wa2ise
Super Member
This is a circuit for creating DC for heaters. Main feature is that this circuit produces very little waste heat.
It uses a MOSFET (sort found in PC switching power supplies, a zener diode (to set the voltage for the output, and a LM393 comparitor chip. The comparitor chip determines when the rectified but unfiltered supply voltage is, further from zero, or closer to zero, than the zener reference voltage. (think of absolute value here). This in turn switches on or off the MOSFET via a buffer transistor. When the MOSFET is switched on, the supply voltage is closer to zero than the zener reference voltage is. The output filter cap, having discharged some since last time (a cycle of AC waveform ago), will then be recharged when the rectifer diode AND the MOSFET conduct. The MOSFET is switched off when the rectified but unfiltered voltage exceeds the zener reference voltage. Thus the output cap is charged up only to the same voltage as the zener reference. Note that we need the rectifier diode to keep the input AC waveform source from reverse draining the cap voltage charge. And there's a separate rectifier circuit to get voltage to run the comparitor circuit.
Note that the circuit is "upside-down" from the usual viewpoint, that is, it produces a regulated (albeit with ripple) negative voltage with a positive ground. This is because most MOSFETs I had on hand were N channel, and I'd need a P channel to create a regulated (albeit with ripple) positive voltage. But a tube heater isn't going to care.
Pro: little waste heat from this circuit vs a linear voltage regulator.
Con: there is ripple, just like you have with a simple filtered rectifier. But the overall voltage won't vary if the input powerline voltage varies. And you can select this voltage with the selection of the zener diode.
It uses a MOSFET (sort found in PC switching power supplies, a zener diode (to set the voltage for the output, and a LM393 comparitor chip. The comparitor chip determines when the rectified but unfiltered supply voltage is, further from zero, or closer to zero, than the zener reference voltage. (think of absolute value here). This in turn switches on or off the MOSFET via a buffer transistor. When the MOSFET is switched on, the supply voltage is closer to zero than the zener reference voltage is. The output filter cap, having discharged some since last time (a cycle of AC waveform ago), will then be recharged when the rectifer diode AND the MOSFET conduct. The MOSFET is switched off when the rectified but unfiltered voltage exceeds the zener reference voltage. Thus the output cap is charged up only to the same voltage as the zener reference. Note that we need the rectifier diode to keep the input AC waveform source from reverse draining the cap voltage charge. And there's a separate rectifier circuit to get voltage to run the comparitor circuit.
Note that the circuit is "upside-down" from the usual viewpoint, that is, it produces a regulated (albeit with ripple) negative voltage with a positive ground. This is because most MOSFETs I had on hand were N channel, and I'd need a P channel to create a regulated (albeit with ripple) positive voltage. But a tube heater isn't going to care.
Pro: little waste heat from this circuit vs a linear voltage regulator.
Con: there is ripple, just like you have with a simple filtered rectifier. But the overall voltage won't vary if the input powerline voltage varies. And you can select this voltage with the selection of the zener diode.
