I'm trying to find a 12v ignition switched power source, but one that is not interrupted by the engine cranking process. I know things like radio and accessory outlet are only powered when the key is in the ignition, but power to them is interrupted when starting the car. Does such a power source exist?

 

Any of the circuits associated with the engine (ECU, ignition, fuel pump) will work that way. Note that these circuits do not come on with the first click of the switch.

Most electronic devices, even if connected directly to the battery, will reboot when starting the engine due to the drop in voltage.

 

Thanks for the info. I'm trying to see if it would be possible to hard wire install a dash cam. I have one of these dash cams that doesn't use a battery, so it performs better in extreme weather. But I have it plugged into the accessory outlet; so if I turn the key to accessory and linger, the camera turns on, but then what I turn the key to start the car, the camera turns off and won't turn back on by itself.

I figure trying to tap into a source that is key controlled, but not interrupted by the car starting would be the solution. The camera's specs says it uses 5v and 1 amp.

 

If your dash cam has a parking lot mode / motion detection it may be better to just give it constant power and let it turn itself off. Inside your under dash fuse box there should be a line of 5 or so bare connections. Depending on your vehicle i at least two of them should be 12v constant regardless of the ignition position, then one tied to illumination and another tied to acc position. Find a fusebox diagram find your vehicle and locate its 12v constant optional output and then run that through a a "cig lighter usb plugin" to step it down to 5v 1-2amp or so.


On this diagram you can see the connectors i mentioned listed as "optional connectors" with an index at the bottom stating there purpose. If using the box pictured you would want optional #1 or #2 then step it down to 5v of course using something like whats pictured. Any usb car charger should work as all usb run on 5v. If lack of a power is a problem when starting the vehicle and is forcing restarts of the device, consider splicing in an inline capacitor to keep the current constant.





PS. I broke this down far more than necessary but hopefully it will inform future users, as OP obviously already knows some of this stuff.

 

Thanks for the response, you do seem knowledgeable. The camera itself doesn't have a battery, it uses a capacitor, so I think if it's hooked it up to a constant 12v source, it will drain the car battery. Do you know if as someone else has said, if I tap into a fuse, such as the one for the fuel pump, power won't be interrupted by the engine cranking process? I have already ordered a hard wire kit for the camera that will step the 12v down to 5v for me.

 

My uniden dash cam turn on with key turn to ign and start. Turn itself off after couple seconds when key is removed. I plug it in the cig lighter. Dont know what dash cam you are using but it seem like a software issue.

 

It's a A118c dash cam; it doesn't use a battery. So when it detects a loss of power, it automatically shuts off. Cranking the engine is enough for the camera to detect a loss of power, since radio and accessory outlet is shut off during that process. But the loss of power is only for a half second or so, whatever it takes to start the engine; which is not enough time for the camera to shut down AND notice that power has resumed.

 

Add a couple large capacitors to buffer the power loss

 

I honestly don't know, but a multi-meter on the probe point of the fuse would. If you could do this without robbing too much power from whatever you decide to splice into (please dont use the ecu). I would say this is the best option.

If your cam has a "motion activated" or sometimes referred to as a "parking lot" setting that would automatically turn off the camera after you park. Then the 12v constant shouldn't kill your battery with the added benefit of also recording any rogue shopping carts, thieves, ex.

If the cameras software lacks that option. Make it come on w/ acc and simply add capacitors till it doesn't turn off when you turn start the car anymore. Though even with the capacitors, if you didn't drive the car for a week or so the cam would most likely turn off on start as they lose there charge rather quickly.

Also have you considered a manual switch? Just turn it on after you start the car, problem solved.

 

How do I calculate how big of a capacitor I would need? I need just enough to keep the camera going while the engine is cranking. I'm hoping the drop in voltage to the fuel pump wouldn't be enough to shut down the camera, but I can't test it until I receive the hard wire kit later this week. I tried it out real quick with a radar detector, and it did shut off the power to the detector for a split second; but the detector requires more electricity than the camera.

If nothing else works, I'm considering adding a momentary switch to disable power to the camera, to reset it. I know I could use a toggle switch, but I really want the convenience of not having to toggle anything.

 

Well depends on where the capacitor is wired. I also don't know exactly how long your car takes to start, or resistance of the wiring so these numbers are skewed. Though i am hoping this gives you the information necessary to figure out exactly what you do need.

Option #1:
12v - what does your step-down wiring draw?

or

Option #2:
Your step-down wiring is likely putting out 5v 1.5-2 amp range as many usb such as phones can pull 2 amps no problem. So these are the numbers we will use for the math

5v - 1.5 amp is 7.5 watts at 3.333 ohms resistance (ohms law). Then convert our watts to joules and assume starting the car takes 5 seconds = 37.5 joules (Watts to Joules calc). Meaning you would need a more than one farad capacitor (30joules @ 5v found using ½*C*V² = energy) . Though any capacitor in this part of circuit will be inefficient as its on the 5v side, so it will need to be larger. This same 1 farad capacitor on the 12v side would give you 72 Joules, more than enough.


Clarification:

A capacitor goes off a unit known as a farad. The capacitors potential changes depending on the voltage its charged too. 1 farad charged to 12 volts has 72 joules of energy just as a 1 farad charged to 1v has 6 joules. ( 1 farad is equal to 1000000 microfarad )

energy = ½*C*V² or .5 x farad x voltage ^2 = joules

Ohms law calculator: Watts/Volts/Amps/Ohms conversion calculator

Watts to Joules calculator: Watts to joules (J) conversion calculator

 

This is insanity.
An ignition wire by definition is one that stays on while cranking. You want that the black wire with a yellow stripe in the ignition harness. Virtually any black/yellow wire in the car will do. It's a simple test with a multimeter to verify it.
Accessory power by definition is powered with key on, but cuts out during cranking. The option outlets on the fuse box provide constant, accessory, and headlight power only. For ignition power go to the ignition harness.