Single Shot heater control
There may be times where you need single shot control for a heater. The principle being that you can start the heating with the push of a button, and it will then run until a set temperature is reached, before shutting off completely until the next time it is manually started.
- A mains double pole relay, with a 230V coil, and contacts rated for a load of at least 13A
- A momentary action or "Push to Make" press switch
- A normal "strap on" cylinder thermostat.
Principle of operation
Pushing the spring return push to make "on" switch energises the relay which closes its contacts This powers the heater.
A feedback is taken from the relay's switched live output back to the input of the the second set of contacts. The output from this set is connected via the normally closed contacts of a cylinder stat to the coil of the relay. This is used to "hold on" the relay, allowing the heater to remain on after the push to make on switch is released.
The system will now stay in the heating state until the cylinder stat is satisfied, and opens its contacts. This will de-energise the coil, turning everything off.
Feeding the cylinder stat via a second set of contacts makes sure that there is no time where the heater could attempt to draw a high current through the contacts of either the on switch or the cylinder stat.
Note that earth connections are omitted from the diagram for clarity, but should be included. Mains filter networks could also be included across the relay coil to reduce mains interference created by switching inductive coils. See this article on using relay controls
Note most immersion heaters include their own thermostat, it is better to leave this with direct control of the element, and rely on the external cylinder stat to control the "single shot" system. That will allow the internal immersion stat to cycle if required (say when operating with a scaled up heating element), without tripping off the heating cycle before the whole cylinder is at the desired set temperature.
IMPORTANT: The temperature on the internal immersion thermostat must to be set higher than that set on the cylinder thermostat, otherwise the circuit will never be able to reach the selected final temperature, and will never turn off.