pulsed to steady output

I am using the "Pulsed to steady output" diagram for a vehicle turn signal application and ran in to a small snag. My factory turn signal wire, instead of switching from +12v to open, actually switches from +12v to ground, which discharges the capacitor. Is there a modification I can make to the circuit to prevent this?

Insert a diode

i am an idiot wrote:

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https://www.the12volt.com/relays/relaydiagram22.html

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I am using both diodes just like that diagram. It works if the turn signal just switches from hot to open, but mine switches from hot to ground and when it does it discharges the capacitor and the relay clicks off.

Replace the diode or check that it isn't shorted.

It cannot work yet discharge. If the diode is in the wrong direction, the relay won't turn on.
If it turns on, the diode won't allow current to return (discharge) to/thru the can side.
And that's true even if the circuit does switch to GND {which I doubt - I think you are seeing the other bulb(s); otherwise what's the circuit r flasher can you are using?}.


Otherwise check your wiring or that you have a BIG capacitor (electrolytic, not a geencap etc).

10,000 microfarad is the capacitor value in the diagram. Is that what you are using?

I am using a 4k uf cap. It gives me the perfect delay time to keep my relay on without staying on too long. After trying this on the car and it not working correctly, I'm now bench testing the circuit without any load on the relay. If I apply a pulsed positive/open to the circuit, it works perfectly but if I apply a pulsed positive/negative, as soon as the trigger goes to ground the relay opens. I'm not even using a flasher right now. I'm tapping the trigger wire on a constant hot and constant ground wire. As long as I don't tap it on the ground wire it works fine and the relay stays closed until I quit tapping the trigger wire on the constant hot.

In that case your diode is shorted.

It has to be shorted.

Unless the cap is NOT on the line-end of the diode...
(I know - the assumption is correct wiring - but just in case...)

Use a second relay to feed that one.

New relay wired as follows. Ground to 85, blinker wire that goes positive to ground to 86. Constant power to 30, 87 feeds the other relay.

Check the diode first, it has to be shorted.

Ok. Did some more tests this morning. The diode between the flasher and the cap was shorted so I replaced both diodes. As soon as I apply 12v the diode before the cap shorts. I am using 1n4742A diodes. Now I assume these are not compatible? They are 12v 21ma. Is the current rating too low?

Use any 1N400x diode - I suggest 1N4004 or 1N4007 as they are the most common and can be used for other things.

The 1n4742A is a Zener diode and totally inappropriate.

Thanks for the help. I found a 1n4001 in my parts box and tried it. Seems to work. I'll just have to go get some more.

What is the purpose of the second diode across the relay coil? The circuit seems to work fine with or without it.

Parallel 3 to handle the current required to charge the capacitor.

The one across the coil of the relay is to quench the backlash that happens when you remove power from the coil. Without it, it may make a pop in your audio system everytime it disengages.

Thanks. Picked up some 1N4004's today. They work great. Now I have a new issue. I'm using the above circuit to turn on 3 other relays. One controls the positive side of a bulb. The other 2 control a second bulb. One for the positive side and one for the negative side. When everything is connected, it all works but the relay with the cap never shuts off after the turn signal wire turns it on, unless it goes to ground for a period of time. If it go's open it stays on. I can't figure out what is causing the relay to stay on. There must be a backfeed somewhere.

Why are you switching both sides of a bulb? Only one side is required to be opened.


The circuit you are using requires the input +12V to be removed. Then the cap will discharge thru the relay coil and eventually the relay will de-energise - probably after ~0.2 seconds or longer, and maybe without chatter.

Controlling both sides of the bulb because it serves double duty as a side marker parking light and a turn signal. When the parking lights are on, I use the 12v + in the turn signal feed to kill the bulb so it flashes opposite the front turn signal bulb and when the parking lights are not on I use the ground in the turn signal feed to turn the bulb on to flash opposite the front turn signal. Basically I wanted the side marker to flash opposite the front turn signal bulb anytime the turn signals are on, but I also want it to remain lit when the parking lights are on which requires it to be controlled on the ground side. I hope this makes sense to you. It's kinda hard to explain.

I think I know what you're trying to do, and I think it won't work. But if you see a current path thru the side bulb as desired, it should work.
But usually all switching is done on the +12V side without any GND switching.   If GNDs can be separated (isolated from GND/chassis) then the bulb is often placed between parker +12V & the flasher can, thou that causes out-of-phase flashing.   

But dual purpose bubs that include the flashing/indicator get quite tricky...

Good luck.

Thanks. You guys have been a great help.