How do I measure very high voltages? I have previously written about an electrostatic voltmeter that measures up to 500V, and have another that that measures up to 5000V (5kV.) What if I want to 20kV, or 100kV? A single instrument that can measure such a voltage directly may not be very useful when measuring much smaller voltages. Rather than acquire lots of different instruments to handle different voltage ranges, the approach I am taking is to use one or two instruments that measure relatively low voltages, and use a voltage divider to extend their range.
Some time ago, I acquired an ex-equipment power supply, which supposedly has an output voltage of 90kV. I built a driver circuit for it and fired it up – the way pieces of dust and small objects flew around my bench told me that, yes, it was certainly producing a substantial voltage, bit I had no way of knowing just how substantial, especially since I wasn’t using the original driver circuit. Fast forward to this year, when I decided to build an instrument to measure high voltages for another project. Since I had the 90kV supply, I thought it appropriate that whatever I built should be able to answer the question, once and for all, as to whether the advertised 90kV really is 90kV.
Originally just called the Big Divider (I onlycame up with Great Divider when writing this article,) my design was a total of fifty 22MΩ resistors, tapped at various points, giving me division ratios of between 1:5 and 1:50. The resistors I selected (a Vishay product, Element 14 part number 190-1890,) have a rated working voltage of 3500V, so the entire string should – in theory – be able to work up to 175kV. Due to issues with insulation and corona discharge, I have no intention of operating the divider much above the 90kV of my supply. (At one point, I considered flooding the divider with sulphur hexafluoride, an insulating gas, but deemed this way to expensive.)
This image is a capture from the CAD software (Cadsoft Eagle) used to lay out the divider boards. I made five copies of this design on a single sheet, then printed to polyester drafting film, and, using an ultra violet light-box, contact-printed onto copper-clad circuit board stock, which is coated with a photosensitive material. After exposure, boards are developed in a sodium metasilicate solution, which removes the exposed photosensitive material. The developed
boards (right hand board in picture) are then etched using a solution of ammonium persulphate. Unfortunately, I ended up having to do this twice. I tried out a cheaper type of board, which uses a phenolic substrate, rather than my usual FR4 fibreglass. When I guillotined the board to divide it into the five boards which constitute the end product, the substrate shattered. With the second batch – made on FR4 – things went as planned, so I drilled holes for the component leads and mounting bolts and proceeded to the next step.
The resistors used are specified as having a value of 22MΩ, with a tolerance of ±5%. This means that any given resistor could have a value anywhere between 20.9MΩ and 23.1MΩ. In order to get the division ratios as close as possible to the 1:50, 1:25, 1:12.5, etcetera, I had to measure (twice) and record the value of every single resistor. I keyed all this data into a spreadsheet and then went through the painstaking process of ensuring that the lower board in the divider was as close to one-fifth the total resistance as possible, and that each tap on
that board was as close to the correct ratio against the total resistance. Painstaking doesn’t even begin to describe the process but, once I had done all the matching, overall tolerance was better than 1%, so well worth it.
Measured resistors were inserted into a marked sheet of paper so that I could find them again for assembly. The following picture is the last that was seen of the pretty colours of the resistors.
The boards are mounted on an Acrylic frame using nylon stand-offs and fasteners. With the high voltages involved, I tried to use as little conductive material in the construction as possible.
Anticipating issues with corona discharge, and having rules out the use of sulphur hexafluoride as an insulator, I opted to use a spray-on conformal coating (Electrolube DCR modified silicone conformal coating SCC3,) applying several coats. How well this performs is yet to be seen, as the maximum voltage with which I have tested the finished assembly so far was a measly 100V.
Initial testing involved connecting the divider to various reference voltages and measuring the voltage at each tap point. Due to the relatively low input impedance of the multimeter I was using, connection was via a very high impedance op amp buffer (input impedance > 10^12Ω) so as not to distort readings by having a relatively low value resistance connected in parallel. Voltages were recorded in a spreadsheet and the actual ratios calculated. As I can’t be entirely sure how accurate my multimeter is, the errors against the calculated ratios came out to be negligible – the 1:50 measuring as 1:49.91, the 1:12.5 measuring as 1:12.49, etcetera. The effort of resistor matching having paid off, as far as I am concerned. I will be performing further measurements with a voltmeter with greater precision to get more exact ratios – but not until said instrument goes beyond being a bare circuit board, a bag of bits, and unwritten software!
The last step in Building the Great Divider was to give it a case. I considered various ways of mounting the acrylic tube, and decided that the best approach would be to design a case and get it laser cut out of acrylic by Ponoko.
I do my laser-cutting design in SVG, using a simple text editor (vim) and a web browser to see what I’m doing – not being a fan of using what is effectively art software for CAD work. I uploaded the SVG file to Ponoko on a Sunday, paid an extra $5 for one-week in-factory handling, and was delighted when it arrived all the way from New Zealand just seven days later. The photograph shows the divider in its tube, sitting on the cut sheet just received from Ponoko.
Which brings us to this full-sized image of the Great Divider.
The connectors are 4mm banana jacks in Bakelite – vintage ex-Soviet military. Cables are silicone insulated EHT cable, rated to 20kV (so it doesn’t matter that some are touching, down below.) To reduce corona discharge from the terminals, they have been insulated with clear nail polish, which I am advised makes a good insulator. (I didn’t have any paint-on conformal coating, so used the next best thing.)
I now look forward to testing it in anger on my (allegedly) 90kV supply.