I am currently designing a part that should always be on when the key is turned to On/Acc, and only sometimes on when the key is set to Off, or removed. My car is a 99 Camry, but I have a good understanding that this should be the same in any car.

The question I have, is the only difference between a car 12v always on circuit, and a switched 12v circuit, a physical switch (the key)? If I place a jump/short between a 12v always on wire and a 12v switched wire, is there any real difference?

The wiring Diagram makes the key switch look like a simple physical switch, while the 12v lines, both Always On, and Switched, look like they are electronically the same?

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Update: This is the general idea I am working with. The Relay will self latch when SW1 is press, or when the car is on ON/ACC. When the relay latches from the 12v On\ACC, the diode should go into reverse bias so the circuit would then be powered from my fused Always On circuit, not the On\ACC circuit, while preventing backfeeding from Always On to the Switched circuit. I know a second relay would make things easier, but part count is important.


So, I guess the real question at this point is simply, not counting the fuses (just for theory), the Switched 12v and Always On 12v are the same exact lines, but the switched 12v has a physical key switch in the say, right? Shorting the two is bad practice, but nothing that will have major problems right?

  • You want your additionnal circuit to be always on or on when the key is switched to Acc/On? I've add an extra power outlet using a relay that is activated when the key is at ACC/ON as I tapped into the cigarette lighter circuit and the load is coming directly from the battery. I rather have this as I would forget to switch off the circuit and drain the batteries. Nov 20, 2013 at 13:33
  • On when Acc/ON, and when the car is off, I can turn it on with a button press, then have a timer circuit disable it 30m/1h/2h later, to prevent battery from draining.
    – cde
    Nov 20, 2013 at 19:58

2 Answers 2


If you place a jumper wire between a always-on 12V source and a switched 12V source, you've now turned your switched wire into an always-on wire (with power supplied from your original always-on source). This is not advisable, and is almost certainly not what you want. It could also create a risk of fire if the fuse for your unswitched source is too large for the wiring of your switched source.

As an alternate approach, consider an aftermarket GPS with a power cord connected to a switched source. The GPS turns on when you switch the power source on, and when you switch the power source off, the GPS begins running off its own internal battery, and displays a prompt saying it will shut down in 30 seconds unless you press "stay on." Perhaps this is the behavior you're looking for. If so, you'd need an internal battery in your device, and you would only connect to a switched source on the car.

  • Yea, I have a appropriate sized diode on the 12v switched line to fix that. Because Ideally, the 12v switched should just be a signal to the circuit, while my individually fused Always on line should provide the load. As for a battery, that's why I am considering this in the first place, sometimes I need to charge my phone but won't have the car on. I am trying to prevent the car battery from draining from being unattended. See my update.
    – cde
    Nov 20, 2013 at 20:38
  • @cde i gots an idear...it uses only the cigarette lighteror some other always-on source...detect the voltage coming in--if it's over ~13V, assume the car is running, and power your accessory. If it's less than 13V, assume the car is not running and start a timer to shut off your accessory after some period of time. No modification of your vehicle wiring necessary!
    – mac
    Nov 22, 2013 at 21:52
  • You know mac, I was thinking something along that line, but threw that on the back burner and forgot about it. Hmmmm. Great idea. :D
    – cde
    Nov 22, 2013 at 23:55
  • except that creates a chicken or egg issue. Unless the button is pressed, the microcontroller controlling the transistor/relay won't be powered, even when the car is on. And if I power the microcontroller separately, then i need another regulator, and it will have some power drain on the battery, even at <1 mA. Maybe I could use a coin cell battery... Hmm. Still a good idea, I'll just need to work on it. Thanks.
    – cde
    Nov 23, 2013 at 1:04

The problem with a short if I understand what you mean by that term is that anything else on the circuit that is being shorted now goes on when your secondary switch is turned on. Only you know if that actually IS a problem.

One other thing to consider is whether this switched circuit is fused at the same current rating as the 12 accessory circuit. If you when the switch is on the acc will be able to pull current through both the switched circuit and the acc circuit potentially allowing much more current through unless you wire them into the same fuse.

  • 1
    A parallel switch/relay where either the switch or the relay will activate the circuit is probably the easiest route. (With the relay activated by ACC.) Nov 20, 2013 at 16:26
  • Yea, I thought of that. But I already have one relay in there (wired for self-latching), and am trying to keep the part count down. I can figure out the electrical most often, I just want to try to confirm if the only difference between the two 12v rails is simply a physical key switch. See my update and schematic.
    – cde
    Nov 20, 2013 at 20:35
  • Your updated question used the wording "ignoring fuses". Ignoring fuses is not a trivial problem. Fuses are a safety mechanism for times when shorts and other unexpected failures occur. You might simply reduce the fuse size in the acc circuit to account for that, but you certainly won't be certified by the Underwriters Laboratories. If you're just doing this on your own car, you'll probably be fine. If you're trying to create a product for sale, that's going to be a liability issue. Nov 20, 2013 at 21:05
  • I meant ignoring fuses, just to understand if the only thing the key switch is doing, is a physical make or break of the connection. Both the lines will have their default fuses, as well as a 2 amp fuse in my box.
    – cde
    Nov 20, 2013 at 21:22
  • Let me try a different approach to explain: If some other device on the on the ACC circuit shorts and the ACC fuse blows, what is to stop that short from drawing another 15 amps directly through your device, and what would the consequences for your device be if something like that happened? Nov 20, 2013 at 22:04

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