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I want to build a thermostat and I'm not sure what power resistor to choose. The thermostat will be build inside a broken microwave oven. The resistors will be connected to 230V AC and they will be controlled by an arduino pro mini for AC PWM using a triac + other components.

I'm thinking to go for 2 or 3, 50W 1.5k ohms power resistors connected in paralel.This is how they look: https://www.tme.eu/en/details/ax50wr-1k5/50w-resistors/te-connectivity/1-1625984-9/

According to my calculations, I will have:

I = 230/1500 = 0.153A

P = I x V = 35.19W

So 35.19W is ok, given that the resistor maximum power is 50w. But in the data sheet is mentioned that without a heatsink the maximum power drops to 20W. So I don't know how the metal plate of the microwave oven would compare to a heatsink. It would be safe to use these resistors?

I want to make fermented yogurt from bacteria. So I need a temperature between 40-45C so that the bacteria wont die. Also, in the instructions, it says that in order to make ticker yogurt, it would help to keep the milk at 70C for 30 mins. That's why I'm doing it. But in the future, I might use it for other things as well.

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  • \$\begingroup\$ You can calculate it, but thermal model of oven will be very complex. IMHO, instead of earning a PhD degree in thermodynamics just mount resistors using some thermal grease and make test. \$\endgroup\$
    – Jakub Rakus
    Commented Jul 7, 2018 at 19:34
  • \$\begingroup\$ Yeah, I think that most likely they would work. When I will receive them I will test and see. I asked because I didnt't wanted to order the wrong resistors. \$\endgroup\$
    – Novak
    Commented Jul 7, 2018 at 19:56
  • \$\begingroup\$ What you're doing is a bit iffy, since you're making no effort to keep the resistors cool - rather, you want them (and the oven) to get hot. However, microwave ovens aren't well-insulated, so you may be all right. Of course, you may not get the oven very hot, too, but you haven't mentioned your target temperature - which you should have done to begin with. \$\endgroup\$
    – WhatRoughBeast
    Commented Jul 7, 2018 at 20:17
  • \$\begingroup\$ For now, the temperature I want is 45C, but in the future I might want 70-80C. But the metal plate wont act kinda like a heatsink? It will dissipate some heat, although not as efficient. \$\endgroup\$
    – Novak
    Commented Jul 7, 2018 at 20:28
  • \$\begingroup\$ There could be a significant shock hazard with this design. You need to give careful consideration to grounding strategy. Any exposed conductive surface which could possibly come into contact with line voltage must be well grounded with a low-impedance ground connection. \$\endgroup\$
    – user57037
    Commented Jul 7, 2018 at 20:48

1 Answer 1

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One of the best forms of power resistor to use for this sort of '10s of watts to modest temperatures' mains heating application is a filament lightbulb.

Wire a bulb-holder inside the space to be heated, and arrange a fan to circulate air in there. Obviously these have to be rated for your target temperature, but you get types for intended for ovens.

Consider the advantages. They're already designed for mains, and to get hot. They're self-indicating. They're inexpensive and available. The bulb holder is already insulated to mains standards from its mounting. And there's a danger of broken glass, which encourages you to be careful.

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  • \$\begingroup\$ Some incubators for chicken eggs rely on incandescent bulbs for heat. Seems to work well. \$\endgroup\$
    – user57037
    Commented Jul 8, 2018 at 6:28
  • \$\begingroup\$ I tried my circuit using a bulb. I used my arduino with a light dimmer. So I used a 100W incandescent bulb. The yogurt was fine. But I tested it in my big, normal, gas oven. As you know, in the microwave oven the space is more limited, so the risk of breaking the bulb is serious. But it could be a solution if I plan it carefully using smaller bulbs designed for oven. Would it matter if I put the bulb up, instead of down? If it is up the risk of breaking is lower. But it would affect the temerature distribution. The fan should always be powered? Or briefly switched on for a period of time? \$\endgroup\$
    – Novak
    Commented Jul 8, 2018 at 6:35
  • \$\begingroup\$ I would not think you would need a fan. \$\endgroup\$
    – user57037
    Commented Jul 8, 2018 at 6:38
  • \$\begingroup\$ @Novak it depends how uniform you want the temperature. With a fan running all the time, it will be more uniform. It might be uniform enough for your purposes without a fan, try it and see, it's obviously simpler without. \$\endgroup\$
    – Neil_UK
    Commented Jul 8, 2018 at 9:12

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