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Stable High Voltage (3kV) Photomultiplier Supply
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David Hess:

--- Quote from: Yansi on February 02, 2019, 08:18:06 am ---
--- Quote from: David Hess on February 02, 2019, 02:15:03 am ---
--- Quote from: Yansi on February 01, 2019, 03:38:58 pm ---Putting a non-regulated supply in series with a regulated one does not create a regulated high voltage supply. Or what did I miss?
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Feedback from the combined high voltage is used to control the regulated supply.  In the designs I am thinking of, the regulated low voltage output was combined with the unregulated high voltage through a circuit much like a high voltage DC restorer.
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Do you have any schematic on hand to give an example? I can't still think how one would regulate two supply voltages with just one series pass element. That seems impossible.
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The examples which come to mind are the Tektronix 7704A and 7904 oscilloscope mainframes.  These were their first to use a switching power supply to drive a separate high voltage transformer producing a semi-regulated high voltage output; previous designs regulated the high voltage about by adjusting the high voltage inverter drive.  Since the high voltage negative cathode supply had to be tightly regulated for good deflection accuracy, a separate low voltage circuit with feedback from the rectified high voltage injects DC into a DC restorer which rectifies the high voltage AC to high voltage DC.

As shown below, R4240, R4241, and adjustment R4123 are the feedback divider which uses a +50 volt reference.  Operational amplifier U4110 is the low voltage error amplifier.  Q4115 and Q4105 are a level shifting stage to produce 0 to 150 volts DC.  CR4205 and CR4206 with associated capacitors and resistors are the combined DC restorer, rectifier, and filter.  The output voltage in this case is a very closely regulated -3000 volts which tracks the +50 volt reference.  I measured stability on my 7904 as better than 100 millivolts.

An interesting feature of this circuit at least to me is that U4110 is a plain Jane C-grade 741 despite the high input impedances but they used capacitive bypassing to limit noise and a bias error cancellation resistor.  I would have used a low input bias current 308 or similar but Tektronix was very parsimonious about using premium parts or changing designs once they had something working.
ycui7:
That was in my grad college time. There was no kilovolt-withstanding AC coupled probe in academic environment, at least for my lab. You cannot just put oscilloscope to AC coupling and connect 3kV to a GHz oscilloscope input. If you do, boss would be super mad, and you may never graduate. So, I did what I could, although no ideal.

Measuring sub-volt noise on a kilovolt signal is always difficult, particularly due to availability of probe.
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