contactor switched by remote contacts

Dont really need a contactor a - mains relay should be enough.. You will need to supply some mains to the remote contacts tho

I’m trying to imaging a scenario where you would need to use a relay to switch another relay. Maybe if you could explain what you’re trying to achieve folk might be able to advise?

Tim

One example would be a time-switch with a 3A rated relay needing a

John

Sounds unnecessarily complicated. Wi-Fi relay controlled from your mobile phone. Job done.

Tim

That was what I wanted to avoid. I don't know if the contacts are rated at 250V although they can handle 8A.

It is an off grid solar system with a 48V battery supplying an inverter and I want to dump excess into the immersion when the battery voltage exceeds 54V.

I have a 12V relay which will do the job but again I will need to derive a 12V supply for it but as the charger has a built in relay and timer I was hoping that would be easier.

A contactor comes in a handy DIN rail format and are about a tenner (see ebay), so I wouldn't worry about relays. Although many do have 250Vac contacts.

Be aware that 'dumping the excess' is not done by switching mains: what a solar diverter does is modulate the immersion coil so that the load into the immersion matches the excess generation. If you switched the immersion it would take 3kW even if the excess is only 1kW, meaning a 2kW draw from the grid. By treating the immersion as a resistor and modulating it you can drop the load to 1kW and not draw more than the generation.

Diverters can work in a number of modes:

I imagine a diverter has its own switching inside, so you can also energise the immersion at full power when you do want to heat the water.

Theo

Exactly the set-up I have.

I have a dual channel central heating timer, with one end giving a signal, via the cylinder thermostat, to the motorised valve to heat the cylinder. The other end controls a relay, which switches the immersion heater on (that end is normally set to permanently off, but can be switched to auto if there is any problem with the heating system or can be put on for one or two hours, after heavy usage, by pressing boost).

I am aware of that and of the fact they are expensive and often not long lived.

So my next option is to find a mains contactor with a 48V coil as the charger can supply 48V and the timer can maintain the contacts closed for 10 minutes, then rather than having the complication of modulating the excess it can supply ~250Wh using the battery as a buffer and then reset if the battery voltage has fallen.

Could you just use the mains immersion at 48V? If you had a 3kW immersion and power it at 48V not 240V it would consume 600W - would that be suitable to skim off the battery?

I havee a relay switching my boiler pump on and off, mains coil, mains contacts. DIB rail mounting.

Thats all that is needed

That's interesting thinking but this is a 1.5kW element so the power would be too low.

It would actually be even lower. A 3kW element rated for 240V operation would dissipate 120W at 48V. A 1.5kW element would dissipate 60W.

John

Well, I used to switch big relays on the site of the devices with much smaller low voltage ones a lot, at least then you only need a low voltage supply to your first relay. There are loads of relays like that available,. Brian

Good point - brain fart, got my currents and powers mixed up!

Theo

somewhat over 30A supply current!

A typical 48V LiFePO4 battery for PV storage can charge, or discharge at

Yup, but wiring gets a bit agricultural at those sorts of currents

Not unbearably so. A typical 3kW 48V LiFePO4 setup is handling 80A currents. Needs some chunky wiring but only at meter-tails kind of size.

Of course if you're running that wiring a long distance because the battery is in the garage at the bottom of the garden, that gets more annoying. (and the voltage drop is more of a concern too). In that case using the inverter would make more sense.

Theo