difference between relay coil for AC / DC 2

[aherbas]

Someboy can explain the difference between a relay coil working in Direct Current and Alternating Current.

What can I do to make an AC relay work with DC.

For example a I have a relay with coil for 220 VAC and i want it to be energized with 125 V DC.

Thanks

Alvaro

 

[Skogsgurra]

The differences are how current is limited and how the force/pull characteristics look. The AC relay usually needs more pull to start with while the DC relay has a more constant force over the entire pull.

The DC coil has a high and constant resistance that limits the current to the correct operating value.

The AC coil has an inductance that is low when the cores are separated and gets higher as the cores are attracted to each other (that is what activates the relay). So the AC coil has a rather large current before the gap between the cores closes. After they have closed, the current decreases because the coil impedance goes up (higher inductance = higher impedance).

So you have to do two things to make an AC coil work with DC: Limit the current with an external resistor and Make the initial current high enough to operate the relay.

I would start with the resistor. Connect it in series with the coil and adjust it so that the holding current is just right - or so that the coil does not overheat.

Second, you have to make the current high enough to start the core moving in. You can either use a capacitor parallel to the resistor. I would guess that any value between 100 or 470 microfarads at 150 V is about right. Smaller relays need smaller capacitors. Or you can use a normally closed contact on the relay and connect it parallel to the resistor. The contact bypasses the resistor so that the core starts moving and when it has started moving, the contact opens so that the resistor is placed in series with the coil. A combination of capacitor and NC contact is often the best solution.

It takes some experimenting to make the combination work reliably.

 

[advidana]

i don't think it will work. the same reason as a tranformer will not work with dc.

 

[Skogsgurra]

advidana,

Think again. A relay is not a transformer. It does not work on the same principles.

 

[RRaghunath]

Very well explained Skogsgurra.

I have also seen DC coils being used in AC system and these coils come with a full wave bridge rectifier attached to the coil AC input terminals. The advantage is obvious that with DC coils there is no problem of chattering.

 

[RalphChristie]

Nice skogs...



Ralph