The Machine

So I am trying to wire up some lights to go on with my dome light circuit and have tapped that circuit wire for my signal wire to a relay. It works fine but on initial startup and power off, the relay buzzes briefly. After a ton of reading I have determined that that circuit uses PWM or pulse width modulation to achieve the dimming effect that coincides with the dome light. This pulsing of current is turning on and off the relay super fast which in turn is causing the relay to "buzz". My question is... Is there a simple way to correct this without a crash course in resistors, capacitors & diodes or is there another wire in the circuit without the pwm, or another circuit which perform the same way (turns on when you unlock the car or open a door and then shuts off upon locking or closing the door)?

Thanks!

raceman

Mine does same thing

schmart

The simplest way would be to use a diode and a capacitor. The diode can be pretty much anything such as a 1n4001. The capacitor will need to be "big enough" to act as a surge tank to keep the coil energized while the PWM signal is off. If it gets too big, it may delay the time for the relay to turn off until it is noticeable. If it is huge, the current surge when the PWM signal turns on may act as a short and cause problems with the driver.

Calculating the exact size would require the course as you mention
However you can experiment and come up with a capacitor value pretty easily. Just start small and keep increasing the size of the capacitor until the buzzing goes away. I'm pretty sure you will want a capacitor in the microfarad range (as compared to picofarad). I'd probably start with something like 100 uF and then change out by doubling the value until you get something that works.

--Rick

The Machine

Hmmm. Ok. Is there another circuit maybe that acts the same as the dome light that I can use instead?

The Machine

Would the puddle lights be the same way, on pwm or is that a different circuit I can use? If not, where do I place the diode and capacitor in relation to my signal wire and relay? Thanks for your help!

The Machine

Anyone out there in F150 land help me out here ?

schmart

Take a look here for a schematic of how to wire the relay:
http://www.the12volt.com/installbay/....asp?TID=10287

As far a puddle lights... I don't have them, but I suspect that any light with a dimming feature, even if only used when turning on and off use PWM. PWM can be done with software as compared to physical components for analog dimming.

--Rick

The Machine

Thanks Rick!

So I just need a capacitor spliced into the signal wire to my relay? I didn't add any diodes to my relays as none of them are really controlling a lot of amperage and from what I read is when that would be called for, correct? I'm learning as I go here so again appreciate your guidance!

schmart

You need both the diode and the capacitor.

Using a plumbing analogy, the capacitor works sort of like a surge tank or bucket. When the circuit is on, in addition to turning the relay on, it fills the capacitor. When the circuit goes off, the capacitor "drains" and provides power to hold the relay on.

The diode works like a one way valve. It keeps the capacitor from "back feeding" and trying to keep the dome lights on as well as the relay.

--Rick

The Machine

Good analogy, which goes first from the signal line splice to the relay, the diode or the capacitor, or does it even matter?

Thanks again!