Electronics Guru's- PWM and Relays

[biggy238]

I've read a couple of times here that most relay coils can't handle the PWM (Pulse Width Modulation) opening and closing the contacts rapidly.



Can a sufficiently sized capacitor be used to "levelize" the PWM? I'm generalizing this as i feel it would apply to any modulated circuit of sufficient current to power a relay.

 

[CKelley1]

i am lost on this one , give me an example please

 

[biggy238]

O. k.



Say an existing circuit is using PWM. I wish to attach a relay to said circuit, so that the primary circuit can trigger another circuit. However, the relay coil cannot cycle at the rate the PWM does or it will fail prematurely.



Will a capacitor counteract the switching affect of the pulse width modulation in order to provide constant polarity to the the relay's coil?

 

[p-Bar]

I thinks it's going to be more like an rc network resistor and capacitor in parallel. I did this for my hi beams , I will try to find the values for you.

Here is the web page: Automotive Lighting FAQ - headlamps and HID harness
go to the bottom of the page with this heading:How do I wire up solenoid of the 2002+ Bosch Bi-Xenon projector unit.

you eill see the circuit with the 1000uF/ 25v cap and the 10k resistor to the relay . Hope this helps. I used this setup on my hi beams for my metal halides.

 

[biggy238]

That is exactly it I think. So I wasn't that far off pace when I came up with that rig?



I want to trigger my fogs with the highs so that I have the flood pattern in front of the truck and the long range pattern.



Elaborate on "RC Network Resistor" for me. At work, we had a discussion about a resistor in parallel for this circuit, but thought that it would trigger the idiot light.

 

[Gary - K7GLD]

If it was me, I'd use a polarity diode followed by a capacitor to power an added relay - to keep the added capacitance from loading and defeating the function the PWM is being used for in the first place...

 

[LHirshman]

You cannot add a relay to PWM circuit

You cannot add a relay to a PWM circuit. The inductive kickback voltage from the relay coil is much much higher than 12 volts and can blow out something.



The circuit with the 1000uF cap and 10K resistor just allows the relay to opperate momentarily.

 

[Gary - K7GLD]

You cannot add a relay to a PWM circuit. The inductive kickback voltage from the relay coil is much much higher than 12 volts and can blow out something.



The circuit with the 1000uF cap and 10K resistor just allows the relay to opperate momentarily.

Which is why I suggested the polarity diode to act as an isolator/block against voltage feedback into the actual PWM circuit - the PWM circuit wouldn't even "see" the added relay circuit at all.



I use something roughly similar in my headlight circuit - I have added Sport Lights, and a Danial Stern relay/wiring setup to control them. I wanted the low beams to light whenever the high beams were on - so used a single diode from the relay coil of the high beam relay to the low beam relay coil - whenever the high beam circuit is activated, so is the low beam - but when only the low beams are on, the polarizing diode isolates the high beam relay - and the high beams don't light...

 

[biggy238]

It seems like I looked into doing that some time ago and couldn't find the poles I needed on the relays.



It would have been almost a year ago now I think.

 

[abrankod]

You could look into a 30-40A Solid State relay. They have no inductive kick and can handle the high switching speed of a PWM input. Digikey or mouser electronics should have something in the right amp range for about $30

-Dan

 

[JKloppe]

I've read a couple of times here that most relay coils can't handle the PWM (Pulse Width Modulation) opening and closing the contacts rapidly.



Can a sufficiently sized capacitor be used to "levelize" the PWM? I'm generalizing this as i feel it would apply to any modulated circuit of sufficient current to power a relay.

The headlights only get PWM when the lights are burnt out. Once the FCM has no resistance from the filement breaking (or bulb gone in my case) it PWM between 8 and 11 volts. If you are keeping your headlights the way they are you can use hi, low or both to trigger your relays.



I have a lot of time on my relays and have not had any problems.



#ad

 

[biggy238]

What am I seeing in your picture there?

 

[CKelley1]

he is using resistors to simulate bulb draw, i spent one full day on this the higher the resistance you can use and not set the code off the less energy you waste, i have geno's easy circuits on my truck so i went a different route i had a spare switch and i wired it to a 40 amp relay and use it to turn on the hid totally eliminating all circuits from the truck . my kit only had the low beam i ordered a new kit that has a magnetic shifter with both beams i am going to see how it is wired and see if i want to use a mounted dash switch for that also , i think i will need the momentary trigger as shown in this thread this is what i like about this forum i get the benefit of of not having to make as many mistakes getting to where i want to go

 

[JKloppe]

Those are resisters just as CKelley1 stated.



There is no reason to add another switch for the headlights, Dodge already put a headlight switch on the truck. With resistance on each circuit there is no PWM. Use low or high circuit to switch relays, use diode if you want additional light on both high and low beam.





CKelley1- If you wanted to keep you high beams on, you will have to hold that momentary switch down to keep the high beams on (not fun or safe).

If you used the oem light swithch you already have a trigger for you high beam solenoid, momentary or on.

 

[rbattelle]

The headlights only get PWM when the lights are burnt out. Once the FCM has no resistance from the filement breaking (or bulb gone in my case) it PWM between 8 and 11 volts.

I didn't know it did this. Why?



Am I reading this right - you guys aren't trying to pulse the headlights, you're trying to counteract the behavior from the computer when it senses a bulb out?



Ryan

 

[JKloppe]

rabattelle-

I dont know why the FCM acts this way.

I needed a constant 12v to keep the relay switched, so I added resistance to keep the "lamp out" off and a constant 12v for the relay trigger.

 

[Gary - K7GLD]

I didn't know it did this. Why?



Am I reading this right - you guys aren't trying to pulse the headlights, you're trying to counteract the behavior from the computer when it senses a bulb out?



Ryan

Yeah - it sorta puzzles me too - don't understand the reasoning or design...