# Modifying HV supply for digital control

(source: desmith.net)

I've built the HV supply shown above (from here http://desmith.net/NMdS/Electronics/NixiePSU.html). And it works great. I would now like to be able to digitally control the output voltage. This is currently controlled by VR1.

I've tried replacing VR1 with a BJT or a MOSFET. However I couldn't get this to work. Should this approach work or is there something I'm missing?

If this doesn't work I was considering putting a high voltage opamp on the output (400V). Are such Opamps available?

## 2 Answers

It's possible to get digital potentiometers. These are chips that have a digital control input (usually SPI or I2C) and act like a variable resistance or potentiometer (pot). Maybe you could replace VR1 with a digital pot, and control it from a microcontroller?

It might be a good idea to add some protection components around VR1, to prevent the high voltage from damaging the digital pot and/or microcontroller, in case of fault or accident (shorts or disconnections).

• Hmm, yes I was also considering a digipot. But I don't understand the electrical difference clearly enough. It seems that transistors and pots both effectively limit current? So a transistor should work here? Commented Dec 16, 2014 at 9:49

You wrote:

I've tried replacing VR1 with a BJT or a MOSFET. However I couldn't get this to work. Should this approach work or is there something I'm missing?

Maxim's data sheet for the MAX1771 shows that a voltage divider must be used to drop the desired output voltage down to 1.5 volts, and that 1.5 volt signal used for feedback in order to regulate the output voltage at the desired point.

Looking at the HV supply schematic, the divider comprises R2, RV1, and R1, with the 1.5 volt feedback to the chip coming from the junction of R2 and RV1.

Since RV1 and R1 can be summed to equal 10k when the pot is set to zero ohms, and 15k when the pot is maxed out,we can write:

which means that the circuit works and that the supply's output can be adjusted so that when the pot is set at zero ohms the HV output will be at about 226VDC, and when the pot is set at 5k, the HV output will be at about 151VDC.

Now what we want to do is modify the circuit so that with a transistor turned ON we'll have 10k from the bottom of R2 to ground, and with it turned OFF we'll have 15k from the bottom of R2 to ground.

That can be done by setting the pot to 5k and shunting R1 with 10k in series with an N MOSFET, like this:

And that should do it!