*** Switching Power Supplies for Tube Heaters ***
The switching power supply is a viable alternative to the conventional power transfromer
for supplying power tube heaters. A typical switcher will be lighter, perhaps smaller,
and provide better than 1% regulation over all reasonably expected line & load variations.
However, it is not practical to connect a "stock" supply directly to a tube heater.
The cold resistance of a heater is a fraction of the hot value and excessive current will flow on the
order of 3-4X the rated value. Even though this condition will persist for only a second or two,
it may be deleterious to both the switching supply and the (expensive!) tube.
Some older discreet component switchers may be modified for an extended turn on ramp.
Unfortunately, as switching supplies have become more integrated, it has become more difficult,
if not impractical, to modify their turn on characteristics to be suitable as heater supplies.
There is a simple external solution.
The circuit shown above provides a twenty second turn on ramp.
The power supply used was originally a 5 volt, +/-15 volt unit but, it had enough headroom
to output 6 volts which increased the tracking outputs by a corresponding percentage.
A brief operational description referring the circuit diagram above.
C2 is charged to the full heater potential at "power on" thru D2.
A slow ramp generated by R1, & C1, is applied to the gate of the power FET, Q2.
As the FET reaches its threshold voltage, circa 3 volts, the drain voltage
will begin to decrease which will delay the gate ramp by discharging C2
into the gate ramp, level shifted by Q1, a 2N5210 transistor.
This delays the ramp for approximately 18 seconds around the FET's threshold
and the heater current is thus limited to approximately its final value -- 10 amps
The effective heater voltage rate of rise is 1/3 volt per second
R3, the 1K gate resistor precludes the power FET from oscillating and
D3, a 15 volt zener provides an upper limit to the ramp voltage.
Observed with a current probe & storage 'scope, the heater current is actually seen
to decrease slightly, after the initial surge, as the heater's resistance increases
at a faster rate than the applied voltage.
The upper trace is the heater current -- 2 amps per division.
The lower trace is the heater voltage -- 1 volt per division.
The time base is 2 seconds per division.
The ramp completes just a skosh off screen right.
The power FET employed has very low on resistance, circa 3 milli-ohms, when driven hard on.
During the power on ramp, the FET will dissipate about 30 watts max.
A small heatsink is advisable to avoid an unnecessary thermal excursion.
After the power on cycle completes, the FET will only dissipate a fraction of a watt
