# Build a high voltage probe to calibrate a 1.5 kV oscilloscope CRT supply voltage

I want to build a high voltage probe to calibrate my 1.5 kV oscilloscope CRT supply voltage.

Will just use this probe once to calibrate the scope so I don’t want to spend \$200 on a probe and I mind the safety risks that it may involve.

Have seen several project examples. Some use several small resistors in series with capacitors in between. Others just resistors. Other just one HV resistor.

But before building anything I should consider what fits my multimeter DC Input Impedance which is 7.8 Mohm, right?

So what I need is basically a voltage divider capable of reducing the voltage 10 times. A 70 Mohm probe should achieve the 10x attenuation and bring 1.5 kV down to 150 V if I am not wrong.

I am planning to use one 50 Mohm and one 20 Mohm in series. Both high voltage resistors, rated at 1.5 kV and 1/2 W. Not planning to use any capacitors as well.

Frequency wise will I have any issues considering just this one time CRT calibration?

Is it worth a shot?

• The key safety spec is the voltage rating for each R and its actual voltage. You can daisy-chain a string of 10 x 10M 250V resistors with a 10k load and expect 1.500V – Sunnyskyguy EE75 Apr 23 at 20:36
• – Huisman Apr 23 at 21:00
• Are you sure the input resistance of your meter is 7.8MOhm? More commonly it is 10MOhm. – Kevin White Apr 23 at 21:37
• They make high voltage precision resistor dividers that would be well suited for a task like this. – laptop2d Apr 23 at 21:40
• I would use a bigger number of resistors of equal value to keep the voltage at each resistor lower. Check the max voltage ratings of the wiring you're using. You may have to build something to make sure the wires are physically apart. Don't worry about the capacitors if you're simply going to measure the DC voltage with a voltmeter (shunt capacitors are helpful to cancel the effects of the input impedance of oscilloscopes). And never forget: FOLLOW THE APPROPRIATE SAFETY GUIDELINES WHEN WORKING WITH HIGH VOLTAGES! IF YOU DON'T KNOW WHAT YOU'RE DOING, SIMPLY DON'T DO IT! – joribama Apr 24 at 2:09