Problem with PWM hot wire cutter

Discussion in 'The Projects Forum' started by Geoff_S, Dec 16, 2012.

  1. Geoff_S

    Thread Starter New Member

    Dec 16, 2012
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    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 !
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Good Idea, but some issues.

    1 24vdc is too high for the gate voltage, it exceeds the +/- max gate volt rating. An easy fix would be a 12 to 15 volt regulator, for the gate supply. Either a Zener/resistor, or a 7812 or 7815 type, but don't need much current.

    2. Put a small value resistor in series with the gate 22-47 ohms. May not be needed for 50HZ PWM but doesn't hurt.

    3. Remove the 4700 filter caps, not needed. I think I get the intent, but the wire itself stores energy.

    Have you checked the voltage across the wire when you perform this test ?

    And this too:
     
    Last edited: Dec 16, 2012
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    The max gate voltage is 20V for that fet, use a gate resistor of 4.7K to try and lower the input signal.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    3,229
    An obvious issue is the capacitors. The MOSFET has to discharge that large capacitance every cycle which dissipates about 135W in the MOSFET for a 50HZ PWM. Remove the caps or connect their negative sides to ground.

    Also your "flyback diode" is not. You need to connect the cathode to the power supply and the anode to the MOSFET drain for it to work as a flyback.
     
  5. Geoff_S

    Thread Starter New Member

    Dec 16, 2012
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    Thanks for the help. I'm currently looking at revising the circuit:
    - replace the 4n25 with 6n138 (to reduce loading on the parallel port). The parallel port is actually a PCI card by STLabs, and I haven't been able to find any relevant specs on it.
    - use a zener to provide 12V supply to the opto and gate of the mosfet
    - add a gate resistor to the mosfet
    - remove the caps (but I'm confused by the comment about the mosfet discharging them on every cycle - surely they will discharge through the hot wire itself ??)
    - remove the flyback/freewheel diode (in wrong spot anyway, but probably not needed - the hot wire is a straight piece of wire so inductance should be negligible ??)

    I'll post a revised schematic later today for your expert comments ! Thanks again.
     
  6. Geoff_S

    Thread Starter New Member

    Dec 16, 2012
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    Here's the updated schematic, with
    - capacitors removed
    - 6N139 optocoupler to reduce input current requirements on parallel port pin
    - mosfet gate (and opto) being fed from 12V supply (from 7812) instead of 24V.

    I'm uncertain about the selection of the load resistor (R4) for the detector side of the optocoupler. I've got some datasheets which show various combinations of Rl for inverting and non-invertng circuits driving either CMOS or TTL logic devices, but I don't know how to deal with the output being a mosfet (and the presence of R2 and R3).

    [​IMG]
     
  7. crutschow

    Expert

    Mar 14, 2008
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    I misspoke. The capacitors suddenly charge when the MOSFET turns on, not discharge. But the power dissipation is still the same. A value equal to the capacitor stored energy is dissipated in the external circuitry when a capacitor is charged and also when the capacitor is discharged.
     
    Last edited: Dec 18, 2012
  8. crutschow

    Expert

    Mar 14, 2008
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    The value of R4 is not critical. A larger value dissipates less power and a smaller value allows faster charging of the MOSFET gate capacitance. As long as R4 is much smaller than R2 it will have little effect on the gate voltage.
     
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  9. Geoff_S

    Thread Starter New Member

    Dec 16, 2012
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    Thanks Crutschow. Much appreciated.
     
  10. Geoff_S

    Thread Starter New Member

    Dec 16, 2012
    10
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    The app notes for the 6N139 optocoupler showed the Vout taken at the transistor collector, pin 6.
    [​IMG]


    Is there anything fundamentally wrong with me connecting the gate of the mosfet to the GND (pin 5) of the 6N139 instead ? Like this:
    [​IMG]

    This will avoid me having to setup the PWM circuit logic as active-low(which is no problem, but I wanted to avoid the possibility that the hot wire would be powered up if its power supply was turned on whilst the PC wasn't).
     
  11. crutschow

    Expert

    Mar 14, 2008
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    I see no problem. Your circuit should work as shown.
     
  12. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Just add a NPN transistor. You don't even need +12V supply.

    I don't have a Darlington OPTO part so I just use a common one.

    The dissipation in the MOSFET in following drive circuit at 50Hz PWM is only 3mW ~ 45mW for duty cycle 0% to 100%, full load current 2A.

    [​IMG]
     
  13. crutschow

    Expert

    Mar 14, 2008
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    3,229
    To eliminate the 12V supply, he doesn't need to add a transistor. Just feed the 12V from the two 1KΩ (1/2W) resistor divider to the opto-coupler collector in his last circuit.
     
  14. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Yes, but the OP wants the heater current to be off if there is no drive coming from the primary side of the optocoupler.
     
  15. crutschow

    Expert

    Mar 14, 2008
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    His circuit will do that. When the opt-coupler is off, there is 0V at the MOSFET gate.
     
  16. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Yes, but switching off a MOSFET using a 10K gate resistor will take quite a considerable time, and this will show up as heat dissipation in the device.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    True. But I figured that at a PWM frequency of 50Hz, the average dissipation would still be well under a watt.
     
  18. Geoff_S

    Thread Starter New Member

    Dec 16, 2012
    10
    0
    Thanks for all your help. I built and ran up the circuit today and it worked beautifully. I ended up using the last schematic that I posted, with just a few tweaks to resistor values. Thanks again - it's been quite instructive.
     
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