Nvisable

I just install two new pairs of Hella driving and fog lights. The problem is when I turn them off I get a loud "pop" heard in my stereo. How can I get rid of this "pop" sound?

I'm using a relay to power these.

jrs_dodge_diesel

That pop is most likely the relay's coil de-energizing.

A little electronics theory here. An inductor (coil of wire) stores electrical energy in a magnetic field. When the coil is turned off, that stored energy has to go somewhere. This is usually in the form of a rush of current as the magnetic field collapses. This problem also happen to a lot of commercial audio amplifers.

This is also the same principle that automotive ignition coils operate on.

What I think is happening is when the relay coil is turned off, that surge of 12V current is getting coupled into your stereo. The simplest, cheapest solution I have is just to turn the radio off BEFORE turning off the lights. Then turn the radio back on afterward.

Another solution would be to add a small capacitor across the power and ground lines going to your radio. That involves more work and some soldering though.

Nvisable

I'm not bothered by soldering so I'll add a capacitor to the power leads of the head unit. I'm looking for one of those disc capacitors right? I had thought about putting a capacitor across the relay terminals. Would that make a difference?

hotdram

You could also add a flyback diode across the coil.
Click Here, see page 2
The diode is reverse biased (meaning no current flows through it) during normal circuit operation (when the relay is energized) but when the relay is de-energized, the diode conducts (for the duration of the spike due to inductive kick).

~Rob

Rollin'_Coal

Hotdram has the right idea. I'm an electrician and my opinion is that a capacitor will stop any flickering if there is any which i doubt. a diode, more commonly refered to in this application as a "free-wheeling diode", will give the spike caused by the coil field collapsing a discharge path and should cure 99% of the popping sound. When you hook up the diode the side with the line (cathode) needs to go to the positive wire of your lights and the other side (anode) needs to go to the negetive.

1-2-3

"You could also add a flyback diode across the coil."

We used something similar at a cabinet shop I used to work at. We'd add them to the motors. While the current was applied, and the motor was running, they did nothing. However, when a three phase motor is de-energized 9turned off), yet still spinning, it becomes a kind of generator. It can make some big voltages. The giant diode/resistor pack across the leads acted as a big brake. Some of our motors would coast for 15 to 20 minutes. Several restarts a day meant hours of work time lost by the end of the week. We figured they paid for themsives in just a couple of motor stop cycles.

Another advantage. Put your lunch on top of one of them. It would be warmed just right by the time the noon bell rang. But there was the situation when they'd burn up on a hot and heavy use day. They would let out the nastiest green tinted smoke. And there would be at least one person get sick and chum from the smell. It would make your lunch taste awful bad too.

We also would add flash sheilds on the contactors to ground out the arc when the contacts dropped out.

But in your case a capacitor will fix your issue. Just stop in any car stero shop. We called them snubbers. They might not know what a snubber is, and if you ask for capacitor, you might get a giant fat boy that the steros folks put on thier power amps to feed them these days.

Maybe if you ask for a noise filter you'll get what you need. A Radio shack would have them for sure. It should look like a small can, about an inch around and about 2 to 3 inches long. It'll have a metal jacket with a ground/mounting clamp around it, and a connector on each end. Simply put those connectors inline on your power wire and bolt the cap to a something metal for a good ground.

hotdram

Small point (since the lights and relay are probably both being powered from the same +12V), but I would hook the diode up directly across the coil side (the lights are on the armature side).

~Rob

hotdram

Got bored and simulated a relay:

Here is a relay being switched on and off WITHOUT A DIODE:


Here is a zoom in of the large spikes from the above image:


Here is a relay being switched WITH A FREEWHEELING DIODE:
Notice the big reduction in spikes (and the spike is not oscillatory like what is shown above)


~supernerdRob

jrs_dodge_diesel

Some really good ideas here. The cap, will work, however it appears that the diode would work even better. I didn't know you could use a diode like that. learn something new everyday.

Jim Lane

It could also be a shielding issue with the input of the amplifier.
How close are the wires for the switch in relation to the audio cables.

When the field collapses you are also creating an RF signal and the switch lead is the antenna.

I use a quench diode across all of my relay coils just because I always have.

Bosch relay #0-332-019-150 most commonly one used by everyone.

Bosch Relay #0-332-019-155 that has an internal quench diode.

You can get these at most Truck Parts.

These relays are polarity sensitive Term. #86 (+)

If you connect diodes across your relay coils and you are not to sure of yourself it is a good idea to connect a tail lamp bulb in series with the battery lead when you first connect your circuit.

If you connect it to the battery backwards it will become a dead short and the diode will explode, but with the lamp in series it will light and the diode will live another day.


Quote:
I had thought about putting a capacitor across the relay terminals. Would that make a difference?

You just inadvertently created a time delay L/C circuit (inductor/ capacitor).
I use this circuit to delay the break of a DC relay circuit.

Flyback now that brings back some good shocking memories.

Jim

Nvisable

Thanks for all of the help! I didn't expect to geek out on this. Anyway if I'm reading everything correctly I need to connect the diode from connector 86 to 85 on the relay with the stripe of the diode connected to 85.

What kind of diode do I need? Is the current something I need to pay attention to? Can I get this at Radio Shack?

I'll also check the polarity for connectors 86 to 85 in the morning. I think mine are wired backwards.

Nvisable

Besides delaying the break of a circuit, it wouldn't make a difference in my case, right? The "pop" would still be there just later in time.

I enjoy learning this kind of stuff.

KBennett

You can purchase the diode at Radio Shack.
Diode link
Place the striped side of the diode on the positive lead to the coil and the black side on the negative.

Unless there is a diode on the relay already, coils are not polarity sensitive. This means that it does not matter which side you apply positive or negative.

hotdram

The diode will spend most of it's time "reverse biased" (not conducting). This means that it will have 12V of "reverse voltage" across it. Diodes data sheets call this "Peak Repetitive Reverse Voltage" or "Working Peak Reverse Voltage" or "DC Blocking Voltage" or "Peak Inverse Voltage". I usually derate by 50%, so you will need a diode that has a Peak Repetive Reverse Voltage of 24V or higher. 50V is a standard value, you should be good with that.
The other parameter to be concerned with is: how much current will be flowing when the diode is conducting (only conducts briefly when de-energizing the coil). To figure out what the current spike (or surge) would be, one would need to know what the voltage spike magnitude is. I can't tell you because that depends on the inductance of the relay's coil. Diodes are rated for a much higher non-repetive surge current than average current. For instance, a diode that is rated at 1 Amp continuous current might have a 30 Amp surge current rating. Click Here to see a data sheet of a diode, you will see what I mean.

I'm not a nerd in real life, I just play one at work
~Rob

DaveB.inVa

hotdram,

What software are you using in your simulations above?

Thanks,

Dave