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Good passive HV scope probe?

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LAN_403

speakerguy79

Tue Mar 17 2009, 12:32AM

Registered Member #1460 Joined: Thu May 01 2008, 12:45AM
Location:
Posts: 19

Hi,

I have a Tek DPO3000 scope. It has an input limit of 300VRMS. Since I will likely be using 338VDC on my SSTC, with spikes and noise and garbage on top of that, I would like a HV probe to keep it 'safe'. Does anyone know of a good HV scope probe that doesn't break the bank? Scope is 300Mhz but the probe doesn't have to go that high, just high enough for a SSTC.

Steve Conner

Tue Mar 17 2009, 10:30AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I have two of the Isoprobe 100:1 probes from Multi-Contact.

Sulaiman

Tue Mar 17 2009, 05:55PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Commercial high voltage 'scope probes aren't too expensive,

Commercial high voltage 'scope probes that can accept high voltage
at high frequency are VERY expensive.

Check the specifications of any HV probe that you intend to buy.

kilovolt

Tue Mar 17 2009, 06:06PM

Registered Member #2018 Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74

I had some time ago a Tektronix P6015 high voltage high frequency probe, but I just rarely used it. Main problem with these probes used together with scopes is the earth grounding of the circuit.

Greets kilovolt

Proud Mary

Tue Mar 17 2009, 06:31PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I've found that there's no keeping company between digital circuits and the more unruly high voltages, and so use a probe of my own invention employing an acorn pentode cathode follower to make a really good firewall that can't be punctured by flukey spikes coming down the line. Gain is about 0.95 up to a few hundred megacycles.

I have some sub-miniature pentodes, and intend to try these out in similar probes when time permits.

tesla500

Tue Mar 17 2009, 07:50PM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

I use these at work and at home, Digkey part # 290-1052-ND. 100x 250MHz 1kV, ~$67 CDN. You can get them cheaper on ebay, ~$75 for 2.

Harry is right about higher voltages, you just can't get good off the shelf probes for measuring very high voltage high frequency signals.

David

Sulaiman

Tue Mar 17 2009, 09:06PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I use a P6015 at home,
although the bandwidth extends flat (2%) to 50 MHz
the voltage rating drops off -6db/octave above 100 kHz.

If you consider the P6015 3pF input capacitance at 20 kVrms 100 kHz
that's 37 mA rms loading !

Proud Mary

Tue Mar 17 2009, 11:09PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Sulaiman wrote ...

If you consider the P6015 3pF input capacitance at 20 kVrms 100 kHz
that's 37 mA rms loading !

A carefully chosen miniature electrometer valve can let you have Cin<250fF with meticulous design - g1 lead out must form the actual probe tip for such a figure to be possible. DC impedances of good electrometer valves can equal or better any solid state solutions of which I'm aware, so long as no effort is spared in attention to detail, but valves will look kindly on very steep high voltage interference transients of the kind that will puncture, I suppose, all but special military 'hardened' FETs.

Steve Conner

Wed Mar 18 2009, 10:35AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Harry, are you suggesting we make our own scope probes out of vacuum tubes? :O How does that solve the 100:1 divider problem?

Proud Mary

Wed Mar 18 2009, 12:53PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve McConner wrote ...

Harry, are you suggesting we make our own scope probes out of vacuum tubes? :O How does that solve the 100:1 divider problem?

There's a place for fringe opinions, even unsound ones, if they are well meant.

From my beginner's perspective, the problems of very high voltage measurement seem so fraught with difficulty as to need, on occasion, equipm,ent created to solve each individual case. Where the obvious solutions, resistive and capacitive dividers, rotary electrostatic force meters, calibrated attenuators and such and are unsuitable one may have no choice but to construct equipment to make a particular measurement.

I'm not so batty as to expect more than polite interest in my antique engineering proposals, which, in any case, I usually present with comedic good grace, but here in 4HV I do have the chance to learn what has happened since the rise and fall of the 807, and am currently somewhere in the 1970s, a great leap forward for me.

I should much rather have my many misunderstandings of the art pointed out in common honour than be left with misconceptions, and poorly thought out analyses that provide no exercise for synapses heading towards Omega.

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