Re: Extend the total voltage input on a oscilloscope
Bjørn, Fri Mar 23 2007, 03:10PM

Have a look at the third picture for a very accurate circuit: http://www.probemaster.com/helpful.htm

If you simply connect the resistor in series with the input you will extend the DC voltage range by approximately 10 times if you consider 10% error to be good enough. You can always calibrate your oscilloscope to show the correct value. When it comes to AC it depends on the resistor and the frequency how well it will work. Some resistors are remarkably inductive.

Re: Extend the total voltage input on a oscilloscope
Steve Conner, Fri Mar 23 2007, 03:54PM

I like these 100:1 HV scope probes:

Re: Extend the total voltage input on a oscilloscope
Hazmatt_(The Underdog), Fri Mar 23 2007, 06:39PM

Digging up another prehistoric thread:

Vmax = 40KVDC from terminal to gnd.
The adjustable pots allow the divisor to be anywhere from 1000:1 to 1,000,000:1, and if you want to meter a TC while operational, you need a really high division ratio for when its ringing because it went way off scale for me at even 1000:1.

Re: Extend the total voltage input on a oscilloscope
ragnar, Sat Mar 24 2007, 05:55AM

Hmm, I've a question that's appropriate here.

According to the reference manual on my PM3092, the scope's isolation is frequency depedent -- that means it can take 500V on the input at DC and audio frequencies.. but at 10MHz, it will only take about 10V.

Does that really mean I'm supposed to use a 100:1 divider probe, such that the scope is exposed to no more than 5V at this frequency? Won't the divider stuff my waveforms around?

Re: Extend the total voltage input on a oscilloscope
Fusion, Mon Mar 26 2007, 01:22PM

If you need a high frequency measure you can use a capacitor divider instead of resistor divider.
I am going to use resistors in parllel with capacitors in order to have DC and AC response

Re: Extend the total voltage input on a oscilloscope
HV Enthusiast, Sat Mar 31 2007, 02:16AM

Make sure you add a compensation network as well at the scope input end. You'll need to compensate the network to get flat response across the measurement band.

Re: Extend the total voltage input on a oscilloscope
Fusion, Mon Apr 02 2007, 11:07AM

Ok, I have simulated now, thanks:

Re: Extend the total voltage input on a oscilloscope
Steve Conner, Mon Apr 02 2007, 11:54AM

347uF?! Don't you think that's a little high for a probe input capacitance? The last X100 probe I got was about 5pF.

Re: Extend the total voltage input on a oscilloscope
Fusion, Mon Apr 02 2007, 12:55PM

Yes it is very big, that is due I used one of the compensating resistor of a serial-parallel capacitor bank as sensing resistor. I supose you can scale it down if capacitances are well over 5pF scope input and resistance well under 1Mohm scope input.

Re: Extend the total voltage input on a oscilloscope
Sulaiman, Mon Apr 02 2007, 08:03PM

I think you should rethink the resistor values so that the input resistance is as high as possible
e.g. for x100 use 99Mohm in series with the 1MOhm 'scope input resistance
Use a high-voltage 0.5pF capacitor in parallel with th 99MOhm resistor,
and a 50pF variable capacitor across the 'scope input.

Usually we want a 'scope probe to load the circuit being monitored as little as possible
Imagine the impedance of a 347pF capacitor at 10MHz (45.866 Ohm)
Use several low voltage resistors in series to make similar high-voltage high-resistance.
(e.g. I have 45 x 2.2MOhm in series for 99MOhm.)
This also allows several cheaper low-voltage capacitors to be used
(e.g. 45 x 10pF)
and produces a resistor long enough to ensure no flashover.

Re: Extend the total voltage input on a oscilloscope
Fusion, Tue Apr 03 2007, 09:56AM

I simulated it and was good also (scaled down waveform rises and falls within 1ns with measured signal). But 1Mohm catch more noise from environment.

I have seen the scope probe specs and said 10Mohm and 9.5pF (the older was 1Mohm 4.5pF), but I tried measure it and is infinite, so I do not know exactly the impedance. Anyway is better measure 400V with a multimeter and compare.

Re: Extend the total voltage input on a oscilloscope
Sulaiman, Tue Apr 03 2007, 02:28PM

My HP High-Voltage-Probe has a 'gain' of x1000 with 100 MOhm and 40 kV maximum input
roughly it has two stages, 100:1 and 10:1
giving a source impedance as seen by the 'scope of about 100 kOhm
and several adjustable potentiometers and trimer capacitors for gain and frequency response adjustment
I haven't got or traced the actual circuit so I'm not sure of the exact circuit.

Re: Extend the total voltage input on a oscilloscope
Fusion, Tue Apr 03 2007, 04:06PM

I have modified the parallel capacitor at simulation from 0.096pF to 0.056 and signal felt to half!

So I think your probe must be too sensible to flat metal surfaces close to test point.

I think I have the solution: if you set a parallel capacitor with scope probe input of 100nF and then with 990Meg of 1nF and then I have a good response. Now the scope probe is less sensible to other capacitances:

Re: Extend the total voltage input on a oscilloscope
Sulaiman, Tue Apr 03 2007, 10:14PM

My homemade probe is actually for dc, it IS a little sensitive to nearby objects

I use my Tektronics probe for hv-hf
The majority of the 100 MOhm resistor is shielded inside an earthed metal tube
only the first 1 inch or so is exposed.

P.S. the probe originally had CFC gas filling, I now use white spirit vapour - seems ok

Re: Extend the total voltage input on a oscilloscope
Steve Conner, Sun Apr 08 2007, 10:34AM

Sulaiman, what model of Tek HV probe do you have? Does it mind being filled with white spirit?

Re: Extend the total voltage input on a oscilloscope
Sulaiman, Sun Apr 08 2007, 10:45AM

Steve - It's a P6015 - very old.
I don't know if it 'minds', but I've had white spirit in there for years with no leakage.
It's not full, about 10cc of liquid only, the rest is vapor.
I store it at all angles and so far the seal seems good
and the voltage standoff is definitely better than just air.

For others - the old voltage probes used to have CFC gas/liquid for insulation/arc-suppression.

Re: Extend the total voltage input on a oscilloscope
Steve Conner, Sun Apr 08 2007, 09:05PM

Wow, the P6015 is a serious high voltage probe. 40kV and 75MHz, IIRC Terry Fritz has one, and once he ran out of freon, he filled it completely with mineral oil and readjusted all the trimmers on the compensation box to get it working again. I'd like one, but they seem very expensive even for an old beat-up example.

PS: Terry said that his arced over at about 22kV when filled with air, IIRC. How much better do you get than that with white spirit vapour? Aren't you worried about air getting in it and making the vapour catch fire if it arcs?

If white spirit vapour is a good insulator, why aren't we using it in our HV projects? I have seen old textbooks recommend carbon tetrachloride vapour, probably because it's not flammable, but that's probably not too good for you...

Re: Extend the total voltage input on a oscilloscope
Sulaiman, Sun Apr 08 2007, 10:36PM

My hv probe had been lying in it's box unused for years as I'd run out of the CFC and didn't buy any replacement,
I wanted to 'scope the output of an ignition coil I was playing with, so I dug out the old probe
it arc'd-over somewhere above 20 kV. Thought I'd ruined the probe but it was ok.
The only likely fluid that I had to hand was white spirit - so, hoping for no ill effects, I used it.
Worked ok up to the 30 kV region with no problems, so I left it like that.
(the other idea I had was butane lighter-refill but I thought it too flammable)

I was impatient so I'm sure there's still air in the probe - so far no explosions!

I may try butane one day (if you don't hear from me again you'll know what happened)
but I like the idea of completely filling with mineral oil - may try that also.

It is a lovely probe - good even at low voltages due to 100 MOhm//3pF input.

For low-frequencies I use a string of 45 x 2.2 MOhm metal-film resistors (VR37-type) with 1 MOhm load
(no special reason for the quantity/value - I just happen to have 1000 pieces form an eBay purchase)
For low frequencies and up to 10 kV I use 3 x 33 MOhm resistors (VR37-type)
Incidentally the VR37are commercially rated for 3500V, but only 250V for safety-critical - so my probe isn't safety-critical