Microwave Incubator Build

Step 4. Thermostat, lights, and fan

I've already documented the electronic thermostat that I built for my little polystyrene forced air 'bator. My original plan was to borrow it, along with the 12 V 10 W halogen lamps, light fitting, and fan, and put the whole lot in in the microwave incubator.

A few things made me reconsider. For neatness and safety I wanted to mount all the electronics inside the microwave, but I didn't want to permanently rehouse the thermostat because I might want it for some other purpose (for example, to use in a brooder). Also, by taking it out of its box I would lose the calibration settings. And in any case the other incubator might come in useful as a hatcher. So I decided to start again from scratch.

One issue with using halogen bulbs for heating is that they do burn out without warning, and do so quicker than usual if they are turning on and off a lot. Another potential problem is that the radiant heat from the bulbs does not reach all parts of the incubator equally, causing hot spots and shadow regions. Heating tape does not burn out and heats more evenly, but I didn't want to use mains voltage for the heating. Apparently you can buy 12 V heating pads to plug into your automobile cigarette lighter, but the ones that I looked into were needlessly expensive and had very large power ratings. I wanted a cheap, reliable solution: how about running 12 V through a big fat resistor? That would generate heat. Better still, I could use the body of the microwave as a massive heat sink to spread the warmth around.

So I bought myself a chunky aluminium-clad resistor rated 6.8 Ω / 50 W that could be fixed directly onto the metal ceiling of the oven. These are sold in automotive outlets as "12 V load resistors" or "ballast resistors" and are intended for use in conjunction with LED indicator lights and fog lamps. Passing 12 V through a resistance of 6.8 Ω would generate 21 W or so of heat, the same as two halogen bulbs. I chose a resistor with a considerably higher power rating than that. It was going to be in continuous operation for 3 weeks, so I didn't want to be pushing it to the limits.

A consequence of using this different heating arrangement was that I would need a different design of thermostat, too. The regulator I used for the halogen lamps was a so-called "bang" thermostat: it cycled on and off with no in-between state. The usual problem with this kind of thermostat is hysteresis -- that is, you have to set the turn-off temperature to be higher than the turn-on temperature, resulting in temperature swings. In my forced air incubator it was not too much of a problem because the lamps were mainly transmitting heat to the circulating air, which had very low thermal mass. The eggs in the incubator had higher thermal mass, and so their temperature remained steady while the air temperature was cycling up and down. But if I was going to heat up the whole of the microwave oven, several kilograms of metal, hysteresis would most definitely be a problem. The large thermal mass of the oven would heat up slowly, and would continue to impart heat to the eggs inside long after the thermostat had shut off. There would likely be a big overshoot in temperature on the upswing, and a similar undershoot on the way down.

The large thermal mass of the oven, and consequent temperature lag, would require a controller that could turn the heat full on when the temperature was low and progressively reduce the power as the temperature approached the target level. This sort of "proportional" control is impossible to achieve with halogen lamps, which cannot be dimmed without reducing their life span considerably. But it is perfectly feasible to cycle a resistor on and off with no such consequences. During each cycle, the length of time that the power is on may be different from the length of time that the power is off. The proportion of time that the power is on is called the duty cycle: when there is no heat applied the duty cycle is 0%, when the power is full on the duty cycle is 100%, and when the power is on exactly half the time the duty cycle is 50%. The amount of heat that is dissipated by the resistor is proportional to the duty cycle. I used a kind of thermostat called a "PID controller" to vary the duty cycle. If it is well-calibrated, it should turn the heat up and down by just enough to reach the target temperature without overshooting.
With the thermostat mounted on top of the microwave, I fed the output wires to the lights as in the picture below. I also routed the temperature sensor to the interior where the eggs would be. I connected the 12 VDC power line to the thermostat, fan, and timer circuit in parallel.

Parts list:

Watlow 965 series PID controller
12 VDC 3A power supply
Aluminium clad resistor 6.8 Ω / 50 W
12 VDC 0.5A CPU fan

I also needed:

Soldering iron (25 W)
Solder (multicore 0.7mm)
Wire stripper/cutter
Hook up wire (the wire connected to the heating resistor is taking a couple of amps so it can't be the really thin sort)

Heatsink compound for mounting the resistor (zinc oxide diaper cream will work just as well)

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