help wiring a capacitor to filter noise

**kuo** · 06-03-2009, 05:38 PM

with the help of the information in this thread i successfully hooked up my xenarc's brightness control wire to my car's dimmer knob. because my car's dimmer knob outputs from 0V to 12V and the xenarc's brightness control wire only goes from 0V to approximately 3.7V, i wired up a voltage divider to get into the desired range as follows.

the problem i have after doing this mod is that i get rolling lines in the screen whenever i dim the backlight using the knob. when i dimmed the screen using the lcd's menu i didn't get lines. Also the interference was present before i wired in the resistors, so the interference is definitely coming from the knob. i tried to wire up a capacitor to filter out the noise but i had no idea what i was doing and all i did was screw things up so i took the capacitor back out. i've done a lot of googling and reading here on the forums for guidance on how to do this but i'm still at a loss. can anyone tell me if i'm even on the right track with the capacitor idea, and if i am, what kind of capacitor do i need and where do i put it into the circuit? when i tried to wire it in, the dimming did not work at all and stayed at full brightness with output at 0V so i was obviously doing it wrong.

**Sicarius** · 06-03-2009, 06:05 PM

Is your dimmer really outputting a lowered voltage or is it pulsing the output (ie. producing a square wave with varying duty cycles)?

**kuo** · 06-03-2009, 10:16 PM

how would i go about figuring that out?

i was also thinking...i have one of those noise filters from radioshack. could i wire that into the circuit somehow?

**kuo** · 06-05-2009, 10:53 AM

so i got some smaller ceramic caps wired in but i still have noise. i'm stabbing in the dark here since i know very little about the physics involved in all of this but my next move is to try bigger resistors in my voltage divider. i'm thinking since this is just a signal wire maybe i've got too much juice flowing...

**Pathos** · 06-05-2009, 12:10 PM

Is your dimmer the rheostat type or is it digitally controlled?

**Sicarius** · 06-05-2009, 04:31 PM

Originally Posted by Pathos

Is your dimmer the rheostat type or is it digitally controlled?

Which is the same question I asked and it really needs to be answered.

If it's a rheostat lowering the voltage then you should be good and won't need a cap.

If the output is pulsed, then no amount or type of capacitor will do what's requested.

**kuo** · 06-08-2009, 09:40 AM

i have the service manual for my car but it doesn't say anything about the dimmer being a rheostat. i googled for a while and found a few instances of people calling the dimmer a rheostat but nothing definitive...could be people just making assumptions. is there a way to test the dimmer to see if it is a rheostat or not? i have a cheapo multimeter.

**colin** · 06-08-2009, 01:07 PM

For the most part, dimmers in cars are simple rheostats with voltage from 0-12v measuring in 255 bits. Your car actually knows how far your dimmer is turned, although it doesnt need to.

The xenarc display is probably using a square wave pulse to control the lighting. Both LEDs and CFFLs are hard to control with varying voltage.