A little electronics guru help?

[v_tach]

I am constantly amazed by the depth of knowledge and expertise the members of this forum display. My education in anything electrical usually comes after I've burned something up.

Anyway, I'm looking for suggestion for a little project I have not EV related and thought mining the extensive knowledge base here would be worthwhile.

I have a 12v electrical device that draws about 250mA when powered up. It's powered from the manually switched circuit of a 12v light. However I need the device to continue being powered for a couple of seconds after the power to the light is switched off. I wanted to avoid anything like a time delay relay and running another 12v power wire.

Can a simple capacitor type circuit be used?

 

[fechter]

Maybe, but it would need to be a pretty large capacitor to power it for a few seconds. You'd also need to do something to prevent a huge power spike when you turn the circuit on.

Not very expensive to try it though...

You'd need a diode in series with the 'device' so that the capacitor doesn't try to run the light circuit.

 

[swbluto]

You'd need a diode in series with the 'device' so that the capacitor doesn't try to run the light circuit.

What would be the problem of the capacitor trying to run the light circuit?

Here's what I'm thinking you're thinking.


View attachment lightdiodecircuit.jpg

EDIT: Yes, okay, this was meant to be deleted after I realized the light wasn't meant to be powered but just the device after the light usually turns off.

 

[dnmun]

i think he means that the light will soak up any power you set aside on the capacitor to run the little motor.

maybe he could add a resistor to slow the spike buildup and would not matter when the light was turned off since the current to power the motor would go out the other leg of the circuit.

i am totally curious about the application though.

 

[swbluto]

Yeah, I just reread the post. *doh* I tried to delete it as soon as possible, but someone replied to it. :wink:

 

[v_tach]

Thanks for everyone's input, I think you guys reassured me my idea may at least have been feasible.

The device is actually a small video camera used as a backup camera for a vehicle. Its currently powered by the reverse light circuit in the taillights. When the transmission is switched to reverse, the lights are powered along with the camera. The issue is, the video display is in the rear-view mirror and continues to display for about two to three seconds after taking the transmission out of reverse. With no video signal, the display changes to an annoying very bright blue for that brief period. If I could manage to continue to power the camera for a few more seconds, the camera image would stay on the video display until the video monitor circuity shuts it off.

I already figured a diode into the mix so again that suggestion reassured my idea. I've been looking at electric double-layer capacitors used as battery backups in some low current applications and portable LED light power. I just didn't know if they could be used stand-alone (adequate capacity and voltage of course) or if I needed some sort of regulating circuitry.

In the end, running a separate power wire for the camera may be the best solution. The video display in the rear-view monitor is activated by a signal wire that goes to ground when the transmission is in reverse. A small relay and a few feet of wire back to the camera seems to be the obvious simple solution. The vehicle is available with the exact same setup as a factory option. I added the components after purchase but the factory wirings harness does not have required wiring in place if the camera option isn't factory installed. The factory harness provides power to the camera all the time via an ignition switched lead according to technical documents from the manufacturer. It is only the rear-view mirror display that is activated when the transmission is in reverse.

 

[heathyoung]

250ma at 12V = 3W, for ~5 seconds = not much current. A pretty solid capacitor (10,000 uf, 25V or so) could provide enough current, just put it in parallel with the power supply leads to the camera (with a diode so you dont feed back across the bulb). Supercaps are usually 5V or so, so you would need a few in series (4) which would reduce the capacitance and would be overkill.

 

[Matt Gruber]

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