High Voltage Probes

My first probe is a DC-only resistive divider with a ratio of 1000:1. It was thrown together with whatever parts I had on hand, so its input resistance is only 200 Meg-ohms (20 10M carbon resistors in series). A neon bulb is in parallel with the 200k output resistor (2 100k in series) to clamp any nasty voltages that might find themselves on the way to the multimeter if a flash-over occurs, or I accidentally connect it to a 50-90 kV source.

The calibration was checked with lower voltages and found to be excellent for carbon film resistors, within two percent. This is quite acceptable for my purposes and I will leave it untrimmed. If anyone has a local source of HV, high value, precision resistors I'd love to know!

The probe tip is a normal probe tip taken from a junked multimeter lead. The tube is two tough plastic drinking straws from the local Subway. They have an excellent brand of drinking straw, transparent, very large diameter, strong, thick walled and fairly long. Last time I was there I thought to my self 'how useful' so I asked and they gave me as many as I could carry.

The grounding lead is a gator clip lead that one end had fallen off. The banana jacks are from the junk box, as is the wire (just some ribbon cable that the girlfriend bought me years ago, when we were first going out, still haven't used all of it yet!). The resulting structure is electrically sound enough for all the work I am doing right now. It is a bit on the flimsy side mechanically, but in practice it is quite safe and usable.

The lack of shielding gives the probe reasonable frequency response, at the expense of stray signal pick-up, but I plan to use it for DC only anyway. I will build a capacitive divider for AC measurements and CRO use. I can live with having separate probes for DC and AC. It makes designing them many times easier as a DC to daylight HV probe is a very non-trivial piece of equipment to homebrew.

DC probe



2003-05-10: Diagram Error
Fred Kennedy noticed the circuit diagram might be wrong.
2003-03-20: Circuit Diagram
Richard Solis asked me to post the circuit diagram.