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I'm building a space heater, not for continuous use, but just as an experiment.

I made a nichrome wire coil that's 3.5 Ω and a fan that blows on it so hot air comes out of the other side of the tube. Everything is enclosed and both electrically and thermally isolated.

For power, the choice was to use a power brick I had salvaged from a junkyard that would output 40 V with 20 A capacity hoping to draw 460 W, but before I could use it, I tested it and it's a dead power brick.

This part is completely hypothetical.

What if I take 240 VAC mains and connect a diode with the heater coil to cut the current?

With the fan blowing on it, and the resistance will also go up as the coils heat, what would be the hypothetical current draw?

My ideal current draw would be 500-800 W. What are my options apart from buying an expensive power brick that's 800 W?

ocrdu
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zan13898
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    Fixing the dead power supply would probably be easiest if you don't want to buy a new power supply. Though that depends on what's wrong with the dead one. – Hearth Nov 23 '22 at 04:43
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    Are you aware of the basic calculations like Ohm's Law and the derived power equations (P= V*2/R , P=I^2 R) ? That's your starting point. The final temperature and thus resistance is going to depend on the particular setup. – Ian Bland Nov 23 '22 at 07:34
  • nichrome wire has a very low thermal coefficient, the resistance will go up maybe 5% by the time it's orange hot. – Jasen Слава Україні Nov 23 '22 at 10:55

4 Answers4

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240VAC (that's rms equivalent) ^2 / 3.5ohms / 2 (for the diode) = 8.3kW!

The current draw may go down a bit as the coils heat up, but not by a factor of 10.

I recommend you redesign the heater to have a much higher resistance.

Drew
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240 VAC into 3.5 ohms is 68 amps, and 16 kW. If you use an SCR controller that can handle that current, and your 240V mains can handle that peak current, you could possibly apply a 5% duty cycle with 1 cycle ON and 20 cycles OFF. This would give you the 800 watts average you want. Of course your heating element would also need to withstand the electrical and mechanical strain of such high current and power. A more practical solution might be a phase modulated SCR controller, although running at 5% may be rather "touchy" and unstable. Perhaps a 25% phase firing for 4 kW bursts, with 5 cycles ON and 20 cycles OFF, would be more manageable.

Using a diode to get half power is not recommended, as it will cause a net DC current in the mains and a lot of distortion which may adversely affect other devices on the line, and may result in an unpleasant visit by the power company.

The most practical solution, outside of making a proper 800W heating element with 72 ohms resistance, would be a 50V 15A transformer, which would give you 714 watts. 48V and 24V transformers are pretty common on the surplus/scrap market.

PStechPaul
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My ideal current draw would be 500-800 W. What are my options apart from buying an expensive power brick that's 800 W?

Buy more cheap nichrome wire (or canibalise an old hairdryer etc). make a bigger resistor.

Jasen Слава Україні
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An assembly consisting of: resistive wire mounted on heat resistant support, fan, air duct, 2-3 speed switch, power cord is commonly sold under the name "hair dryer".

Besides that, as the others have said, with a 3.5 ohm resistor, there's not much you can do because the current will be huge. If you use a dimmer that lets through whole cycles once in a while it will have to be designed for the current (thus a large triac) and produce large current peaks. If you use a phase cut dimmer you'll have to set it to a very low duty cycle which is not always practical.

In other words, a common hairdryer will most likely be much less expensive and safer than a DIY solution.

If you need hotter temperatures at the cost of lower airflow, a heat gun from the hardware store should do nicely.

In both cases, if you require it, you should be able to use a triac dimmer on the heating element. Since its resistance value is designed for mains voltage, you will be able to use the whole range of the dimmer, which is much more practical.

bobflux
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