PWM and fermentation heater

Hey guys. Question about PWMs and lightbulbs for my fermentation fridge. This being the winter, I’m having a hard time maintaining warm temps in my fermentation fridge, which is located in an uninsulated garage. My 42W light bulb can’t keep the vessels warm. I’m experimenting with some other heat sources. One of them is 250W Infrared heat lamp. The problem with this is it gets so hot that it can melt the plastic inside the fridge if I’m not careful. I currently have the PWM on the heater set to a period of 10ms with a maximum power constraint of 70%. Is this an ok application for PWM and such a high wattage bulb? Is there a risk that I’m not considering when using such a high frequency PWM with a 250W bulb? I was trying to get the same effect has having the bulb on a dimmer, but it appears that 100Hz is the minimum frequency available.

10ms period for a heater operating on AC is too short. 1 AC cycle (minimum resolution):
50Hz = 20ms
60Hz = 16.7ms

I think the duty cycle number it 12bit so
4069 x 20ms (50Hz) = 81.9seconds
4096 x 16.7ms (60Hz) = 68.3seconds

so about 1minute period is better. @Elco correct me if my logic is totally off?

As a side note I like these heaters (works pretty well with my small fridge with 30L fermenter):

Thanks. I’m using a lightbulb, so as far as I understand it, the period is just the amount of time the light is either on or off. With a very short (i.e. 10 ms) period, the lightbulb appears dim (when <100%), but visibly flickers. When I have it at a 1s or 5s period, it flashes on and off briefly. Both of these configurations seem to work, but my understanding of circuitry is cursory, so I’m just trying to get some info on whether I’m doing something that’s either bad for my equipment or dangerous.

Most SSRs switch on an AC zero crossing to minimize high inrush currents.
Those ac crossings happen 50 times per second. So your PWM period should be at least 2 seconds.

I think for an infrared bulb, 10-20 seconds would be great.

If your period is 1s, and you have a duty setting of 80%, then your element spends 800ms on, and then 200ms off (or tries to - periods can be stretched for various reasons).

I’m not really qualified to weigh in on the electrical side of things, but if your solution is liable to melt your fridge, it seems like you may want to go for plan B anyway.

What is rate of change difference between no heating and with the 42W bulb 100% on?

Plan C is to stop trying to do kveik beers in the winter. I’m the only idiot who doesn’t pay attention to the seasons and tries to ferment at 30C when it’s 0C outside.

Actually, I have some settings dialed-in currently, and the PID curves are attenuating nicely and converging on a constant output of about 20%. I’m not worried about that melting my fridge unless the period were very long. 20% of 250W is 50W, so I’d imagine that if I had the small bulb on at 100%, it would struggle to keep up and the temps would start to drift downward. I’m sure the actual physics are more complicated than that, but it’s an approximation…

My take on this is that if it is stuck at 100%, nothing bad should happen. Safety should not depend on software.
For a fridge, stay below 100W for heating.

Hi Aaron,
You should be aware of the fridge cooling compressor does not necessarily works well when the ambient temperature is below 15°C. I have installed a fan below the heating element to ensure uniform temperature distribution around the Fermzilla during fermentation. The fan only runs when the heating element is turned on.

BR Ken