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Auto dimming problem


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I put the circuit together to dim my headlights but everytime I hook power to the board one of the pots smokes. This is the first time I have tryed to put something like this together and I am not sure where to look for problems. I have checked the wiring out several times and it looks right. Any comments would be helpful. Thanks

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Hi and welcome. ;D
You picked a project that I think has mistakes in it.

Maybe this is the project you made that has the pots directly across the car battery so of course they smoke. The 2N3906 transistor will also smoke because it is missing a protection diode to arrest the high voltage spike from the relay's coil.


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The link to this project is here:

What is the rating on the pots you have used? In a car circuit, you must use high watt rated pots or you will have the problem you are discussing. These pots are limiting voltage to a car battery. Since this circuit only causes relays to go on and off, a simple fix would be to add a voltage regulator to the circuit before the battery. This would limit the available current to this part of the circuit.

To all:
Please note that many of the projects in our projects section have come from various places on the Internet. There is no guarantee from this site that any of them work...thus we have this forum for Projects Q/A where we discuss how to make them work.


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What rating for the pots do you recommend for R3 and R4 to be able to set them at zero ohms in the car battery circuit when S1 is closed?

As I said, I recommend the use of a regulator, ante.

I am sure the author never expected anyone to put a pot in the zero ohm positon, which of course, is a direct short. Aaron Cake admits that he is not an engineer. It is just a circuit that worked for him. I do not know what he used for pots.

Handcannon: Use a voltage regulator. Even on a voltage regulator, a pot in the zero ohm position is a direct short. However, you can overcome this by connecting a 1N4004 or similar diode across the input and output pins of the regulator in a reverse position to protect the circuitry from shorts. Let me know if you need more explanation of this.

Unlike others in this forum, I am trying to help you find a solution instead of useless grumblings about the circuit. Hope it helps get you there.

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Here is what I meant with the diode.
The pots are for the proper adjustment of the circuit. You should start with them in the center position. Usually, the designer will add a fixed resistor in series so that full adjustment will not allow a direct short. You can calculate how much current draw will be required by using the ohm's law formulas. Calculate high and low resistance requirements, which means the span of which you will adjust the pot. You do not want this to exceed the rating of the regulator. If it does, you can add the addition of a series resistor to limit current. It is easier to do this after a regulator than to do so with full battery power. Otherwise, a lot of current is dissipated in the series resistor, requiring a high watt rating.

Sorry audioguru. This forum is for discussion of projects on this site. I have moved your google find to the proper forum, Project Design/Ideas. If handcannon wants to dump this project and use that one, it is in that forum under "Auto Dimmer".



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Hi AG,

This is all in the wrong board because as soon as you divert from the original circuit it’s not a “Projects Q&A” anymore!
See this: http://www.electronics-lab.com/forum/index.php?topic=5433.0

The use of a regulator disqualifies the project from this board doesn’t it?

Hope this is valid for everybody!

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What is the voltage regulator for? What voltage?

What is the diode for?
It is normally used to protect the regulator if it has a big output capacitor and the input is suddenly shorted. Then the big output capacitor discharges into the diode instead of into the regulator's output.

The pot R3 is 5k. The relay won't work if the pot is adjusted to more than 3% of its rotation.

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