HIGH VOLTAGE METER!

(please click on any picture to enlage)

20KV DC Meter

Inside View

This my HVDC Meter that I use while my Tesla Coil is operating. It is about as simple as it can be, and cost about $20 in surplus parts. The box is polycarbonate plastic, but could easily have been acrylic.

The meter movement is a 1ma full-scale ammeter with a face that was marked 0-20 Kilovolts DC, with a green "typical operation" area marked from 12kv to 14kv ... perfect! except ...

Let's see now, 20KV at 1ma is ... 20,000*.001 = 20 Watts! This is a lot of power (to say nothing of heat). It's a good thing I was lucky and found two 10-watt resistors that could handle it. I mounted the resistors on a piece of phenolic though just to make sure. The clips are funky though ... as soon as I can find better ones I'll replace them.

The really, really good thing about the 20watts (1ma actually) is that since I hook this meter to my tank capacitor (my coil is DC driven and the capacitor is charged with DC voltage) when I turn power off, the meter safely discharges the primary tank circuit and shows me when it is safe - Yeah!

The schematic shows the highly complex and elegant design ... two components ... the resistors and the meter. The resistors are IRC HV resistors that are type "MVP" ... 20MEG Ohm, 10watt, 5% film resistors on a ceramic core. I noticed that Surplus Sales of Nebraska is selling these now for $20.00 each!

Theory of Operation:
The meter is effectively a short to ground. This is good since the meter is not rated for high voltages.
The two resistors in parallel are 20Meg ohms. The maximum input voltage is 20KV.
So, 20,000V at 20,000,000ohms is 2^4 / 2^7 = 1^-3 = 0.001Amps or 1ma to get a full scale movement.

I calibrated the setup against my Fluke HV probe and DMM. I was able to choose a pair of resistors that gave me the most accurate reading ... within a hundred volts or so cold.

Here are some additional pictures of the parts, including the high-voltage connector out of an old copier that I often use for my high-voltage wire connections. All the wire is rated 30KV with silicone insulation:

20Megohm Resistors

Side View

Side View with HV Connector

That's all, Thanks for Looking!
Last updated: July 29,2001