I have a simple relay circuit which consists of a 12v DC coil relay switching a 110v AC motor load. The relay is controlled by a pic through a ULN2803 Darlington array and there is also a 1N1004 diode across the relay coil to snub the flyback voltage from the coil.

The problem I have is when I switch the relay the AC load causes some sort of noise or interference which travels all the way back to the Pic and locks it up. This noise can happen both when I turn the motor on and when I turn the motor off. The motors are small 20 watt motors.

So what can I do to filter or suppress or eliminate this noise? What exactly is this noise?

One more thing, when I switch the relay with no AC load it works fine, it also works fine with resistive loads such as lights.


Thanks to all.

 

Do you have a 100nF capacitor connected across the supply pins?

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No I do not. Should there be a capacitor across the supply pin of the relay coil?

 

Also a zero crossing circuit might help.

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Bill
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Most improtant for me is if someone can help me understand what is happening in the circuit. Why does the AC load effect my DC circuit when there is no direct connection between the two. And why does it only seem to be happening on motor loads?

For crying out loud, I am just switching a relay, what is going on here???

thanks again

 

Inductive EMF switching loads can be a very noisy affair. Proper power decoupling is a must.

PS the 0.1uf go across every digital IC power pin pair.

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Bill
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A snubber network across the motor might also help but if you've not got a 100nF capacitor across the supply pins of the PIC then you're asking for trouble.

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The short answer is that you need to filter out all the noise. Get rid of as much as you can at the motor and at the relay contacts. Better get as much as possible at the source. Then filter the power supply to the control electronics. Then filter some more right by the chip. Then pray that everything works.

Learn and understand differential and common mode noise and how to prevent them with differential and common mode filters. You will see these installed at the input of many electronics and you can salvage them and reuse them.

 

Yes. The three magic words are filter, filter, filter. Switching an inductive motor load not only causes power line noise, but can also radiate high frequency noise into sensitive circuits.

Is the PIC decoupled to a ground plane on a circuit board? It should have a power supply decoupling capacitor with short leads right at the power pins(preferably surface mount). Also try a small decoupling capacitor (say 0.01uF) with short leads across the relay coil.

And physically separating the motor and relay from the PIC as much as possible may help.

If all else fails you can try putting the PIC in a metal box with feedthrough filters for all lines.