I have a simple circuit in my car. A +12V signal wire that comes from the ignition is used to tell an amplifier to power on.

When +12V is present here, the amp turns on and is powered by a separate, higher rated +12V feed from the battery.

The issue is that my car has start stop technology, so when I roll up to lights, the engine dies, and when I pull away, it starts again.

The +12V signal is present when the engine is not running, but when the car starts itself, it drops to 0V for a couple of seconds, before returning. I can only assume this is because the car routes the current through the starter relay instead.

My question is this. Is there a means of maintaining that 12V signal at an output for a couple of seconds after the input goes off? It literally only needs to be a couple of seconds, the input is back before long. I was thinking some kind of capacitor bank?

Let me know your thoughts.

  • \$\begingroup\$ Diode plus capacitors. Without the diode your capacitors help start the car. For calculations we need an estimate how much current your radio uses. Mine (4x50W) I just today found out is specified as 10A max. \$\endgroup\$ – Oldfart Apr 19 '18 at 14:31
  • \$\begingroup\$ Thing is, the amp shouldn't really pull any current down that cable, it's merely to act as a signal for a relay. \$\endgroup\$ – Jacob King Apr 19 '18 at 14:36
  • \$\begingroup\$ The rail drops to zero so the brownout (due to cranking) does not affect anything on the switched system; a controlled drop and re-power will simply force equipment on the switched system to do a cold start and reset. \$\endgroup\$ – Peter Smith Apr 19 '18 at 14:56
  • \$\begingroup\$ The car has a start-stop... no-one would buy a car with a stereo, GPS and computer which hard reboots at every traffic light! I highly doubt that it browns out to a low enough voltage to bother OP's sound system... so let's clear this up: Jacob, can you measure the +12V which supplies your amp while the car cranks? I mean, not the line that you already know goes to zero, but the main power supply. \$\endgroup\$ – peufeu Apr 19 '18 at 15:08
  • \$\begingroup\$ It's worth clarifying that the car's headunit does not shut off, just the aftermarket amp in the boot. I had assumed that the headunit was switched by the ignition but perhaps it is on a different circuit. \$\endgroup\$ – Jacob King Apr 19 '18 at 15:10

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