What purpose does this capacitor/resistor serve?

[marmite2012]

Hello, I have recently had my hot tub connected to the home boiler. The hot tubs control circuit used to power a 3kw 240v (UK)electric heater, through a simple relay/contactor on the circuit board. We have removed the heater and used this switched feed to feed a coil on a 240v contactor, then used the volt free contacts to switch the hot tub zone on at the boiler. This then allows the hot tub to control the temp via the boiler. It turns the boiler on but then the hot tub is up to temp and turning off, the contactor is staying on. I am still getting about 18v on the contactor. On closer inspection I can see that there is a 1w100 resistor and a 473/630v capacitor connected in series over the in and out on the hot tubs relay/contactor. This is obviously letting some voltage pass through, enough to keep the second contactor in the on position.

My question is what is the actual job of this circuit?

Thanks in advance, Steve

 

[marmite2012]

Also just to add if it isn't obvious, when the hot tub is not calling for heat, this capacitor and resistor is in series with the heating element, then it calls for heat the relay/contactor bridges a circuit over these components.

 

[KrisBlueNZ]

Sounds like a snubber circuit - see https://en.wikipedia.org/wiki/Snubber#RC_snubbers

 

[marmite2012]

Am I likley to damage anything if I remove this circuit. Obviously I am not using a heating element load anymore but simply powering the coil of a contact from it. Regards Steve

 

[KrisBlueNZ]

The snubber is there to reduce arcing between the contacts in the relay on the hot tub control board when they open. Arcing causes early contact failure and can cause the contacts to weld themselves together.

You have changed the load that those contacts drive. Previously they directly switched a 3 kW heater; now they switch the coil of another relay. The new load draws a lot less current, which means less arcing, but it is a coil, which is inductive, which means more arcing. It's hard to say which factor will be greater!

In this application you definitely don't want those contacts to weld themselves together. So I would not remove the snubber. But you can connect a resistor across the coil of the relay that you are driving from those contacts, so that you don't get 18V AC across it when the driving relay is open.

The next step is to calculate the resistance and power rating of the resistor(s) you'll need to connect across the coil of the relay you are driving, to reduce that 18VAC voltage low enough for it to drop out. This may be outside my area of expertise; in this case I'll ask duke37 and Steve to advise. But we will need to know the details of this relay/contactor you are driving from the switched 240VAC from the hot tub's relay. This is the relay/contactor that stays ON when it shouldn't, because it has 18VAC across its coil.

Can you give us all the information you have on it. A photo might be helpful too.

 

[marmite2012]

OK sorry for the late reply i've not had time to look into this until tonight. I measured the voltage with a more accurate meter and its actually 25v ac. I also measured the resistance of the coil and it is 3.3k ohms. I've currently added a 25w lamp in parrellel with the coil so the control circuit operates, but obviously I would prefer a more permanent solution like the resistor you mentioned. Regards Steve

 

[KrisBlueNZ]

I think that's a pretty good solution.

If you want to use resistors, you should get some 10 kΩ 10 watt wirewound resistors and connect them in parallel across the contactor coil. You'll probably need more than one, maybe as many as six. You can keep adding them until the contactor turns OFF when it should, then add one or two more, or just bite the bullet and use six.

Each one will dissipate about 5W while the controller output is active, so they will all get pretty hot. You can try to heatsink them, or leave them free-standing. It's probably best not to solder to them; they won't get hot enough to melt solder, but they could form dry joints after a time. Use crimp connectors or "chocolate block" strips (temperature rated) or similar.