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ac relay drive problem .Help needed

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Junior Member level 3
Aug 28, 2004
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problem of ac relay

I have a problem with the drive of a 220 V ac relay . I have an 8051 mcu with which i drive (with a uln2003 interface ) a 12 vdc relay. This in turn switches a larger 220V ac /6W relay . I have a snubber in shunt with my 12v relay board consisting of a .033 uF X2 cap. in series with 100 R resistor . The problem is that the arc during turn off is sufficient to reset the cpu sometimes and sometimes to blow the cap. What can i do ?. Is zero crossing detection the solution?.
I don't want to change something in my circuit . Any opinions would be appreciated .

ac snubber for ac relay

If the 250Vac cap in the snubber circuit gets damaged you may have problems with transient overvoltage ..
Try to add MOV in parallel with the snubber circuit .. see picture below ..

If you don't want to further experiment with DC-Relay<=>AC-Relay<=>Load configuration go for Solid-State-Relay ..
Not only it can be driven directly from MCU pin, but it switches at zero-crossing, so most of your problems should disappear ..



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ac relay driving


I think a simple diode in reverse direction on 12v relay coil is sufficient


Thanks guys for your answers .

Now regarding mov the problem is that in case of mov failure they usually short so this would turn on permanently the exit which would cause a series of other problems . One time i've seen that so i can't go for that .
The other solution with the diode shunt to 12 volt is already there in the ULN chip that drives the relay.I've also tested that and came to the conclusion that it is EMC based problem .
Has anybody experimented with zero crossings and relay drive ?.

I had this problem before. It drove me up the wall. Spend much time and money on it. Much of it like what's being discussed here. Here is my opinion.

1) Do a 4-layer PCB.
2) Try to change the way your program clock/latch signals to the ULN drivers.
2b) Use active low as much as possible.
3) Re-examine your PCB routing. Keep the clocking signals short. Especially on the sensitive lines.
4) I also changed fr AC contactors to DC24V contactors.
5) Use xxuF Mylar caps and In4004 at the output of ULN(ya there is a built in diode, so what)


Thank's for the reply Mcu Man

It has also driven me mad . The thing is with no AC load everything works ok.
So i suspect that the problem is not in the winding of the drive relays rather than the winding of the load relays. So i'm oriented towards the zero crossings solution .

I thinks u have to 2 things to correct:
1) With DC 12V Relay, u needn't use RC snubber. U only need a Diot in parrarllel with DC Relay Coil.
2) Your problem is in AC Relay. If u want to solve it, u can do following:
+ Isolating the AC portion and Microcontroller board. U should set AC portion away from your board.
+ Connect the load in parallel with a RC Snubber circuit.
+ If u can, u should use DC Relay. 220V line always contain a lot of reference.

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