On 11/27/2020 02:55 PM, Caecilius wrote:
I've got a large house with gas central heating which I use during the
day. It gets cold overnight in the bedroom though, so I use a 2KW
electric convection heater with the thermostat set to about 18 C on a
timeclock which switches on between midnight and six am (boiler comes
on again at six).

This was a great improvement last winter, and it only uses one or two
KWh overnight which must be cheaper than heating the whole house with
gas.

This year I want to improve the solution to keep the temperature more
constant (there's a big hysteresis on the convection heater
thermostat) and remove the bimetallic thermostat clicking noise.

I think I want electronic switching to remove the clicking noise and
allow a lower hysteresis plus an external temperature sensor. Maybe
something that's closer to a temperature controlled dimmer switch
instead of a standard bimetallic thermostat.

My plan is to put this temperature controller between the timeclock
and the heater, and set the heater thermostat to something like 25C as
a fail-safe.

Has anyone found or built something like this? I'm sure I can't be
the only person who want a constant overnight bedroom temperature
without running the main house heating.

Have a look at triac datasheets.
For the 2kw heater you'll need a rather large one. (18 amps? BTA20 maybe)
and a heatsink.
Not all triacs are insulated,

as to how to drive them... you'll need some power supply, xfrmr or resistive.
and generate pulses or pulse train with correct timing.
There are application notes.

Microchip AN958
Low-Cost Electric Range Control Using a Triac
(very much what you want)

ST has a lot of appnotes. search their website.

On semi
AN1048/D (Snubber networks)
HBD855/D (Thyristor Theory and Design Considerations) Handbook, large
There's a zero point switch shown on pg 137. supposedly there's IC versions.


OTOH
There are perfectly fine thermostats and heaters available
wot utilize triacs. maybe get one of those.