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Last update 12/08/2010

Induction Heater Operating Instructions

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since 08/13/09


This heater requires some restraint in its operation, particularly with regard to the transistor temperature, to be reliable.  It was not designed as an end-user appliance.  Thus it is possible to over-stress and burn out the transistors and other parts of the heaters.  Please follow these instructions exactly.  Use the lowest setting on the Variac that will get the job done. Particularly important: Do not operate the heater unloaded except as instructed below!  That is, do not operate the heater without work in the work coil.  The unloaded tank voltage excursion is extreme and may damage the tank capacitor and overload the FET transistors.

Pre-Operation Setup

Before energizing the heater for the first time, set the Variac to 0 volts.  Set the frequency control dial to about 90.  That is, about 9 turns clockwise, one turn from being wide open.  Always start tuning from a high frequency and come down.  Operating the unit at a lower frequency than resonance is hard on the transistors and causes excess heating.  Set the meter sensitivity pot to about mid scale.

A foot switch is shown on the schematic.  I highly recommend using one so that you have full control over the heater even when both hands are occupied.

These instructions assume that your Variac has the standard 0-140 volt scale. If you have some other scale, adjust the specified values accordingly.

Initial Tuning

  1. Turn the main power switch on
  2. Set the Variac to about "30".
  3. With the work coil empty, press the foot switch NOTE: This is the only time that you can safely operate the unit without a load.
  4. Turn the sensitivity pot to the right until you get an up-scale indication.
  5. Slowly turn the tuning pot counter-clockwise until the meter indication peaks.  If necessary turn down the sensitivity pot to keep the meter from pegging full scale.
  6. Release the foot switch.


  1. Insert the work into the coil or place the coil around the work.
  2. With the Variac set to about "40", press the foot switch
  3. Turn the tuning knob to re-peak the meter.
  4. Release the foot switch.
  5. Turn the Variac to about "80".
  6. Press the foot switch and observe the work as it heats.
  7. Increase or decrease the Variac setting as necessary to control the heat rate.
  8. If the ferrous work is to be heated red hot, it will pass through the "Curie Point".  At that point the resonant point will shift significantly.  Retune the tuning pot to achieve resonance again.  Do not operate the heater off-resonance for a significant period of time, as this mode generates more heat in the FETs.
  9. Release the foot switch when the desired heat has been achieved.


  • Burn Warning: The work coil gets very hot during use, not only from heat radiated from the work but also ohmic heating from the high current passing through it.
  • I do not recommend operating the heater at over "80" on the Variac for an extended period of time.  If you do, keep close watch on the transistor heat sink and do not allow the temperature to rise above about 140 deg F.  Rapid thermal runaway and transistor failure occurs above that temperature.
  • There is line voltage and high voltage DC present on the PCB and inside the enclosure when the unit is operating.  Observe appropriate precautions.
  • When in operation, there is high frequency high voltage on the tank.  RF burns are possible.  Don't touch the work coil while energized.