Forums

DIY CRO Protection HV Probe

Started by Unknown March 25, 2014
I have been experimenting with HV circuits that generate up to 10KVAC
within the audio range. The usual oscillator and step-up transformer
configurations.

I would like to check the output waveforms on my digital CRO but don't
have a suitable HV probe, and would prefer not to spend $100+ for this
one time use.

Is there some arrangment of passive components I can wire between the
CRO and probe (set at 10x) to protect the input stage?

I am only interested in an approximation of the HV signal, not a dead
accurate voltage or waveform.

Any suggestions?

Kevin Grant
On a sunny day (Tue, 25 Mar 2014 17:53:57 +1100) it happened kgrant@protec.com
wrote in <ac92j91r63ri1hod0ouerkp2k5nimof5lb@4ax.com>:

> >I have been experimenting with HV circuits that generate up to 10KVAC >within the audio range. The usual oscillator and step-up transformer >configurations. > >I would like to check the output waveforms on my digital CRO but don't >have a suitable HV probe, and would prefer not to spend $100+ for this >one time use. > >Is there some arrangment of passive components I can wire between the >CRO and probe (set at 10x) to protect the input stage? > >I am only interested in an approximation of the HV signal, not a dead >accurate voltage or waveform. > >Any suggestions?
Look how ididit here: http://panteltje.com/panteltje/pic/sc_pic/sc_pic_pcb_test_in_box_img_2452.jpg Just a buch of resitors in series, voltage divider. Thats is not quite 10kV, but the idea is the same. look for maximum flash over spec for such a resistor, and use at least x times that in series. Perhaps put it in a pvc tube (electrickity tube) and thats it. <disclaimer> If you blow up you 10,000,00,000 cents scope I havenotdunnit.
On a sunny day (Tue, 25 Mar 2014 07:56:04 GMT) it happened Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote in <lgrcvs$vns$1@news.albasani.net>:

>On a sunny day (Tue, 25 Mar 2014 17:53:57 +1100) it happened kgrant@protec.com >wrote in <ac92j91r63ri1hod0ouerkp2k5nimof5lb@4ax.com>: > >> >>I have been experimenting with HV circuits that generate up to 10KVAC >>within the audio range. The usual oscillator and step-up transformer >>configurations. >> >>I would like to check the output waveforms on my digital CRO but don't >>have a suitable HV probe, and would prefer not to spend $100+ for this >>one time use. >> >>Is there some arrangment of passive components I can wire between the >>CRO and probe (set at 10x) to protect the input stage? >> >>I am only interested in an approximation of the HV signal, not a dead >>accurate voltage or waveform. >> >>Any suggestions? > >Look how ididit here: > http://panteltje.com/panteltje/pic/sc_pic/sc_pic_pcb_test_in_box_img_2452.jpg >Just a buch of resitors in series, voltage divider. >Thats is not quite 10kV, but the idea is the same. >look for maximum flash over spec for such a resistor, >and use at least x times that in series. >Perhaps put it in a pvc tube (electrickity tube) and thats it. > ><disclaimer> >If you blow up you 10,000,00,000 cents scope I havenotdunnit.
BTW do NOT use you existing probe directly, this goes between HV and probe, and use a low value to ground: scope probe ------------R1 - R2 - R3 - ... - Rn ---> object under test 10x | [ ] 10k | /// If you are still paranoid add a zener over the 10k, or for AC 2 zeners in series reversed, or better some NEON bulb, or whatever.. If you are really working with high power HV that can do real lethal currents get a real HV probe, If you die from it ihavenotdunnit. All this assumes you have some xperience with electrons. If no task somebody who has... Oh that was what you were doing, hell should add more disclaimers. Whataworld Maybe better go sunbathing, oh no, the radiation may kill too. Go to sleep? x
On Tue, 25 Mar 2014 17:53:57 +1100, kgrant@protec.com wrote as
underneath :

Try for info
http://www.youtube.com/watch?v=jUvSP3BQpvs&feature=player_detailpage
http://www.youtube.com/watch?feature=player_detailpage&v=RMuBWuI6Lb8
http://www.youtube.com/watch?v=f1fCPDno224&feature=player_detailpage

>I have been experimenting with HV circuits that generate up to 10KVAC >within the audio range. The usual oscillator and step-up transformer >configurations. > >I would like to check the output waveforms on my digital CRO but don't >have a suitable HV probe, and would prefer not to spend $100+ for this >one time use. > >Is there some arrangment of passive components I can wire between the >CRO and probe (set at 10x) to protect the input stage? > >I am only interested in an approximation of the HV signal, not a dead >accurate voltage or waveform. > >Any suggestions? > >Kevin Grant
If you just want wave *form*, and can do without amplitude, just hold a 
normal 10x probe somewhere in the vicinity.  It'll AC-couple in just fine. 
Or put a single turn sense winding on the transformer.

Making a wideband probe is tricky.  If you use a chain of resistors, the 
capacitances from each resistor to free space (and to each other, and to 
whatever kind of shielding you have nearby) screws with compensation in a 
complicated manner.  You can get a step response that's more-or-less 
correct, but has odd risey-dippy behavior in the 1-100kHz range.  It's 
only give or take a dB or so, but it really sucks for a probe.

You can attempt to dominate parasitic capacitances with a bulk capacitance 
(for this kind of ratio, really just having a high voltage wire near the 
divider node -- a gimmick capacitor), but you can quickly run into 
unfavorable ratios: say, needing 10nF to ground to compensate it -- and no 
way to adjust it, because you can't just buy 10nF trimmer caps...cheaply, 
anyway.

The pros do it by keeping things small -- under oil if possible, like the 
sealed Tek (6026??!?) probe, which uses a small resistor and a guard ring 
/ shield / capacitance hat to help with compensation.

I've done it before for lower voltages (~2kV) by stacking chip resistors 
and capacitors, say 200V each.  That's about as much as you'd want to risk 
with them.  Still needs a big compensation cap, and you can't vary AC gain 
because of it.  You'd want a better approach for 10kV.

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

<kgrant@protec.com> wrote in message 
news:ac92j91r63ri1hod0ouerkp2k5nimof5lb@4ax.com...
> > I have been experimenting with HV circuits that generate up to 10KVAC > within the audio range. The usual oscillator and step-up transformer > configurations. > > I would like to check the output waveforms on my digital CRO but don't > have a suitable HV probe, and would prefer not to spend $100+ for this > one time use. > > Is there some arrangment of passive components I can wire between the > CRO and probe (set at 10x) to protect the input stage? > > I am only interested in an approximation of the HV signal, not a dead > accurate voltage or waveform. > > Any suggestions? > > Kevin Grant