I'm trying to drive a hot wire foam cutter with a PWM signal (to modulate the heat output/temperature of the cutter).
The schematic below shows what I came up with. It seems to work (in that the heat of the wire does vary as the duty cycle of the PWM signal is changed), but my probem is the mosfet is getting extremely hot - too hot to touch (but no smoke, yet) after a few seconds.
The power supply to the hot wire is a 24 VDC SMPS, rated at 10 amps.
The hot wire cutter bow itself is a length of stainless steel wire, approx 1m long by 0.3mm dia. With 24 VDC applied to it directly, the measured current is around 2 amps.
The PWM signal is coming from a CNC controller, via the parallel port of my PC. The output voltage of the parallel port card is 5V. I've checked this with a DVM (5V when the pin is high, 0 when low). The frequency of the PWM signal is 50 Hz.
I set the value of R1 to give me 10mA drive current to the optocoupler.
The mosfet (IRFZ40) is just something I had lying around. I figured it had more than ample current capacity, and reasonably low Rds (0.028 ohms). With peak of 2A, I figured it would only be dissipating I*I*R = 0.1W. I've put a small heatsink on it regardless.
I added a flyback diode just in case there were some inductive spikes when the fet switched off (even though I expect the inductance of the hot wire to be very small). I added some biggish capacitors on the output to smooth the output.
I'm at a bit of a loss to explain why the fet is getting so darn hot. Do I need a proper mosfet driver, or have I made a simple mistake somewhere else ?? Any help would be greatly appreciated !
The schematic below shows what I came up with. It seems to work (in that the heat of the wire does vary as the duty cycle of the PWM signal is changed), but my probem is the mosfet is getting extremely hot - too hot to touch (but no smoke, yet) after a few seconds.
The power supply to the hot wire is a 24 VDC SMPS, rated at 10 amps.
The hot wire cutter bow itself is a length of stainless steel wire, approx 1m long by 0.3mm dia. With 24 VDC applied to it directly, the measured current is around 2 amps.
The PWM signal is coming from a CNC controller, via the parallel port of my PC. The output voltage of the parallel port card is 5V. I've checked this with a DVM (5V when the pin is high, 0 when low). The frequency of the PWM signal is 50 Hz.
I set the value of R1 to give me 10mA drive current to the optocoupler.
The mosfet (IRFZ40) is just something I had lying around. I figured it had more than ample current capacity, and reasonably low Rds (0.028 ohms). With peak of 2A, I figured it would only be dissipating I*I*R = 0.1W. I've put a small heatsink on it regardless.
I added a flyback diode just in case there were some inductive spikes when the fet switched off (even though I expect the inductance of the hot wire to be very small). I added some biggish capacitors on the output to smooth the output.
I'm at a bit of a loss to explain why the fet is getting so darn hot. Do I need a proper mosfet driver, or have I made a simple mistake somewhere else ?? Any help would be greatly appreciated !
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