fsackur/Brief for electrical relay circuit

## Brief for electrical relay circuit

Relay circuit design & spec for domestic electrical heating
============================================

I have multiple Tado Smart Thermostats, model RU01, and multiple rooms with electrical underfloor heating. I require a power-switching circuit for the heating.

The heating in the largest room has a resistance of ~14 ohms and should run at 230VAC (I'm in the UK). All other rooms have higher resistance. This is a purely resistive load.

Tado do not provide great datasheets, but they have three pins for simple NO/NC/COM. I found a downloadable user guide at http://www.free-instruction-manuals.com/pdf/pa_1184164.pdf which states:
    - Relay: max. 240V AC 6(4)A / max. 36V DC 6(4)A

Obviously, I want to stay well under that!

Switching will not occur more often than every few minutes. (There is no PWM.)

I initially installed the system with SSR-25AA relays. The relays got unacceptably hot, and I have blown up two thermostats. I used a naive "design" where I ran mains voltage through the Tado relay on the input side.

Brief:
    - Design a circuit to be powered from 240VAC, using only components available in the UK (e.g. from https://uk.farnell.com or https://uk.rs-online.com or amazon) with less than a week lead time
    - Provide a complete parts list
    - Parts cost not to exceed £50 per thermostat - I hope it will be a lot less than that!
    - Maximum power to be dissipated by circuitry, per thermostat: 5W, but ideally under 1W
    - Designs including a DC PSU for the input side are fine, as long as the components are on the parts list
    - Please state whether you can provide a design that offers:
        - No audible solenoid clicking
        - Entire design fits within a single pattress box (70 x 70 x 45 mm)

Deliverables:
    - Circuit diagram
    - List of parts by mfr part code and/or link to component on retailer website

I will need to know what license you offer the intellectual property under. I will not be doing commercial development, but I may wish to share the design.

	Relay circuit design & spec for domestic electrical heating
	============================================

	I have multiple Tado Smart Thermostats, model RU01, and multiple rooms with electrical underfloor heating. I require a power-switching circuit for the heating.

	The heating in the largest room has a resistance of ~14 ohms and should run at 230VAC (I'm in the UK). All other rooms have higher resistance. This is a purely resistive load.

	Tado do not provide great datasheets, but they have three pins for simple NO/NC/COM. I found a downloadable user guide at http://www.free-instruction-manuals.com/pdf/pa_1184164.pdf which states:
	- Relay: max. 240V AC 6(4)A / max. 36V DC 6(4)A

	Obviously, I want to stay well under that!

	Switching will not occur more often than every few minutes. (There is no PWM.)

	I initially installed the system with SSR-25AA relays. The relays got unacceptably hot, and I have blown up two thermostats. I used a naive "design" where I ran mains voltage through the Tado relay on the input side.

	Brief:
	- Design a circuit to be powered from 240VAC, using only components available in the UK (e.g. from https://uk.farnell.com or https://uk.rs-online.com or amazon) with less than a week lead time
	- Provide a complete parts list
	- Parts cost not to exceed £50 per thermostat - I hope it will be a lot less than that!
	- Maximum power to be dissipated by circuitry, per thermostat: 5W, but ideally under 1W
	- Designs including a DC PSU for the input side are fine, as long as the components are on the parts list
	- Please state whether you can provide a design that offers:
	- No audible solenoid clicking
	- Entire design fits within a single pattress box (70 x 70 x 45 mm)

	Deliverables:
	- Circuit diagram
	- List of parts by mfr part code and/or link to component on retailer website

	I will need to know what license you offer the intellectual property under. I will not be doing commercial development, but I may wish to share the design.