How to Prevent a Circuit Affected by Back EMF of AC Relay?

What kind of output circuit are you connecting to in order to add your relay? Back EMF is usually a problem for DC circuits where some output transistor is sensitive to a high voltage spike.

If your AC circuit is affected by the additional impedance of your relay, then it must be one big relay! But if the relay is rated for the voltage then it should work in all likelihood.

Since your AC circuit is some sort of discrete signal, the question to answer is what is its rated current output and can it stand the additional few milliamps your relay is likely to require. You might want to measure the amount of current your relay pulls at normal rated voltage.

Any energy left in the field of your relay coil when the circuit is turned off will be dissipated in the (parallel) load with little chance for a spike condition.

Get one of these AC snubbers or build your own, it's just an resistor and capacitor in series, then paralled across the coil. For DIY component values use Google.

And then there's use a spare contact of the first relay to switch the second relay.

How is your original circuit protected (snubber diode, RC network, etc) and is this existing protection robust enough to handle the second relay coil?

If it is then there is no point redesigning the original circuit, especially if you want to make major changes to existing printed circuit board artwork and bills of materials which can get very expensive.

I once had to do this in a circuit which had on the same board audio, signalling and a low voltage ac relay that caused clicks and triggered nearby CMOS when disconnected.

Putting a Zobel network in parallel with relay coil cured problems. Values based on CR=L/R. R was same as coil resistance and C was calculated from measuring the impedance at 10kHz (arbitrary frequency easily obtained with test gear on hand).

When measuring impedance, manually hold the relay in the operating position as the inductance is somewhat higher then.

Worked for me but if you have a high voltage relay you might have difficulty obtaining non-electrolytic caps of suitable value.

an MOV and/or capacitor across the coil and possibly a choke in series with it.

forget the choke-would add more inductance! Sorry-did not think it through.

If I'm reading your problem correctly, your worried about the back emf from the relay causing problems with your original circuit, correct?

Lets go back to physics 101 for a second. A relay is simply a coil (control) which pulls the contact(s) closed on your load. The contact is pulled via a magnetic field that is produced by the alternating current. As electrons move through a conductor, a magnetic field is created around the conductor (right hand rule tells you which way the magnetic field is oriented). Now, look at your AC wave, for simplicity sake, we'll assume it's a sine wave. As the voltage reaches the peak, the magnetic field is also reaching it's peak. Then the voltage starts to decrease and the magnetic field decreases as well, but the magnetic field will impart a slight voltage to the conductor - think of this as the difference between a generator (moving a magnetic field to generate electricity) and a motor (using electricity to create motion). The collapsing magnetic field creates this "back emf", which is simply inductive resistance.

The stronger the magnetic field, the more inductive resistance you will have. Since a relay's job is to move a contact, only a small magnetic field typically is needed. If you need a higher load voltage, the relay will have a stronger spring to insure the contact doesn't get stuck closed, thus you need a stronger magnetic field to overcome the larger spring.

I don't think you will have a problem with the relay in your circuit. If you do, you will need to isolate the inductance via an RC filter. The easiest way is via a capacitor and resistor in parallel across the coil.

Since I don't know what your original circuit is, so I've provided general information for you. If the circuit is very sensitive, you will need to look into the effect of altering the impedance.

I hope this helps.

All A/C relays have a shading coil incorporated into their design to prevent chattering or buzzing of the relay when the voltage crosses zero.It works by using the back-emf to energize a small shunt winding(shading coil) to generate a magnetic field to hold the armature in place.Thus,most of the back-emf is absorbed by the shading coil itself.There is very little spike left unless it is one huge coil.Look closely at an a/c relay and you will find the shading coil.Sometimes it is in the armature, sometimes it is around the coil core.It will be usually made of copper, and easy to find.