I was faced with the situation of needing a 12.6V DC heater supply from an 'odd' sized transformer and using a simple regulator would have dissipated huge amounts of heat and been very inefficient. So, I devised this circuit that can be adjusted to generate any DC voltage as long as the transformer puts out a larger voltage.
Basically, the MOSFET is normally on and turned off when the incoming AC waveform passes a threshold set by R15 and the zener voltage (see waveforms below).
During the first part of the AC cycle, the incoming voltage is less than the charge on C3 and no current flows. As the waveform exceeds the charge on C3 current flows and is then cut off as the waveform continues on past the threshold point.
R12 adds a modicum of load regulation by sensing the 12.6V level, which is then added to R14 sensing the AC waveform. Load regulation is not terribly tight but sufficient to prevent excessive heater voltage if a heater fails or the tube is removed. Ripple is on the order of 200mV.
Zener D7 protects the MOSFET gate and D13 prevents the zener from going forward when Q5 conducts.
An interesting characteristic is the MOSFET is triggered twice per half cycle, 4 times per full cycle, since it triggers on both the rising and falling quadrants.
Another interesting characteristic is waveform distortion, which normally occurs at the peaks with the typical rectifier/filter supply, occurs here on the slope.
This circuit is the"as built" 12.6V heater section for an amp where the same transformer also provided opamp, -100V, bias supplies, and 20V for powering 10GK6 heaters in series.
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