High-voltage, low-capacitance probe.

[Ranganok]

Hello everyone,

For a project, we need to measure the output of a commercial device that is no longer manufactured ( ). In theory, it should produce a step-like output from 0 to 1200V in less than 30us. However, when using the differential probe (PICO TA042), the output observed on the oscilloscope is 1000V and 50us (approx.)

I assumed that the probe’s impedance was overloading the device’s output, so I was looking for a probe with very low input capacitance (the device is grounded, so I suppose the probe doesn’t need to be differential).

The other option is to design the probe myself, but it surely has many flaws that I haven’t noticed  .

Can anyone help me?

Thanks.

P.S.: It’s not very clear in the image, but the 1G\$\Omega\$  resistor would be the input, and the 1M\$\Omega\$  resistor would be the measurement point.

[PartialDischarge]

Don't waste your time doing this, that 1GOhm resistor needs capacitive compensation to provide a nice step response which is another problem in itself (a big one)

[mtwieg]

It's possible that the source has a large output impedance. 800kohm would explain measuring 1000V with the TA042 (input impedance of 4Mohm).

Instead of building an entirely new probe, I would try and test that theory by measuring the output with additional resistance added. For example, add an additional 4Mohm between the probe and the DUT. If its output impedance is actually 800Kohm, then you should measure 545V.

What is this device meant to drive with this high voltage output?

[Ranganok]

Don't waste your time doing this, that 1GOhm resistor needs capacitive compensation to provide a nice step response which is another problem in itself (a big one)

You're right, but I can't find any sensors capable of measuring this, so I have to deal with all the problems myself.

The problem I see with connecting a capacitor in parallel with the 1GOhm resistor is that I’ll increase the input capacitance (and that parameter is already very tight). Even if I split the 1GOhm resistor into several 250MOhm resistors (4) and add a small capacitor to compensate for them, I’ll still be adding too much input capacitance to the probe.

It's possible that the source has a large output impedance. 800kohm would explain measuring 1000V with the TA042 (input impedance of 4Mohm).

Instead of building an entirely new probe, I would try and test that theory by measuring the output with additional resistance added. For example, add an additional 4Mohm between the probe and the DUT. If its output impedance is actually 800Kohm, then you should measure 545V.

What is this device meant to drive with this high voltage output?

The device measures how fabrics behave in electric fields (for safety clothing), so it doesn't power anything. But we have to make sure (somehow) that the device generates that pulse.

Thxs,

[MariuszD]

Do you need your probe to transmit DC? Making a divider with a large resistor compensated over a wide frequency range is difficult. Making a capacitive divider is easy.

[Ranganok]

Do you need your probe to transmit DC? Making a divider with a large resistor compensated over a wide frequency range is difficult. Making a capacitive divider is easy.

What I need to verify is the rise time of the signal. In other words, the signal reaches 1200 V in less than 30 μs.

[mtwieg]

You're right, but I can't find any sensors capable of measuring this, so I have to deal with all the problems myself.

The problem I see with connecting a capacitor in parallel with the 1GOhm resistor is that I’ll increase the input capacitance (and that parameter is already very tight). Even if I split the 1GOhm resistor into several 250MOhm resistors (4) and add a small capacitor to compensate for them, I’ll still be adding too much input capacitance to the probe.

If you want to preserve the rise/fall time of the pulse, there's no way around the compensation capacitor. The overall input capacitance should still be very low.

btw your schematic makes no sense to me. There are no 1G/1M resistors...

The device measures how fabrics behave in electric fields (for safety clothing), so it doesn't power anything.

It has to have some sort of drive capability, otherwise it effectively doesn't even exist...

[Ranganok]

If you want to preserve the rise/fall time of the pulse, there's no way around the compensation capacitor. The overall input capacitance should still be very low.

I see I’ll have to deal with the compensation capacitors.

btw your schematic makes no sense to me. There are no 1G/1M resistors...

Oh, the 1GOhm and 1MOhm resistors are air-mounted (to try to reduce parasitic capacitance), so in the schematic they’re shown as test points with a small label. I see that's a bit unclear.

The device measures how fabrics behave in electric fields (for safety clothing), so it doesn't power anything.

It has to have some sort of drive capability, otherwise it effectively doesn't even exist...

What I meant was that the device (they won't let me open it) is both the driver and the sensor. It generates the 1200V pulse and reads the fabric's response itself.

[2N3055]

You might have more problems here with 4MΩ load than with 7 pF capacitance with Pico probe.

Try Testec TT-HV 200 x100 passive probe, if signal is single ended..
It will present 100MΩ||5pf load to DUT.

If that does not work you are going to have hard time measuring that..

[jonpaul]

Avoid ICs to measure HV. Read the Tektronix Circuits Concepts books for 1980s,

"Oscilloscope Probe Circuits and
Oscilloscope probe measurements.


https://www.davmar.org/concepts.html

We use Tek P6015A x1000 to 30 kV, 50 MHz BW.


Jon

[MariuszD]

If you want to preserve the rise/fall time of the pulse, there's no way around the compensation capacitor. The overall input capacitance should still be very low.

I see I’ll have to deal with the compensation capacitors.

You can't get rid of the capacitor, but you can get rid of the resistor by making a capacitive divider. You only need a capacitor with a small capacitance and high insulation resistance; such a capacitor is easy to make.

Below is an example of how to make such a capacitor.

[jonpaul]

capacitive HV probe will not show the DC component and has a highpass breakpt depending on the load.

[ballsystemlord]

I've kept an up-to-date list of the various HV single-ended probes on the market.

It doesn't list the probe capacitance, but the price, and datasheets are all listed as are the BW and voltage which fit into several columns as I analyze the probes using multiple BWs/voltages.

Let us know how it works out.