DC High Voltage Line Splitting Problem (Electrode "Tuning" Application)

Question

I am having trouble coming up with a solution to a problem engineering a DC circuit that can take a single line input (Vin) and split into two outputs (Vout1 & Vout2).

The goal of the circuit is to take a high voltage line and supply two parallel electrodes connected to Vout1 and Vout2. By changing the potentiometer setting (R1) one should be able to adjust |Vout1-Vout2| between 0-9V or 0-18V depending on the switch (SW1) settings. The circuit should be able to handle high voltage inputs (Vin: 0-4000 VDC, 1000 VDC typically). The electrodes are floating in ultrahigh vacuum.

The current circuit setup is shown below; however, this circuit does not perform as desired.

^{simulate this circuit – Schematic created using CircuitLab}

The current configuration allows one to change electrode voltages (Vout1 & Vout2) 0-9V or 0-18V by adjusting the potentiometer; however the potential difference between the two electrodes (|Vout1-Vout2|) is always constant (either 9V or 18V depending on SW1 settings). I would like to be able to use the potentiometer to "tune" one electrode relatively. For example, I might wish to use a 1000 VDC input voltage (Vin) and get output voltages of: Vout1= 1000 VDC, Vout2= 1005 VDC.

Does anyone know of a way to modify this circuit or adapt a simple design to implement this idea?

Answer 1

I think this may do what you want:

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Electrode 2 positive-only with respect to Electrode 1.

^{simulate this circuit}

Figure 2. Switchable polarity version.

Note that while the batteries, switches and potentiometer have a maximum of 18 V across them they are possibly all at high voltage (1,000 to 4,000 V) with respect to ground and you. All need to be properly insulated. The metal case of the potentiometer should not be grounded (as you're unlikely to find one with a 4,000 V rating between the carbon track and the case) but should be mounted totally inside the enclosure with only the plastic shaft protruding through the case with a knob on the outside.

Answer 2

I designed one of these for a gas mass spectrometer back in the 90s and it was fairly complex because there was also a 12V heater circuit involved that had to be floated up to the HT voltage (up to 5 kV from memory). 5 kV isn't too much of a problem providing you apply sufficient insulation - I used switching power supplies (home spun) and very-well insulated ferrite transformers (hand made) for transferring power up to the HT voltage. I also used pots and very long plastic spindles connected to the knobs on the front fascia of the box.

There were some logic controls to the HT board from ground level and I used 10 kV rated opto-isolaters. For feedback of the voltages I used a parallel ADC and a bunch of optos but these days I'd use serial ADCs. Watch your hands - I once got a 5kV shock from it and it hurt and flung me a couple of yards.