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I'm building a hot wire setup to do various hobby projects. I've got most of it figured out already, thanks to the wealth of information there is out there. I have a 12V switching power supply with a max current of 10A to heat the wire. One thing I'm trying to work out is fine tuning the wire temperature. I tried simply putting a pot in the circuit after the power supply, before the wire, to essentially modify the power supply's output voltage. Predictably, though, the pot got hot. Really hot. Obviously, I'm not happy with that as a solution, since burning my fingers while trying to adjust temperature doesn't sound appealing.

My next thought is to use PWM to control the temp, and, again, there is some information out there to support this. However, I'd prefer to use a very simple controller. I've already built a small motor controller that uses PWM, the schematic provided by this site: http://makezine.com/projects/the-dial-a-speed/. I'd be concerned that this wouldn't handle the kind of current I'm pushing through the wire (1.5-2.25A by rough calculations), however. Plus, if I want to use a higher voltage with the wire, it would almost definitely fry the controller. Could I control a relay with PWM that in turn shuts power on and off to the wire? Or does someone have a much better idea for me to look into? To preface, I'm a mechanical engineer, so I have a basic knowledge of circuits, but the simpler the better. Thanks for any assistance!

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  • \$\begingroup\$ If you are happy with a manual (i.e. by feel or eye) temperature adjustment, this circuit it is fine. To use a relay controlled by a PWM signal it is not reccomended and never used. What you can do is to use a diferent power source for the MOSFET drain. \$\endgroup\$ – GR Tech Feb 16 '15 at 8:06
  • \$\begingroup\$ Thanks! I generally adjust to cutting feel, so that will work fine. Adding a temperature sensor will be an advanced project for when I get bored. \$\endgroup\$ – dboston Feb 17 '15 at 18:11
  • \$\begingroup\$ A simple solution I used when stuck with a fixed voltage supply was to make the wire longer than required so that it ran a little cold. Moving the connection along the wire made the resistance, and hence the current, adjustable. \$\endgroup\$ – Transistor Apr 3 '16 at 20:55
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The schematic you posted (dial a speed) is suitable for your needs. The stated FET(IRF510) tolerates 100V 4A, so it could handle your hot wire with proper driving/cooling. If needed you can find many alternative FETs with lower voltage rating and higher current rating really cheap.

A relay would theoretically work for driving a low frequency PWM, maybe less than 1Hz (I believe you dont need high frequency). However in practice it is never done and I would avoid that route, especially if you have already built FET PWM controllers.

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  • \$\begingroup\$ Actualy I would use another FET. IRF510 has a Rdson of 0.5R at 10V, you can easily find FETs with lower gate threshold and lower on resistance, improving your circuits efficiency. \$\endgroup\$ – Wesley Lee Feb 16 '15 at 8:07
  • \$\begingroup\$ Thanks for the advice. I'm somewhat familiar with the notion of FETs and gate threshold, but would you mind explaining what I'm looking for in terms of parameters for an ideal FET for this kind of application (for instance, I don't really know what Rdson refers to)? Or have any recommendations for specific ones? \$\endgroup\$ – dboston Feb 17 '15 at 18:15
  • \$\begingroup\$ Rdson = Resistance between drain and source while the device is on. The lower the less power dissipation. You need low Vgs to turn it on with logic levels, but you also need low input capacitance so you can turn it on fast. The higher the current rating for the fet, the higher the input capacitance.. When the fet is not fully on (you havent "filled" its input cap), its internal resistance is really high, so it dissipates a lot of heat. (wont matter if Rdson is low if you spend a lot of time between on/off). Quick search on Mouser gave these: bit.ly/17PKcbG \$\endgroup\$ – Wesley Lee Feb 18 '15 at 2:03

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