Modulating thermostat for electric heater???

[Gazz]

Does such a thing exist,

I am making a chick brooder, the heating will be a 25 watt heat plate above the nest,

But as i will be brooding song thrush and blackbird chicks (from my aviary) i need pretty tight temperature control, as the chicks hatch featherless and for the first 7 days unable to do much more than stick their head up and gape for food, then put their bums in the air and shit.

So i want something a little better than a bang - bang type thermostat (maybe with a 0.1 degree hysteresis it could work) but what i'd rather do is have the thermostat adjust the power going to the heater to keep the temperature at a steady 37.5 degrees C. an analogue thermostat if you will.

Anyone know of such a thing? ideally one that doesnt cost silly money as it'll only be running a very low resistive load.

[Davey]

Is this legal? Just wondering.

[Bob Minchin]

You can get PID temp controller modules on ebay pretty cheaply these days just need to add a thermocouple and a solid state relay for your heater.

[John Rumm]

Sounds like PWM control of the heater is what you need... Raspberry Pi or similar with an I2C temperature sensor, and the PWM output controlling a suitable power transistor. Then a simple bit of code to drive the PWM in response to the temperature reading. You would probably need proportional plus integral control to allow for varying rates of heat loss.

[Chris Hogg]

You want something like a PID* three-term electronic thermostat. They're certainly available, but not cheap, I suspect.

*Proportional Integral Differential. The electronics sense how close the temperature is to your chosen set point, and adjusts the power input to the heater accordingly (varying mark-space ratio, sort of thing). Once it's settled down, very little over- or under-shoot. I know of them from pottery kiln controllers, but a bit above your temperature range!

[nick]

Sounds like a burst-fire type of system is what you need, below x on full time as you approach target the duty cycle of the heater decreases I seem to remember making one ages ago with a U217 chip.

HTH

Avpx

[Chris Hogg]

I've just looked on ebay. I was amazed how cheap they are. Used to be up in the £100's thirty years ago!

[nick]

Also

ftp://ftp.sic.rm.cnr.it/incoming/ifa.rm.cnr.it/Maurizio.Viterbini/Public/ termostatiTRIAC/thermostat-gb.htm,

[Bob Minchin]

Yes it is quite amazing what sophisticated stuff you can find for next to no money including being posted halfway round the world too.

[ARW]

What is the alternative? Release the eggs into the wild?

[Gazz]

Don't worry, i am totally legal,

the song thrushes are standard ones as you see in the wild, but they are properly closed rung and i have proof of their parentage all being bred in captivity, plus DNA sexing certificates (which could be used to match them up to their parents aparantly)

The blackbirds are a silver/blue mutation you do not get in the wild, again properly closed rung with proof of their captive breeding going back generations.

It's just that the blackbirds are on their first breeding year, and have lost 2 clutches so far thanks to the hen deserting the nest, The songies have just been a pain in the arse, one hen i discovered is an egg eater... stupid thing lays it then eats it, but there is a short window of time between these 2 actions, so i may be able to get the next clutch of eggs away from her an in my incubator, to hand rear them if they hatch... hence the need for a temperature stable brooder,

Most brooders are sold for chickens and similar species that hatch self supporting, so just provide heat and it's upto the chicks to move if they are too hot or cold,

[harryagain]

Such tight control is hardly neccesary. Think what happens in the wild.

[Andrew Gabriel]

Interesting - I do remember in biology lessons, we hatched chickens using a biscuit tin with a 25W light bulb mounted in it, IIRC. I don't recall any thermostat. OTOH, only about 2 out of 12 eggs actually hatched. The others were probably scrambled.

[Gazz]

oh yes i know that, but in the wild the hen would sit on her chicks for a few hours, where the temp would be a steady 37.5 degrees, only lifting up to feed them (which the c*ck brings them the food for the first few days)

what i don't want is the temp in the brooder swinging from say 36 to 39 degrees every few minutes as a simple thermostat tries to keep the temp stable by turning on and off, i'd rather keep the temp at a steady setting... failure rate is high when hand rearing this kind of chick, so i want to get the temperature just right to take away one thing that could bugger things up.

[Mathew Newton]

If you don't get anywhere with an 'analogue' stat then something like Honeywell's DT90E might well work well:

With its TPI-based control logic it is able to minimise over/undershoot much better than a simple stat.

Mathew

[Gazz]

This mention of PID's and the like made me look them up, and when i saw the photo's of them, i remembered i had one in my 'doubt i'll ever use it, but will keep it just in case' pile in the garage,

I stripped a fridge/incubator unit down years ago, ex's dad gave it me non working, firm he worked for made them for hospitals, i imagine either for meds storage or growing cultures or something,

The main controller was a 'jumo itron 32', which looks to be a pretty sophisticated PID thingy, the instructions online to program it are rather confusing, but it looks to do exactly what i want and then some. it uses a 3 wire temp probe, and controlles an SSR to switch the heating elements,

am i right in thinking this could do exactly what i want, the SSR allowing the heater to be turned on and off rapidly in a sort of PWM fashion.

There is also a 'ELREHA EAT 100 K002' temperature controller, which operated an over temp buzzer, i think that is just a basic thermostat, you push a button to light the display, turn a knob to set the temp, then the display goes dark, only lighting up when the temp set is exceeded (and activating the relay)

so i think i will try and figure out how to use the Jumo PID, and hopefully get some songies and blackies to fledging stage this year (as the last 3 clutches have all failed due to the parents fannying about)

[The Other Mike]

Yes they will probably do the job and will be built like a brick shithouse but being 'old' there could be problems with aged internal components and the temperature sensor could possibly be more optimal for a higher temperature.

Ebay 281294199646 - PID Temperature Controller Control with thermocouple/SSR-25DA solid state relay

12 quid with free postage from China

Almost certainly a knockoff product of the Japanese original so the instructions will be non existent or in chinglish but the original manufacturers instructions can be downloaded here

[Roger Mills]

As others have suggested, you need a PID controller (or PI at any rate - the derivative function may not be needed).

I was using PID controllers in the 1970's to control automotive test rigs (throttle actuators, dynamometer loads, etc.) - but these were analog, and the output was a variable voltage rather than a switching function. This sort would be best suited to your application if you can find a suitable device to vary the current fed to the heater in response to the controller's output. That way, you would have true modulation - with the heater on all the time and would just control the amount of heat it produced.

If you want to go digital, you could use a digital thermostat intended for controlling central heating systems. But that's not ideal in my view. Firstly the heating element may not like being cycled on and off rapidly, and this may shorten its life considerably. Secondly - depending on the time constants of your setup - the minimum on and off times built into the firmware may be too long, and you may still get overshoot. [When you're heating something with the thermal inertia of a house, these controllers can hold the temperature rock steady - but I'm not too sure about heating a small space. However, since you're only talking about 25 watts, you may be ok, but you'd need to suck it and see.]