High Voltage Test Box is creating spikes that keeps destroying meters

[rwcarter1109]

(Please note: I must keep details about this device to a minimum for the discretion of our client)

I am working on a High Voltage Test Box that uses a HVPS to supply 15kV to a set of resistors. Using a voltage divider, the HV across the resistors is measured using a digital meter that is powered by a 12V DC PSU. The meter will display 10mV per 1KV. The maximum range of the meter is 200mV (thus it can measure up 20kV).

The box has two HV inputs and two HV outputs that are on separate channels. The channels are both operated using a Castel key which commands a 240V AC Ross relay for each channel. When the Castel key is turned off, the relays dump the high voltage to ground to ensure there is no high voltage on the outputs. Turning off the Castel key also turns off the HVPS.

Using a Tektronix scope, I can capture a series of spikes on the input of the 12V DC meters. The scope is set to capture a rising edge at a time base of 10us. With no HV on the channels, the spikes will achieve a max of 23V (11V higher than the nominal input) and with the 15kV on the channels, the spikes will reach around 50V max (over 4 times the nominal input!!!)

I have fitted snubbers across the power inputs of the relays, as well as setting up a system of diodes and capacitors to clamp the voltage going to the meters I have also made sure the high voltage, DC and AC conductors are well separated from each other and all earth are individually going to the main earth nut. I have attached an image of the diode/cap circuit.

Does anyone have any suggestions as to what may be causing these spikes and what I can do to stop them occurring? These spikes keep blowing the meters, and replacing them is starting to become expensive... Thanks, Russell.

 

[MacGyverS2000]

So with no high voltage, you're still seeing voltage spikes? Seems like it would be relatively simple to track down the cause now that the HV is out of the picture. Or maybe I'm not quite understanding what you're seeing...

Also, when you say "dump to ground", are you switching in resistors? If you're just connecting HV lines to ground, expect some serious EMI issues on closely-spaced lines.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

An oscillograph image would be particularly useful, I think

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[Comcokid]

Your meter may have an A/D with a switch-capacitor structure on the input. Such ADC types, like some sigma-delta types (popular in meters because the high resolution) pull current pulses on their input as they connect their input and charge an input sampling capacitor. With the capacitor and diode ladder structure you have, you may inadvertently be creating a charge pumping structure which is magnifying these pulses (just a quick look hunch, and not any in depth analysis by me).

I would research the subject some (google, and look for app notes) or take apart one of your failed meters, determine the ADC used, and look at the app notes and data sheet. Usually to reduce the input switching noise on these converters, and R/C network is used, consisting of two R's (one one each of the differential inputs) and then a C to smooth out the pulses. Generally the R is in the range of 10 to 100 Ohms.

 

[Skogsgurra]

I don't know how Ross relays are built today. But if they are anything like they used to be ten or twenty years ago, there is a solenoid and a plunger with a contact piece that connects two terminals when activated. The travel is long and there is a spread in the make/break times.

You mention that you use a differential arrangement where two Ross relays switch positive and negative potential and it seems that you have assumed that both relays have identical characteristics. They have not. There is at least always a spread in closing and opening time in the 100 us range and it can be as high as one millisecond.

A millisecond is a long time and the best way to see if your circuit can handle that is to deactivate one Ross relay and switch on the other. If you meter survives that - then you are fine and can look for other causes. If it dies - you have to use another voltage divider configuration.

Gunnar Englund

http://www.gke.org

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[MarvintheMartian]

The capacitance of the voltage divider may be factor in passing large spikes of voltage through to the meter input. You may need to add capacitance to ground on the meter input so that the divider properly divides both DC and fast AC transients. This is very similar to adjusting an oscilloscope probe to obtain a proper edge response.