mins after the ignition is turned off, without the worry of the battery being drained if the PC fails to shut down correctly.

Kiwi

Jan 28, 2013
409
Run the PC through a 5 minute Turbo Timer. Most auto accessory shops or auto electricians should be able to supply one.

Does the PC shut down correctly when the power is turned off?

championc

Apr 2, 2016
3
Perfect and simple solution.

The PC has an SSD so there no big issue if it gets turned off without having been shut down properly. The CarPC software has a a built in trigger to watch for the ignition off on the BMW K-Bus and triggers a PC shutdown after 5 mins. So I can trigger the Turbo Timer for maybe 8 mins so that, if something goes wrong on the K-Bus side, it will still kill the PC anyway.

Many thanks

swagguy8

Dec 10, 2014
73
Here's my dilema - I was thinking about a re-chargable battery which would be perfect except for the fact that I have the PC set to auto boot on connecting power. So in the mornings, I can just put the key in the ignition and start the car, and the PC starts up. If I had a battery which didn't fully discharge, then there would still be power and so, the PC would not auto-start, hence the capacitor idea.

A small 12v re-chargable battery would work if I could find some simple way to discharge it completely each time the 5 mins after key switch off had expired.

Maybe there's another approach to this that I'm missing. My main aim is to kill the power to the PC 5 mins after the ignition is turned off, without the worry of the battery being drained if the PC fails to shut down correctly.
why don't you just get an arduino, have some relay circuit that shuts off after ignition is turned off after 5 minutes? you wire the computer directly to the battery, the arduino detects ignition on/off thus controlling a relay powering the pc on or off. and you could also wire the switch of the pc to the arduino, so when you turn on the ignition, the arduino will detect it and turn on the pc for you.