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danielkos
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Joined: 10/07/2021
Location: Australia
Posts: 25

Posted: 05:09am 29 Mar 2022

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Hi all,

I've been toying with a project for a while which involves puttting a Raspberry Pi in my car, to display data on my rear-view monitor which currently sits unused when I'm not reversing.

I want to power the Pi off my car's ACC supply but there's a few challenges there:

1. Possible data loss if I take the key out of the ignition. This one's not a big deal as the SD card can be configured to be read-only. The pi will display stuff but won't save anything.

2. When I start the engine on my car, I have to turn the key through ACC and ON (which will cause the Pi to begin booting) but then when I turn it to START the power to accessories gets cut until the ignition returns to ON. So it would get its power cut halfway through booting up every time.

The second issue is the one I want to avoid. I was thinking something along the lines of a large capacitor and a relay but if there are off-the-shelf solutions that would be easier. Any ideas?

Haxby

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Joined: 07/07/2008
Location: Australia
Posts: 418

Posted: 05:30am 29 Mar 2022

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No need for a relay. A large capacitor on the 12v power supply side should work, with a diode to stop the cap discharging through the vehicle.

Maybe add a 4.7 ohm resistor in series so that the inrush current isn't catastrophic to any vehicle systems.

phil99

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Joined: 11/02/2018
Location: Australia
Posts: 1777

Posted: 07:07am 29 Mar 2022

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With two diodes, one from ACC and the other from ON the capacitor could be much smaller. ON stays on during starting.

danielkos
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Joined: 10/07/2021
Location: Australia
Posts: 25

Posted: 07:10am 29 Mar 2022

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Haxby said No need for a relay. A large capacitor on the 12v power supply side should work, with a diode to stop the cap discharging through the vehicle.

Maybe add a 4.7 ohm resistor in series so that the inrush current isn't catastrophic to any vehicle systems.

Thanks Haxby. Follow up question is how would the Pi respond to the charge/discharge curve of the capacitor? I'd want to have a fairly "gentle" charging curve, perhaps 1-2 seconds or more to reach the threshold needed for the pi to boot, since this is a 35 year old car with a small battery by today's standards -- too many sources of current draw before cranking the engine can make it crank slow.

noneyabussiness
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Joined: 31/07/2017
Location: Australia
Posts: 506

Posted: 07:41am 29 Mar 2022

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you could also try a sepic converter, they basically take any voltage input and turn it into a stable 5v output... so when cranking the voltage will sag significantly, but will continue to supply the needed 5v... a big ass cap on the output would work too ( as has been suggested)..

AU $0.79 22%OFF | DC-DC Auto Boost Buck Converter Module DC 2.5-15V to DC 3.3V 4.2V 5V 9V 12V Step Up Down Voltage Regulator Power Inverter Supply
https://a.aliexpress.com/_mLe6HlM

or a couple of these ( in series to make up 5v rating) on the output of your regulator..

AU $1.45 40%OFF | 2Pcs Super Capacitor 2.7V 1F 2F 3F 6.8F 8F 10F 15F 20F 30F 40F 50F 80F 90F 100F 120F 200F 300F 400F Farah Capacitors
https://a.aliexpress.com/_mNjSd3C

I think it works !!

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