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Protecting O-scope inputs from HV transients...

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Patrick

Fri Feb 04 2011, 12:30AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Since i did not want to skyjack this thread any further "

" I started this one...

Here is where we left off...

Anders M. wrote ...

Patrick wrote ...

Anders M. wrote ...

Patrick, a neon lamp would be excellent here, as long as what you are protecting can stand voltages up to the breakdown voltage of the neon lamp. With a scope, this shouldn't be a problem.

What if I had a 600 Mohm resistor running as a V divider into an oscope, if I wanted to clamp the BNC input at 30 V pk could I use a 72v neon a little higher up on the divider to achieve the 72v, and 30v numbers simultaneously?

With such high impedance (600M) I worry about leakage current a several microamp leak would be perhaps a 10% error. (Hence the desire to avoid zener or TVS's)

Is my reasoning sound? Didnt mean to HiJack this thread, but it is related.

Yeah, if I understand you right, the resistor divider idea should work for getting it to clamp at 30V with a neon lamp, if you put the voltage divider after the neon lamp.

You're right, TVSes wouldn't be much good here, as 30V ones have typical leakage currents of around 5ÂµA according to datasheets.

I found some good news. According to this site

as Resistor , the typical resistance of a neon lamp is in the hundreds of gigaohms range, probably with a lot of variation from lamp to lamp.

I have run the following sim, and will use these screen caps to illustrate.

I want the 30kV wave not to be clipped at the BNC.

I want the clipping to begin at about 33-35 kV. 40kV applied, clipping definately taking place.

Here we see 40 KV applied with the pseudo-neon left open, such that the divider in now unhindered, scope unprotected. (22.5 V - 28.2v = -5.7 V)

So -5.7V of clipping is added to 28.2V to get the wave of 22.5V.

I think I have done all this right, please comment.

Pinky's Brain

Fri Feb 04 2011, 02:39PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

The problem with airgaps is that they are pretty slow.

Also, why use tubes? Surge arrestors cost next to nothing.

Patrick

Fri Feb 04 2011, 03:40PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Pinky's Brain wrote ...

The problem with airgaps is that they are pretty slow.

Also, why use tubes? Surge arrestors cost next to nothing.

Are surge arrestors not tube components ? Surely you dont mean MOV's or TVS's !

Pinky's Brain

Fri Feb 04 2011, 03:59PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Well they are ceramic cylinders, so in that respect they are tubes ... just purpose built to arrest surges as opposed to making light.

James

Fri Feb 04 2011, 04:49PM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

For serious HV stuff an old vacuum tube oscilloscope is not a bad idea. I keep my ancient Tek 531 around for such things. It's huge, heavy, and makes a good space heater but I don't have to worry about frying it with HV.

Patrick

Fri Feb 04 2011, 06:12PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Gas tube surge arrestor....
]littelfuse-src_gdt_cg-cg2.pdf[/file]

Anders an I were discussing the possibility of Neon tubes, that diode was only used because Multisim10 doesnt seen to have NE-2 in its component selection. There is no air gap either.

Pinky's Brain

Sat Feb 05 2011, 01:05AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Air gap, neon gap, nitrogen gap ... all the same, all relatively slow.

Patrick

Sat Feb 05 2011, 05:05AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Pinky's Brain wrote ...

Air gap, neon gap, nitrogen gap ... all the same, all relatively slow.

Shit! really?! Give me an estimate of "slow" please.

Myke

Sat Feb 05 2011, 08:43AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I measured the fall time of the IN-3 neon lamp striking at 80V to be about 2.5Âµs. The fall time for higher voltage neons might be lower though. Normal neons are pretty slow. RC relaxation oscillators with them have a max frequency of about 100kHz. At that frequency, the change in the voltage across the neon is only a couple of volts or so.

Weston

Sat Feb 05 2011, 06:19PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Most scopes have a maximum peak input voltage of several times above the "max" input it says on the scope input. For example, the Rigol DS1052E which many members of 4hv have is CAT I rated to withstand 1000Vpk on the inputs. This means that even "slow" clamping devices should be fine, as long as you don't measure a source with ultra fast rise time, like a marx.

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