Electronics > Beginners
Switching boiler with a 5V / 240V relay
paulca:
I want to use an ESP32 to switch my oil boiler on/off.
From the wiring diagram of the heating controller it simply switches the mains to the boiler spur in the garage. Boiler is 240V.
From the various datasheets and manuals I can find for the boiler the start up currents for the various motors is quite low, in the order of a few amps. In one I found "Electrical charateristics under extreme conditions" which listed the maximum current draw of 3A. The burner unit has a time delay 6A fuse.
Given this I should be fine placing my opto-isolated relay in series with the controller as is suggested for wiring room thermostats.
Can anyone see a problem with this?
Personally I'd like to measure the startup current, but I see problems. It's mains and AC so the only thing I have that would register AC current is my multi-meter and I don't fancy putting that in series in the mains. Also, even with that the startup current, most likely for the draught fan or fuel pump might be a short spike and missed by the meter.
paulca:
I am thinking of switching with one of these:
https://www.ebay.co.uk/itm/3-3V-5V-10A-Relay-Module-for-Arduino-Nano-Due-Raspberry-Pi-ESP8266-Optocoupler/272272819712?epid=4021131680&hash=item3f64b98600:g:3W4AAOSwMNxXXo16
capt bullshot:
Basically, there's two or three issues to consider:
- does the relay board provide sufficient isolating space between mains related circuit and your EPS32 circuit - can't tell from the ebay pictures
- can the relay stand the normal operational current of the boiler - apparently yes
- what does the inrush current of the boiler look like and will the relay stand this?
Especially the third one can cause some headaches: For example, an ordinary SMPS (e.g. computer power supply or laptop brick) has a nominal current widely covered by the relay rating, but the relay contacts weld (to a continuous short) after a few cycles. This is due to the inrush current of these supplies, charging their input bulk capacitor. Up to a certain size, these inrush are worse than motors up to a certain size. So you should go and check your boiler for any signs of SMPS units. If it's only the motors and no VFD or other stuff, you're probably fine.
paulca:
I looked at a represetative thermostat device:
https://www.plumbworld.co.uk/documents/1-honeywell-dt90e-digital-room-thermostat-data-sheet.pdf
It seems they rate for up to 8A, but they do note for higher than 3A they have a high energy mode on the relay. Given my relay power will come from an ESP32 I will need to check the datasheet for the relay I use to check it's coil power requirments to ensure it can break whatever the boiler pulls.
On inrush current, all I did was check through any manual or datasheets I could find. None seemed to suggest high inrush currents. The burner unit is mostly mechanical with a small electronics control box. I had said control box replaced this morning, so I can open it and look for any capacitors.
The boiler does have a 4uF capacitor, which I believe is for the HV electrode transformer (to create the ignition spark).
capt bullshot:
--- Quote ---Electric heat operation: If a direct electric heating application > 3A switching current is required, DT90 will use more energy to operate its relay and so ensure the higher current is switched reliably.
--- End quote ---
Interesting setting and interesting description: One could figure they're underpowering the relay on purpose, knowing this might fail at nominal load so they provide this setting to drive the relay at its nominal rating. Looks kind of dodgy to me without knowing the details behind.
Anyway, as long as you use the relay at its nominal ratings (coil and contact), there's no need to increase the drive power. In case of SMPS inrush, this wouldn't save the relay contacts from welding.
BTW: one can do quite a lot of interesting "tricks" with relays, regarding the power required to close the contact and to hold the contact closed, and to reduce the SMPS inrush current (by closing the contact near voltage zero-crossing, but that's real tricky and could be counter-productive if your load is a transformer or motor).
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