Intro: 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.
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.
After collecting all neccessary parts i soldered the resistors in the proper network (in a string), isulated them with heatshrink tubing, connected the tip and slid the whole thing into the housing. After drilling 2 holes into the cap you can either fill it up with mineral oil -or or some thinner resin. You will want to have a clip securing the ground terminal at the other end of the probe, and two general banana plug for the DMM. I used coax cable but anythig else would work. Try to stick with something beefier than standard 22 gauge hookup wire. In addition i put on 2 protective acrillyc discs to avoid direct contact and over-arcing-..