High Voltage Probes

Some time ago I had need of being able to measure high voltages in a very old oscilloscope I was repairing. Specifically, the manual states an AVO meter set to the 2kV range, and an electrostatic voltmeter to check the EHT voltage, are required. After some head-scratching and a little research, I came up with this pair of voltage divider probes which plug into a DMM.

Unfortunately I didn't think to take photos when I made them. I was able to dismantle the EHT probe to take some pictures for you, but the AVO replacement probe is well glued together.

The AVO replacement probe is designed to present the circuit under test with an impedance similar to what an 20,000 ohms per volt AVO meter would have on the 2kV range. It scales by a factor of 10, so 2kV reads as 200v. You have to mentally scale back by a factor of 10.

The electrostatic meter replacement probe is designed so that a DMM will provide a direct reading in kilovolts.

The AVO replacement probe has an integral ground connection because I used non-stackable plugs, however the EHT probe uses normal, stackable plugs so you can just plug your ground lead into one of these

Both probes are based on having a multimeter with 11M internal resistance, however some meters (especially older ones) may have 10M internal resistance.

To find out your meter's internal resistance, first check the manual. If you can't find it there, connect one end of a 10M resistor to a known voltage, such as 12v. Test the voltage with your meter between the open end of the 10M resistor and the other terminal of the supply. If you get very close to half the supply voltage (eg, 6v), your meter is 10M. If it's higher (eg 6.29v), your meter is 11M. Ideally you should measure the resistor to more accurately predict what reading to expect in either case.

2kV "AVO"(tm) substitute probe

• Cheap meter probes. I bought a pair specially for the purpose.
• 1x 47M high voltage resistor
• Assortment of 10M resistors (so you can select the best one)
• Rigid plastic tube
• Contact adhesive
• Heat shrink sleeving
• Wires to connect to meter, banana plugs etc.

EHT probe

• 2x 1G resistors (Ohmite 104E 1717)
• 1x 2.2M resistor
• 1x 500K multi-turn preset
• 2x cases from blank CD cakes, with central "spindle"
• Plastic pieces taken from liquid soap dispenser etc
• 1.5 litre PET pop bottle
• Contact adhesive
• Electrical tape
• Short piece of stiff wire
• Wires to connect to meter, banana plugs etc.

I'll start with this one because it's easier to make and is likely to be useful to more people. The design is based on a DMM having 11M input resistance. It actually scales the reading by a factor of 10, so the meter will read off by a factor of 10. So 100v will read as 10, 1000v will read as 100, and so on. Multiply the meter reading by 10 to get the true voltage.

The reasoning behind it is that old equipment manuals were often written with voltages shown as read on a 20,000 ohm per volt meter, so this probe is similar to that.

You need a meter probe you don't mind cutting up, a 47M high voltage resistor, and a selection of 10M, 5% resistors. You also need some kind of plastic tube to make the casing. I used the cores from two rolls of dog-poo bags, held together with some heat shrink. The tubes are just the right size to fit onto the cut-off probe casing but they are made of about the cheapest plastic you can get. It's not great but it works. Better quality tube would be highly desirable.

I opted to keep the original shrouded plugs from the meter probes, even though this required adding a ground clip, partly because they look nice, but more importantly because the voltage between the plugs is still relatively high.

You will need a calculator (or a pencil and lots of paper) to work out the value you really need for the 10M resistor.

a 20,000 ohm per volt meter on the 2kV range actually has a resistance of 40M, but the extra 10M presented by this probe won't make a significant difference to the reading. As an experiment I simulated the probe in LTSpice using a 2KV supply with a 500k internal resistance, connected to a 40M load or this probe. The difference in supply voltage drop was only 6v, which is negligible for the majority of applications.

You also need a constant high voltage reference you can use to calibrate the probe. At a pinch you can use AC mains for the purpose, however not only is this potentially dangerous, but mains voltage tends to vary so you may have to check a few times. A stabilised DC voltage source is much better and safer.

Cut off the handle part of the probe you are making this out of, making sure there is enough of a stub to fit your tube onto, and also enough of the wire left inside to connect to the 47M resistor.

Before you connect it, measure the 47M resistor and make a note of it's real value. It could be anywhere in the range from 44.65M to 49.35M.

Take R2 as being the combined resistance of R2 plus the "padding resistor"

Where Vt is the voltage you want to measure, Vm is the meter reading, R1 is the 47M resistor, R2 is the 10M resistor and Rm is your multimeter's internal resistance. R is R2 and Rm in parallel.

The value you need is given by R=R1/9. In a world of ideal values and perfect components this would be 5.222˙M. However, no component is the perfect value, and not even the 11M (or 10M) input resistance of your meter is exact.

To demonstrate this, take 1000v input, divide by the combined resistance of 52.22222M, giving 19.14894µA, the total current through the probe and meter combined, and multiply by 5.22222M and you see the value is 99.995V, close enough to 100V. So Vm=Vt/(R1+R)*R

To get the actual resistor you need, use Rm*R/(Rm-R). With ideal values of 47M for R1 and 11M for Rm, this gives 9.9423M

With known real values for R2 and Rm, you can select the closest value to your calculated value, from a batch of 10M resistors. You may need to pick a lower one and pad with a smaller value.

I wrote this Instructable after making the probes, so can't show you the inside of this one as it's glued together. I hope the drawing will suffice.

Whilst an electrostatic voltmeter is a fairly exotic piece of kit, it's easy to make a very high resistance probe using large value resistors. The only caveat is they have to be suitable for the voltage to be measured, and housed in an adequately insulating enclosure.

Ideally it would be frequency compensated, but I don't anticipate using it in a situation where this has much relevance. It goes under the "nice to have" category.

The design is based on a DMM having 11M input resistance. See previous step for procedure to determine this.

The main resistance consists of two 1 giga-ohm resistors in series. These particular ones are rated at 15kV each, so they should be able to handle nearly 30kV altogether. Whether you would want to trust the rolled-up pop bottle construction at this voltage is another matter however. The probe handled my oscilloscope's 9.5kV final anode voltage with no problem and importantly, no "tingle" for me.

The two main resistors are a radial leaded type, designed for mounting on a PCB. As there is no PCB here, some careful lead-bending is required to get them in line. I wanted to create a sound joint before even applying solder, so spend a bit of time on this.

The specific reasons I avoided using a PCB were to create rounded joints, and to eliminate any risk of tracking up the board.

One end is then connected to a piece of thick, stiff wire, which forms the probe tip, and the other end is to the low voltage part of the potential divider. This consists of a fixed 2.2M resistor and a 500k preset, which can be adjusted through a hole in the probe body. Connect the preset to that it's resistance increases when you turn it clockwise. It's preferable to connect the wiper to the 0v terminal if possible.

The probe tip is fixed into some bits of plastic, mainly part of the pump from a liquid soap dispenser bottle. I made quite a long insert for this, to keep it stable. The body is made from a rolled up pop bottle (I used a Lilt bottle because of the nice green colour) and the handle is made from the bases of two old CDRW cakes.

I made this before deciding to publish it so don't have construction pictures unfortunately, only some I took when I dismantled it for the purpose of taking them.

The main thing you need here is a 1.5 litre pop bottle. Get it as clean as you possibly can, you don't want contaminants on it which could allow the voltage you are testing to track up the probe.

These bottles are made of PET, which has the handy property that it shrinks and curls when it gets hot. Cut the top and bottom off the bottle, just before where it starts curving inwards, and cut down the side. The fun bit is then immersing the curved sheet of plastic in very hot water and watching it curl up!

Mine wouldn't curl as much as I needed (maybe hotter water would have done it) so I had to manually twist and squash it in further. Further dunks in hot water made this easier. I made it slightly conical so it fits the pieces at either end, which in turn necessitated a bit of trimming to get it even, as the cone shape makes the layers be misaligned at either end, and the side-seam be wonky. With more planning I could have cut it just right to start with, but as I'm no geometer it was easier to just trim it afterwards.

After rolling it up I made it tighter by re-rolling it with the outer seam on the inside. This eliminates the problem of the not-quite-curled enough outer edge, and takes advantage of the over-curled inner.

Ensure the end piece is a good fit on the end of the body.

I had two types of CDROM cake cases available. One where the central rod is moulded as part of the base, the other where it has a large diameter thread and screws into place.

I cut a disk from the centre of each base and glued them together so that the moulded rod projects from one side, forming a handle, and the ridge with the threads on it forms an attachment point for the body on the other side. The area of disk which extends outwards makes a bit of a guard for your hand. It's main function however is to remind you that high voltages can be very flipping dangerous!

Ensure the wide end of the probe body fits the threaded part, or whatever arrangement you are using. (Thick disks cut with a hole saw might be a good substitute.)

I used the screw top from a liquid soap dispenser bottle to secure the body at the handle end as it's a tight fit through the central hole and the edge of the top can be glued to the handle, so the end result is nice and stable and looks a lot better too.

Fit the probe tip into the end piece and use whatever insulating material you have available to make it secure. I used a bit of rubber sleeving and some glue. Temporarily fit this assembly into the end of the body.

The other probe from the pair I used to make the AVO replacement probe would have provided a better tip, but I didn't have any suitable pieces to make it fit the body.

Fit the preset into it's carrier and temporarily assemble the handle to the body and find the spot where the centre of the preset's adjustment lines up with the outside, avoiding either the inside or outside long edges of the rolled up plastic.

At this point you need to be very sure of the body diameter. Any tightening or loosening of the roll will mess your hole up! Slide on the soap-dispenser-bottle lid so that it's near to it's final position. Use tape to keep the rest of the roll in place. It needs to be assembled, just not finally secured yet.

Take the handle off again, and drill through the spot you marked (it might be better to just melt through with a soldering iron. I did a bit of both).

Trim the wire from the main resistor and connect it to a red wire and the 2.2M resistor. Make a good mechanical connection by folding the wires over before you solder it. Connect the other end of the 2.2M resistor to the 500k preset and connect a black wire to the other end of the preset.

I used thin wires to make these internal connections so that the whole thing would be easier to assemble. They are connected to the main wires, sealed with heat-shrink and curled up inside the handle. The main wires are knotted to stop them pulling through.

Finally, assemble the handle to the body, ensuring the preset is lined up with the adjustment hole, Glue and tape everything firmly in place. Put some glue around the soap-bottle lid edge and push it down so it drags the body down tighter.

Work the tip end of the body around in your hands so it tightens down onto the tip assembly. Secure it with tape.

At no point did I attempt to glue the actual body as it didn't seem to need it and I felt that doing so was to invite problems later on.

Calibrate the probe by measuring a stable high voltage with your DMM, then connect the probe to it and measure the same voltage again with the probe. Adjust the preset to give you the same reading.