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Old 09-18-2017, 02:21 PM   #1
40cpe
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Default school me about diodes

I want to install a 3rd brake light without going all the way up to the brake light switch for power by installing a diode in a wire from each brake light socket feed. I see many different names for the diodes and am confused as to which type I need. What kind/size/rating of diode do I use for this purpose? My system is 12 volts.
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Old 09-18-2017, 02:35 PM   #2
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Default Re: school me about diodes

You shouldn't need a Diode if its just a stop light ,a direct wire should work .if its a multiple flasher combo then yes a diode. 1/4 " od with wire each end ,don't know the # Ted
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Old 09-18-2017, 02:37 PM   #3
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Default Re: school me about diodes

I'm not an electrical expert by any means, but I don't think a diode is required for this. Just tie one of the wires from the brake light switch that goes to the rear lights, to the third brake light and provide a ground from the 3rd light.

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Old 09-18-2017, 02:52 PM   #4
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Default Re: school me about diodes

The brake lamps share a socket & bulb with the tail/turn signal lamps but the 1156/1157 bulbs are dual pin/dual filament so they are separate circuits. As was previously mentioned, just tie into the brake light circuit and you'll be OK. You would avoid the turn signal/tail lamp circuit since it would require diodes to isolate the turn signals.
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Old 09-18-2017, 02:53 PM   #5
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Default Re: school me about diodes

We need to know..........turn signals, or not? DD
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Old 09-18-2017, 03:17 PM   #6
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Default Re: school me about diodes

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If the brake light filaments also act as the flashers you can'y tee off the rear lights, diodes or no. You have to take a feed from the brake light switch or the other end of the wire where it feeds the indicator switch.

Again, this assumes the brake light filaments also act as the flashers.

If the brake lights are separate from the flashers you can just tee off either rear brake light or anywhere in the feed wire to them. Again, no diodes required as the brake lights are connected together anyway.

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Old 09-18-2017, 03:22 PM   #7
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Default Re: school me about diodes

With a diode from each brake/flasher lamp right and left tied togetter feeding the 3rd brakelight it should work...
Whatever brake light that is not flashing will feed the 3rd light and keep it from flashing.
For diode type just about any will work that takes the current needed.

Last edited by flatheadmurre; 09-18-2017 at 03:33 PM.
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Old 09-18-2017, 03:31 PM   #8
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Default Re: school me about diodes

A diode is basically a one-way valve that let's current flow only in one direction you will need to use diodes if you have brake turn-signal lights.Diodes are measured in amperage capacity .Use 5amp or greater diodes wire like the diagram I posted otherwise your third brake light will flash with your turn signal if hooked only to one brake/ light turn signal circuit.
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Old 09-18-2017, 03:33 PM   #9
40cpe
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Default Re: school me about diodes

Yes, I have turn signals and do need the diodes. I need to know what kind to buy.
EDIT: Thanks cordbob, I was typing when you posted. I assume from your drawing I need Zener Diodes? Are they voltage specific? Thanks for your helpful drawing.
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Old 09-18-2017, 03:38 PM   #10
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Default Re: school me about diodes

Not zener diodes.
Rectifier diodes search for SR5100 SB5100.
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Old 09-18-2017, 03:47 PM   #11
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Default Re: school me about diodes

I've heard them called signal diodes or generic diodes or constant-current you do not want a zener diode they control voltage as well .Any diode that looks like the photo posted will work the number like Flathead Murre said designates the amperage of the diode
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Old 09-18-2017, 03:50 PM   #12
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Oops for some reason on the first go the photo didn't upload
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Old 09-18-2017, 04:02 PM   #13
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Default Re: school me about diodes

That is what I need to know. Many thanks.
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Old 09-18-2017, 04:03 PM   #14
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Default Re: school me about diodes

Constant current and signal diodes are working in the mA range so stay away from anything called that.
Just the cheapest rectifier in the 5A or more range you can find.
The versions i posted shouldn´t cost you more then max $2 in a fancy store.
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Old 09-18-2017, 04:58 PM   #15
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Default Re: school me about diodes

Thank you. There are so many terms used to describe them I was confused. I have some 6A ordered.
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Old 09-18-2017, 05:00 PM   #16
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Default Re: school me about diodes

Sorry Guys, I'm going to have to bite my tongue because I do not want to be that guy that is always arguing.

If the lights are combined flasher/brake lights using diodes will not work.

If the lights are not combined and are purely brake lights you don't need diodes.

If combined, and using diodes, if using the left hand flasher and no brakes, the third brake light will flash also. Ditto the right. Obviously if the brakes are on then extra light will work, receiving feeds from both sides.

It's when only the flashers are being used that you will have a problem.

How hard is it to pick up a feed from the brake switch? It's the obvious source that does not require any trickery to make it work.

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Old 09-18-2017, 05:05 PM   #17
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Default Re: school me about diodes

Quote:
Originally Posted by Mart View Post

How hard is it to pick up a feed from the brake switch? It's the obvious source that does not require any trickery to make it work.

Mart.
That's a fact! DD
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Old 09-18-2017, 05:26 PM   #18
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Default Re: school me about diodes

The actual brake switch should be upstream of the flasher mechanism. Tapping off it should give you simple off and on when the brake pedal is pushed without any additional electronics. Should be a lot less wiring than going to the lights. Just follow the brake switch wire to the dash and splice into it before it goes to the flasher circuit. Where are you mounting the third light?

Last edited by 38bill; 09-18-2017 at 05:31 PM.
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Old 09-18-2017, 05:50 PM   #19
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Default Re: school me about diodes

Oops your right...will take more then diodes to make it work...a couple of SCRs and capacitors...and by that time installing a wire from the brake switch was done twice...lol.
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Old 09-18-2017, 05:50 PM   #20
40cpe
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Default Re: school me about diodes

The 3rd light is at the rear bumper. The brake light switch is under the dash and the wiring to the rear is under the carpet/seats. I'm going to use the diodes. I've done this before to a one-bulb indicator. I'll let you know how it works out. Diodes are ordered.

Last edited by 40cpe; 09-18-2017 at 07:21 PM.
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