Kyle asks:

I’m just getting in to the world of homebrewing my own beer. My basement is a little too cold for proper fermentation, so I’ve constructed a simple box out of foam insulation and I want to add a small heat source.

I have sensors measuring the temperature of the fermenter and the ambient air in the box. I’d like to use an Arduino to turn the heat source on and off. My plan was to buy a small space heater (Lasko My Heat ceramic heater for example), and use my Powerswitch Tail to turn it on and off. I’m worried about the life of the Powerswitch Tail, since the heater isn’t a pure resistive load (it has a small fan motor).

Should I just use a a 40-60 W incandescent bulb instead? I remember seeing an app note about how an incandescent load needs to be derated even more than an inductive load. What’s the best way to safely add a controllable heat source that won’t result in me having to replace my relay switch too often?

Mmmmmm…..theres nothing better then brewing your own and I will see if I can tackle all of your questions.

First off, you are absolutely right in questioning the type of load you are putting on your Powerswitch Tail. Lady Ada has referenced a good document that describes the different types of loads commonly connected to relays and why they can affect the overall lifespan of the relay. The document describes 5 different load types, and as you had suggested, your system would be working with either incandescent or motor+incandescent loads.

As the document describes, because the filament changes resistance based on temperature, the contacts on the relay are more prone to wear due to the high inrush current when the filament is cold. The document suggests de-rating your relay by 10% or using a series resistor to current limit the load. This would apply to both your heater and your bulb ideas and your 1800W Powerswitch Tail is now capable of safely switching 1,620W.

Your space heater also has a fan, which would be classified as a motor load (although the fan is probably pretty small). Like the inconsistent incandescent load, there is a large inrush current when starting the motor. This is usually taken care of by attaching a starting capacitor to AC motors. The result is similar to the incandescent and requires derating your relay by ~20% and your 1800W Powerswitch Tail is now capable of safely switching 1,440W….which is a little low for a space heater.

Another concern with the Powerswitch Tail as a heater control is its life expectency. Adafruit states that the Tail should be able to switch 100,000 times with a 15A resistive load. As an example, if you system constantly switches your heater on and off every 30 seconds:

30s * 100,000cycles = 3,000,000s of operation
3,000,000s / 60 / 60 / 24 = 34.7 days of operation

Depending on how long you plan on fermenting and how accurately you want to maintain temp, this might be a bit low. You might want to look at some more industrial controls or solid state relay‘s to do the job.

Also, an alternative to the lightbulb/space heater idea would be to use a silicon heater blanket. I used to work in a lab and we used these all the time to maintain temp on bio-reactors. They are much better at distributing heat then point sources like the bulb/space heater (also are a lot more safe due to their max temperature!) This, in addition to a SSR and a thermal switch, would be a pretty stable system.

I hope this has helped answer your question and good luck with your brew!

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“Ask an Educator” questions are answered by Adam Kemp, a high school teacher who has been teaching courses in Energy Systems, Systems Engineering, Robotics and Prototyping since 2005.