DIY High Voltage Probe for digital multimeters

I was in a need for a high voltage probe for my multimeters to check various capacitor banks, and high voltage DC charging circuits. After checking for professional ones, I realized that I could make one from some spare parts from my lab.

General properties: DMM internal resistance: 10M
DMM max DC voltage rating: 1000V
Desired max measurable voltage with this probe: 20KV

First things first: During construction pay extra attention to proper insulation if you're playing with high voltage. Always discharge caps after use!


Things needed for this instructable:

high voltage resistors from microwave oven caps (I've already had them disassembled for another project) 10M each on ceramic base.

Heatshrink tubing - for insulation

Applicable tube (Xtra big sharpie housing or other blackboard marker) - for the probe body

Double insulated HV wiring

Copper rod - for the tip

Crocodile clip + banana plugs - for connectivity

Some power and handtools for the operation.

Step 1:

Since i needed this probe for the 20KV range I calculated the required resistor values for my voltage divider. In series I will solder 10 pieces of 10M ohm resistors and 11,1M in paralel with the input of the DMM. on the pictures you can see the diagram. These values are giving me 1000V readout at 20KV input on the probe.

First you need to go get some microwave oven capacitors. Be careful with them they are rated at 2100VAC!!! That's lethal if you get shocked. I generally use snippers to cut the upper edge off, then i pour out the mineral oil and remove the resisor. You can put the whole thing back together seal it up with some goop refill the oil, and you have a nice HV DC cap. That way is really cost efficient 'cuz you end up with two useable parts for one move.

For the body I chose to use a old dried out fat ass mega sharpie. I pulled out all the guts and discarded them. Then I made a copper insert to the front to secure the measuring needle (tip) in the nose section (tight tolerances!). It's removable if service is needed.

Comments

[SnyperBob]

Hello, I have a few questions about this instructable... You said you soldered all the resistors in series and then slid them in the tubing. Then slid it all in the marker. So is the oil inside the tube that holds the resistors? Or is the oil inside the marker with the tip, or is the oil inside both? What is the point of the oil, is it an insulator? Because your meter reads up to 1000 volts, and this is designed for 20 kv....do you just multiple your meter reading by 20 when measuring high voltage? This would give you the actual high voltage value then? Thanks

[vdevilmate]

Hey Buddy, I meant to put the whole string of resistors in a shrink tube, or something similar for primary isolation, cuz you obviously couldn't place the bent string sections next to each other whitout shorting them out. Now you make it to form a Z or W whichever fits better. After this, solder the leads and then you are good to slide it inside the marker and if you wish fill it up wth the oil you previously removed from the caps. I think it's a good idea to prevent arcs and increase safety. Yes you just multiple the result in head and youre done. One could make a more professional unit with a tuned analogue meter\ matching modified scale pair. But i needed it fast, and didn't bother buying one. I have a professional one for backup too:) Hope it helped, thanks. Mate

[rdelaplaza]

Dear Friends: I'm sorry for being a party pooper but... safety is always first, an a cancer a few years down the line is not worth a DIY HV Probe.
Opening oil filled capacitors and messing with the oil inside is HIGHLY NOT RECOMMENDED, those oils are usually PCB's or...

Polychlorinated biphenyl (PCB; CAS number 1336-36-3 ) is any of the 209 configurations of organochlorides with 2 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. 130 of the 209 different PCB molecular arrangements and orientations are used commercially.

PCBs were widely used as dielectric and coolant fluids, for example in transformers, capacitors, and electric motors.

Due to PCBs' toxicity and classification as a persistent organic pollutant, PCB production was banned by the United States Congress and by the Stockholm Convention on Persistent Organic Pollutants in 2001.

According to the U.S. Environmental Protection Agency (EPA), PCBs have been shown to cause cancer in animals, and there is also evidence that they can cause cancer in humans. A number peer-reviewed health studies have also shown a causal link between exposure to PCBs and non-Hodgkin Lymphoma, a frequently fatal form of cancer.

Concerns about the toxicity of PCBs are largely based on compounds within this group that share a structural similarity and toxic mode of action with dioxin.

Toxic effects such as endocrine disruption and neurotoxicity are also associated with other compounds within the group. Therefore, the current maximum containment levels as stated by the EPA for PCBs in drinking water systems is 0.5 parts per billion.

So if you ask me I'll prefer to stay away of "capacitor oils", is a lot cheaper to buy those resistors from Mouser, or Advance Electronics or any electronic components supplier.

Be Safe.

[shimo]

hi
i am going to make 40kv so do i us 20 pieces of 10M ohm resistors and 2 of 11,1M in paralel or 22.2m in paralel .
thanks