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Electronics => Beginners => Topic started by: spinach on March 28, 2018, 09:21:54 pm

Title: Switching 3Kw 240v heating with low voltage 24DC thermostat
Post by: spinach on March 28, 2018, 09:21:54 pm

Hi All,

I wanted to ask the opinion of others about this project.

I have a spare Smart thermo (Tado with extension box) that I'd like to use to control 2x1.5Kw 240v heating with a spare Tado thermostat and extension box. 

I was thinking about using the following:

https://www.mouser.co.uk/ProductDetail/528-6240AXXSZS-DC3 (https://www.mouser.co.uk/ProductDetail/528-6240AXXSZS-DC3)
https://www.mouser.co.uk/ProductDetail/528-SSR-HS-1 (https://www.mouser.co.uk/ProductDetail/528-SSR-HS-1)
https://www.mouser.co.uk/ProductDetail/528-SSR-TP-1 (https://www.mouser.co.uk/ProductDetail/528-SSR-TP-1)

putting it inside a large junction box and switching the low voltage side with the smart thermostat.  The mains will come from an RCD already in place and there will be a fused switched spur before the junction box.


What do you think?

Title: Re: Switching 3Kw 240v heating with low voltage 24DC thermostat
Post by: Zero999 on March 28, 2018, 10:20:47 pm

That will work, but why have you decided on a solid state relay, rather than a mechanical one?

The solid state relay you've chosen is much more expensive and will have higher power losses, than the equivalent mechanical relay. The only advantages are: more resistance to vibration and the life is not limited by the number of cycles. If it's not being switched very often and isn't subject to vibration, then a mechanical relay is a better choice.

Title: Re: Switching 3Kw 240v heating with low voltage 24DC thermostat
Post by: spinach on April 04, 2018, 02:18:11 pm

I decided on an SSR because of the frequency it's going to turn on and off.  From what I know about this particular thermostat it works on a demand for heat scale, which determines how frequently it turns on and off.  I've noticed my boiler which is also on the same brand of thermostat can be on for a few minutes then off for a few minutes.

It looks like I've either broken it, have a damaged unit or don't understand the wiring but it doesn't work.

I have terminal 2 connected to the AC live, terminal 1 connected to the heater live, and the AC neutral and earth connected directly to the heater.  When I turn on the power the power led on the heater comes on for half a second then goes off.

Title: Re: Switching 3Kw 240v heating with low voltage 24DC thermostat
Post by: spinach on April 04, 2018, 08:38:20 pm

I managed to figure it out.  After reading around the internet I hooked up my bench power supply to the DC side of the relay, and an incandescent lamp bulb to where the heater connects.  Increasing the voltage slowly on the bench supply, the lamp came on at just over 19v.  So the relay requires quite a bit more than the stated minimum 3v to work.