Hi,
I have designed a circuit is about digital outs. I controll a relay with a logic(12vdc) and then the relay's normally open contact triggers 2nd relay. 2nd relay is triggered with ac signal(220vac). If i triggered 1st relay speedly,i think 2nd relay generates back emf. Because it causes to reset the logic. I have tried varystor between normally open contact and notr. But it doesnt effect. It still resets.
How can i suppress ac back emf in the relay?

 

hi mr.
Welcome to AAC.
Do you have any noise suppression across the relay coils.?
E

 

As in a diode?

 

Hi mr.
I attached my design. In design, NO_2 contact goes to other relay's coil. I tied up 220vac to COM_2 and also connect varystor between this NO_2 and notr. Still it gives back emf.

 

You mention switching another relay, AC coil?, do you have a RC snubber across the coil?
Max.

 

Hi mr.
I attached my design. In design, NO_2 contact goes to other relay's coil. I tied up 220vac to COM_2 and also connect varystor between this NO_2 and notr. Still it gives back emf.

There are lots of things that could cause this problem. The most likely is that the input to the switching circuit is picking up electrical noise radiated by the AC relay when it is activated.
How far apart are the two relays? what kind of wire/cable do you have connecting the contacts of the 12V relay to the coil of the 220V relay? Is the supply driving the 12V circuit connected to earth ground? What is connected to the input of the 12V relay switching circuit? How far from the circuit is it? what kind of wire/cable are you using to connect it to the input?
All of these things will have an effect on how immune to noise your circuit is.
Regards
Keith

 

Yes, i mean ac coil. There is not rc snubber in the circuit. Where do i add rc snubber to?
There is a 0.75mm2 30cm wire between 12v relay and 220v relay. I usually use it.
There is not earth ground in the logic part. There are just dc+ and dc- .
I controll the 12v relay switching with sn74hc595n ic shift register. It controll the RELAY_2 part in the sketch. It controll also other 7 relays. I think it is affected from back emf.

 

Hi,
I have designed a circuit is about digital outs. I controll a relay with a logic(12vdc) and then the relay's normally open contact triggers 2nd relay. 2nd relay is triggered with ac signal(220vac). If i triggered 1st relay speedly,i think 2nd relay generates back emf. Because it causes to reset the logic. I have tried varystor between normally open contact and notr. But it doesnt effect. It still resets.
How can i suppress ac back emf in the relay?

A surge suppressor would need to be connected across the AC coil. A 220 Ω 1W resistor in series with a 0.5 μF 350 V ~ metallised polypropylene capacitor would suffice. The MOV across the driving relay contact would not be required.

Regards,

Nandu.

 

The AC relay coil can be suppressed with a snubber network - a resistor and capacitor in series.
A better way is to use a Varistor. Something like this. They have a resistance that is high until a certain voltage is reached, then the resistance drops. They are "bidirectional" so they work with AC. They are normally specified with a voltage rating that will depend on your application.
Remember that with a AC supply, the peak voltage is √2 (1.414) times the RMS voltage so the varistor must be able to tolerate the peak voltage without going into the low resistance region.

 

I am using MOV-S07K275E2 varistor. But it cant prevent the back emf. Can the error be because of other things?
I will try a rc snubber 220Ω 1w resistor and 0.5 μF 350 V capacitor.

 

I controll the 12v relay switching with sn74hc595n ic shift register. It controll the RELAY_2 part in the sketch. It controll also other 7 relays. I think it is affected from back emf.

What is controlling the shift register? It would be much easier to solve the problem if we could see the whole picture.

 

. Where do i add rc snubber to?

Directly across the AC coil as close as possible.
Snubbers have always worked for me in the past when experiencing EMI etc.
The manuf of the AC contactors I use also supply a snubber that fits directly across the coil.
Max.

 

What's worked for me in the past better than an RC snubber, is a TVS diode. Since you're working with 220VAC at the coil, peak voltage is squrt(2)*220 = 311 Volts, more or less. So a bidirectional 342V TVS such as this one ought to do the job.

 

I have added to Ac coil 220Ω 1w resistor and 0.5 μF 350 V capacitor. But now there is always voltage on the coil. I measured it 40-80 vac when i didnt trigger the relay.
Microcontroller(pic24f) is controlling the shift register. Microcontroller runs normally. It is not affected.
Will i add the tvs diode between ac coils?

 

hi,
Is the R and C snubber across the actual coil of the AC relay.??
If yes, is the DC relay not energised when you measure the 30-80Vac across the AC relay coil.??
It suggests you have a wiring problem.
When posting a diagram its a good idea to include on the drawing the the Common Gnd reference line.
E

 

I have added to Ac coil 220Ω 1w resistor and 0.5 μF 350 V capacitor. But now there is always voltage on the coil. I measured it 40-80 vac when i didnt trigger the relay.
Microcontroller(pic24f) is controlling the shift register. Microcontroller runs normally. It is not affected.
Will i add the tvs diode between ac coils?

hi

I don’t know what your planning to do with this board...but hopefully you’ve taken safety requirements into consideration for the 220 VAC circuit components and traces on the PCB.

Otherwise, I would move the DC relay off the board..

just a suggestion...

eT

 

Hi,
I just quickly drew necessary part for you. I cant send all design.
Is the R and C snubber across the actual coil of the AC relay.?? ( Yes )
If yes, is the DC relay not energised when you measure the 30-80Vac across the AC relay coil.?? ( No, it is ac relays coil)

 

If yes, is the DC relay not energised when you measure the 30-80Vac across the AC relay coil.?? ( No, it is ac relays coil)

Hi mr,
This line of text is contradictory .??
I asked:
when you measure the 30-80Vac across the AC relay coil.?? ( No, it is ac relays coil)
You replied:
No, it is ac relays coil)

Please post a sketch showing which AC has the 30-80Vac across its coil.
E

 

Sorry, i misunderstood. Althougt i didnt energize the dc coil, there was 30-80Vac across ac coil.(If I add rc 220Ω 1w resistor and 0.5 μF 350 V capacitor across ac coil)

 

I have tried your suggestions. But they didnt effect. I have added the MOV between dc relays normally open contact and ground. The back emf(error) is gone when only i did that. I have tested a while. There isnt any problem. Can it be use like that? if i use, Can it cause a problem?