Help Choosing Relays

The relay contacts must be rated for the load current, in this case 4.5A for each heater. Feed each one through a contact rated 4.5A or above to be sure.

The relay coil voltage, and whether it's AC or DC, must match the supply voltage of the control circuit coming off the switch of the Watlow temperature controller.

Okay, i got it. Thanks. Then theres no problem in putting the relays in parallel right?

I think you have gotten the answer you need, but I want to make sure of one thing. You ask if there is a problem with putting relays in parallel, if you mean wiring the relay coils in parallel, your OK, as long as your driving circuit can handle the power needed to switch all the relays, but if you are talking about putting relays in parallel to increase the load that can be switched, the answer is no. You can not use 2 relays rated at 2 amps each to switch a 4 amp load. The relays will not switch in and out at the same time, so they will not share the stresses equally. They may share the steady-state load of the heaters once on, but it's the on/off sequence that causes the most stress for relays.

Tom

PWSlack gave a correct answer. I'd like to emphasize that the relay contact rating should be well above the actual current to be switched, if you want the system to last. I'd suggest 10 Amp rating to control 4.5 actual Amps.

If I understand correctly, you want to control 3-4 rows of 4 heaters each row. This means a total of 12 or 16 heaters, each with its own relay. Common 24V relay coils take around 50-100 mA, so a set of 16 relays all connected with their coils in parallel would require 800-1600 mA. (Yes, they would normally be connected in parallel.)

Notice that the controller was rated 8 Amps resistive. Relay coils are inductive, which means more arcing at the contacts. Buy one more relay, and connect just that one to the controller contacts, and use that relay's contacts to switch the other relays. This protects the more expensive controller.

If you elect to use DC coils, don't forget the back-EMF diode on each one.

Hey, sorry for the late reply. Got a little busy at work. Thanks all of you, really helped.

@Tdesmit : Yup i was going to wire them up in parallel to turn them on at the same time thats it, not to drive a larger load. I;ll jsut get a larger relay .

I'll get higher rated relays. I think my watlow controller can give 800 mA. That shouldnt be a problems. Awesome, thank you so much.

My suggestion is that since you have a temperature controller that is rated for 240 VAC just use the same voltage you are going to use for the heaters to the coils on your relays. The higher the voltage, the lower the amperage to achieve the same power. I am thinking maybe motor starters with multiple contacts would be a good choice and you could use one to a row of heaters. This would simplify your circuitry as you will not need a transformer, etc. for your control circuit.

The one shown above also has overload protection which may or may not be needed for your application.

Ordering tomorrow. Thanks a lot.

My pleasure!