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.
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
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.
- Turn the main power switch on
- Set the Variac to about "30".
- With the work coil empty, press the foot switch
NOTE: This is the only time that you can safely operate the unit without
- Turn the sensitivity pot to the right until you get
an up-scale indication.
- 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.
- Release the foot switch.
- Insert the work into the coil or place the coil
around the work.
- With the Variac set to about "40", press the foot
- Turn the tuning knob to re-peak the meter.
- Release the foot switch.
- Turn the Variac to about "80".
- Press the foot switch and observe the work as it
- Increase or decrease the Variac setting as necessary
to control the heat rate.
- 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.
- Release the foot switch when the desired heat has
- 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.