Continued Power During Cranking
Discussion
I have a device tapped into a fuse location which is ignition switched, it draws up to 1.5amps. When the engine is cranking, this fuse's power is cut. I want to keep power to the device for maybe 10 seconds after ignition cut (i don't want to move the fuse tap to a permanent live location).
It seems a really obvious thing you might want to do but the fact i can't seem to just buy a solution at halfords or amazon suggests i've misunderstood something?
Could i put a capacitor in series with a resistor and a diode, then put that string of 3 in parallel across the device?
I'm thinking the capacitor charges to supply voltage, the resistor stops it charging too fast and blowing the supply fuse and the diode stops the capacitor draining back to the common supply rail when power is cut. I.e. the diode makes the capacitor's charge only available to the device, when power drops on the supply line i don't want the capacitor back feeding power to everything else on that common supply.
This must be wrong because by that reasoning i'd need something like a 5 ohm resistor to save the 5amp fuse (12v/5ohm = 2.4amp which is plenty discharge rate for a 1.5a load and doesn't come close to blowing a 5amp fuse). I've never heard of a 5ohm resistor, is that not going to get pretty toasty every time current flows over it 5amp x 12v = 60 watts?
Put me out of my "knows enough to be dangerous" misery. Please.
It seems a really obvious thing you might want to do but the fact i can't seem to just buy a solution at halfords or amazon suggests i've misunderstood something?
Could i put a capacitor in series with a resistor and a diode, then put that string of 3 in parallel across the device?
I'm thinking the capacitor charges to supply voltage, the resistor stops it charging too fast and blowing the supply fuse and the diode stops the capacitor draining back to the common supply rail when power is cut. I.e. the diode makes the capacitor's charge only available to the device, when power drops on the supply line i don't want the capacitor back feeding power to everything else on that common supply.
This must be wrong because by that reasoning i'd need something like a 5 ohm resistor to save the 5amp fuse (12v/5ohm = 2.4amp which is plenty discharge rate for a 1.5a load and doesn't come close to blowing a 5amp fuse). I've never heard of a 5ohm resistor, is that not going to get pretty toasty every time current flows over it 5amp x 12v = 60 watts?
Put me out of my "knows enough to be dangerous" misery. Please.
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