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Author Topic: DC vs AC heating elements  (Read 1012 times)
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Hello everyone,

I would like to control a kW sized heating element for mashing beer. I've had success with arduino and relays to regulate fridge temperature and now I'm looking to do something a little more complex.

It seems to me that large heating elements require AC. Why is that? Why is not possible or best practice to use DC? It would be alot easier to control with PWM if it were DC. Is there an upper limit where DC is not practicable?
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They don't "require" AC but considering the mains supply is AC and that AC relays/contactor arcing contacts survive better than DC contacts, it makes sense to use AC.  There is no practical reason why you cannot apply DC to your AC element, you will get the same thermal effect (watts of heat).
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It would be alot easier to control with PWM if it were DC.
No it would not.
To PWM a heater with DC requires very hefty FETs. Where as with AC it is easy to use a triac or solid state relay. In effect this is PWM control because you can simply control the number of cycles the heater is on and off. Because of the thermal lag this 50 or 60Hz PWM does not matter, it need not be any higher.
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The idea of PWM is fine for a heater, but the Arduino time base is way off. Becasue of the thermal mass of the liquid you are heating you on and off times will be much longer. By using a triac based device you will get what is called Zero Crossing Turnoff as the triac will turn off when the sine wave is close to zero volts. Reduces the amount of heat the device has to dissipate during the time when it is turning off.

You might want to look into a small temperature controller with PID built in to control the temperature. Either that or you will ruin a few batches until you get the control algorithm all figured out.
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Thanks for the feedback. I followed up that information.  It was new and useful. I'm in the information discovery stage at the moment, so everything is helpful.

I could get a PID controller kf2qd, but where's the fun in that?
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It seems to me that large heating elements require AC. Why is that? Why is not possible or best practice to use DC?

Because normal power supply,  everywhere in the world,  is AC ?

It is possible to use DC,  but before very modern electronics,  it was very hard to change the voltage level of DC,  which could be easily accomplished using transformers for AC.
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