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PWM'ing a 250W heater.

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Terry Bites:
A bit over complex for an incubator heater. Make your own or buy a bang-bang contoller with a narrower hysterisis band.
Home heating thermostats eg  BYC07HE can be set to 0.5'C hysterisis. Cheap as chips. Nice green glow as well!

Medved:
The problem is the long response delay of the bimetal switch vs the temperature in the chamber changing rather fast, making the simple hysteretic cintrol to over- and under-shoot a lot.
Quite nice trick: Add some extra small heater directly onto the bimetal and set it so, its heating power is barely able to overcome the bimetal hysteresis That way you will essentially form an electro-thermo-mechanical PWM proportional regulator, generating rather nice PWM with few minutes period. The gain of this is then dictated by how muc the auxiliary resistor warms up the strip beyond the hysteresis. This trick was often used in room heating thermostats in the past...

But for your case the ready made PID controller should be the best choice. It has its advantage to display the actual temperature there, so you will see it is off (e.g. due to heater fault,...) and be able to rectify the problem before it causes consequent damage.

But don't use any of the hysteretic ones, even with the hysteresis adjustable to zero. Those will over-/under-shoot the same as the bimetallic does, so you will end up the same as you already are.

paulca:
Was just coming back to say I bought a smart 4 plug power bar.  With individual Wifi control over the plugs, so I can control the heater and the extractor fan from software.

I just stuck in 1*C natural hysteresis and it toggles every few minutes.  Saw tooths with an amplitude of almost exactly 1*C.

It don't get more simple.
            if temp < 19:
                self.extractor("OFF") # if it gets really cold and heater can't keep up
            if temp > 20:
                self.extractor("ON")
            if temp < 20:
                self.heater("ON")
            if temp > 21:
                self.heater("OFF")

The plants on the other hand may be beyond saving at this stage.  When I sort of found the extractor and the heater on 100% of the time would balance out around 24-26*C I left it there.  The humidity was 40-70%... it didn't twig that once I stabilsed the temp at 25'ish the humidity would go down and stay down... and keep going down.

I added a humidifier to keep it at 75% and ... well, it rained in the propagator, thankfully just down the walls, so I dropped it back to 65% and 20-21*C.

Maybe they will recover, they have got fresh green growth nodes currently being protected by the seedling leaves, but, they aren't meant to be curled up like that and they are very, very slow.

I knew it would turn into another "hobby" and explode.  The propogator now includes:
A 240V desk fan, on 24/7
A 250W heater, switched automatically
A 5 litre humidifier set to 65% on 24/7
A 12W heat mat for the base, to warm it against the cold floor below (switched manually but remotely)
Extractor fan, switch automatically, on most times, off if the heater can't keep up.


paulca:
The Smart Power bar did valiantly defend it's self from a few hacks, but it was game over when I soldered the serial lines to the board.  Bye bye, Chinese firmware

beanflying:
Nice life hack  :-+

Generally most environmental heating applications don't need the fast switching/pwm control. My coffee Roasters run in the order of seconds for switching even though they are following a fairly tight temperature ramp, moving the both the thermal mass of them and the air inside significantly takes 20-60 seconds not matter what you do even with gross airflow changes.

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