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Author Topic: Diode suggestion  (Read 1521 times)
K8POS
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« on: January 22, 2013, 07:01:58 AM »

I have installed a back up camera in my car.  Works great, wish I had one years ago.
The camera draws power from the back up lights so that it is only powered on reverse.  The camera an monitor share common ground (-12 volts) but the positive is separate.  I want to install a switch to turn on the camera at other times but do not want it to back power the reverse light.  I know how to wire in a diode to do it, I just don't know what size and type of diode to use.
The camera draws less than 300ma

Any suggestions?
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AA4PB
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« Reply #1 on: January 22, 2013, 07:07:03 AM »

A 1N4007 is pretty common and rated for 1A up to 1000V reverse. It should work fine.

The 1N4007 will drop 0.7V so with 12V from the battery you will get 11.3V on the camera - that is probably okay.
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WB6BYU
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« Reply #2 on: January 22, 2013, 07:12:16 AM »

In fact, any of the 1N400x series that you can find will work.  They start at 50V
and work up from there, all rated at 1 amp.

Similarly the 1N5400 series are similar but rated at 3 amps.

The ratings you need are a minimum of 25V and 0.5A, but I'd suggest 50V and 1A
to be on the safe side.  Anything larger than that is fine:  use whatever you can
find conveniently.  Radio Shack might even have something suitable.
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K8POS
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« Reply #3 on: January 22, 2013, 07:22:04 AM »

Thanks,
I think I may have one or two of those in my junk box.

Bob
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KD0REQ
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« Reply #4 on: January 22, 2013, 08:19:47 AM »

just a little mutter... with the engine running, when this camera would do the most good, you have charging voltage on the power bus, which would be between 13.2 and 13.8 volts.  somebody somewhere, someday, might be trying to turn an Explorer into a Dreamliner and need to know the voltage swings.
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KE3WD
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« Reply #5 on: January 22, 2013, 09:05:48 AM »

Most stuff rated for automotive "12V" use is designed to tolerate the kind of voltage swings associated with automotive systems, design max typically being 16VDC, so I wouldn't sweat that issue. 

However, while use of a standard rectifier, 1n4 series or 1N5400, etc. would likely work, one can avoid the .6 volt drop somewhat by using a Schottky barrier diode instead. 

Either choice would likely work here, though, if connected properly.


73
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KK0G
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« Reply #6 on: January 22, 2013, 02:00:41 PM »

Are you sure the camera doesn't already have that feature built in? Every one I've installed had a main power wire that could be switched on at will and a separate reverse signal wire.
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N4CR
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« Reply #7 on: January 23, 2013, 04:53:35 AM »

With a single pole double throw switch, you can toggle between 12v and backup light power.

No diode.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
NR4C
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« Reply #8 on: January 23, 2013, 06:39:55 AM »

Do you really have a -12V "ground" system in your car?  Mine is 0 volts, with pos 12 Volts to radio and accessories.  But if you're correct, separate positive 12 v for camera and display, and a common -12 volts "ground", that's 24 volts.

...bc   nr4c
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KC0RZW
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« Reply #9 on: January 23, 2013, 06:44:42 PM »

I would do it with a 5 post relay.  Pin 85 on the relay to ground, pins 86 and 87 together with your B+ switch.  Pin 87a to reverse light feed, and pin 30 to camera.  But that's because I don't have diodes laying around, and have plenty of relays laying around.
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