RC Circuit idea

[zakotaa]

Hi,

My electronic knowledge is limited so please bear with me. I have recently put headrest screens in my vehicle. I have powered the screen from ACC source. The screens don't have battery backup. The problem is if I stop at a gas station and turn off the engine, when I restart the engine, the ACC power cuts out for few seconds and that resets the screen and whatever was playing on it, which is very frustrating for kids. Some research have indicated that an RC circuit would be a good solution for temporary loss of power (1-2 seconds). I was thinking if good folks here can suggest the values for resistor and capacitor that I need to put in. I believe it takes 9~12v and 1A. Also I was wondering how the circuit would be laid out. option A or option B
thanks,

 

[KlausST]

Hi,

Farad (F) is the unit for capacitors.
1 F = A × s / V ...... (Ampere times esconds divided by Voltage)
A = Current for your device = 1A
s = backup time = 2s
V = delta voltage, starting with "fully charged" until application stops to work = 12V - 9V = 3V (ignoring diode dropout)

So C = 1A × 2s / 3V = 0.667F = 670mF = 667,000 uF

This is a big capacitor, also not that cheap.

The resistor is not important for operation, if you use it to limit the inrush current use 0.1 Ohms .... 0.5 Ohms on circuit A.
For the diode I recommend a 3A/ 30V (at least) schottky diode (for low dropout)

I expect the given "1A" as rated current. Maybe the truely drawn current is less. Thus you may try with a lower value capacitor.

Klaus

 

[zakotaa]

Thanks Klaus,
this is very helpful. Yeah let me see if I can find more accurate current usage.