Controlling Resistance Heaters - General Electronics

Grumpy_Mike:

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but theres not too much running aroudn in these forums about 120VAC electronics.

That is because playing with mains when you don't know what you are doing is dangerous and can be fatal. I am surprised a teacher has given you this to do with so little knowledge of electricity. I would first check that your teacher is fully insured for when your parents try and sue over your death.

I would use a SSR solid state relay and turn it on and off at regular intervals to control the power. Get a relay that is rated at a current at least twice that you need to switch. The ratio of the on time to the off time determines the power. The code looks exactly the same as the code to flash an LED.

Perhaps I should note. This isn't an actual project. Its one of those kind of, end of semester. "Here go with this info and figure out how to do something like this". And get some xtra credit for doing a short essay about how you would do it. On of those deals. My teacher loves doing them. SO i figured I'd ask here, then over the weekend go find the guy who did my house's wiring and harass him for an hour. Basically what i had already was a relay controlled off of a switch, to turn the "heating units" off and on. But as far as temperature controlling. I understand its 1 thing to switch them on and off at certain setopints to keep the temp near constant. But with adjusting their output like a dimmer for lights. I'm sure there's more going on in a dimmer switch for lights in your house then just a potentiometer, so curiosity got the better of me and I didn't feel like waiting to ask the electrician so i threw it up here.

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Gonna go up into the first statement and edit it stating its not actually something to build, shoulda probably mentioned it in the first place. However as it does stand. It seems a bit easier in the forums to get responses when asking about something that's over your head then for hypothetical projects.

Hi,

Sorry for the newbie questions.

Say I have a hard (fixed) requirement of 48V power, up to 20A, it must be DC.

Similar to a hairdryer I want to create hot air, assume the fan speed is independently controlled.

There would be a microcontroller and a PID.

Questions:

How do you design a resistive heater to use 960W?

As you quoted 960W with the given voltage & current, you already know
P=VI.

Knowing the needed voltage & current, you can derive the resistance.
R=V/I
In this case, 48/20 = 4.8 ohms.

960 watt is a serious amount of power, so that 4.8 ohms needs to be pretty rugged, or it will "have a short life but a happy one."

There are almost certainly ways of deriving the specs for such an element, but I would suggest looking at the elements in devices that already have ratings of around 1kw.
Perhaps elements in parallel rather than series, or something like that.

Quote

Does equal wattage of AC or DC in to a resistive heater result in the same temperature?

Yes, this is why we use RMS values for AC, rather than the more intuitive Average.
48v RMS AC will produce the same power dissipation in the element as 48v DC.

Quote

How do you best switch the heater on/off (960W at 100% duty cycle, that would be an issue for MOSFET because it would have to dissipate that)?

The switching device doesn't dissipate the 960W, its dissipation is entirely due to its own resistance.
If you had to dissipate the entire devices operating power, if you switched off a bar heater, you would vaporise the switch on the GPO it is plugged into.

It isn't a matter of power, it is really the current handling ability of the switching device.
Electric kettles used to (maybe still do ) use bi-metal switches, but,of course, they are operating at full Mains voltage, so the current is a lot lower.

If the 48v is ultimately derived from the Mains, I suggest doing the switching there, or perhaps as a function of a SMPS.

Quote

Is there a way you can work out the maximum temperature of the air without building it, I guess it's a function of heater and fan duty cycle (lowest fan speed plus the maximum heater power)?

Thanks.

Richard

You could measure the air temp of a similarly rated Mains operated heater, but, depending on how you organise your 4.8ohms, the elements may not be all exposed to the air in the same way as the comparison device.

As you quoted 960W with the given voltage & current, you already knowP=VI.Knowing the needed voltage & current, you can derive the resistance.R=V/IIn this case, 48/20 = 4.8 ohms.960 watt is a serious amount of power, so that 4.8 ohms needs to be pretty rugged, or it will "have a short life but a happy one."There are almost certainly ways of deriving the specs for such an element, but I would suggest looking at the elements in devices that already have ratings of around 1kw.Perhaps elements in parallel rather than series, or something like that.Yes, this is why we use RMS values for AC, rather than the more intuitive Average.48v RMS AC will produce the same power dissipation in the element as 48v DC.The switching device doesn't dissipate the 960W, its dissipation is entirely due to its own resistance.If you had to dissipate the entire devices operating power, if you switched off a bar heater, you would vaporise the switch on the GPO it is plugged into.It isn't a matter of power, it is really the current handling ability of the switching device.Electric kettles used to (maybe still do ) use bi-metal switches, but,of course, they are operating at full Mains voltage, so the current is a lot lower.If the 48v is ultimately derived from the Mains, I suggest doing the switching there, or perhaps as a function of a SMPS.You could measure the air temp of a similarly rated Mains operated heater, but, depending on how you organise your 4.8ohms, the elements may not be all exposed to the air in the same way as the comparison device.

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