Voltage rating of Agilent N2772A diff probe

[treez]

Can i just check i can probe an offline flyback drain going up to 850V with this diff probe..?
Agilent N2772A

N2772A 20 MHz Differential Probe [Obsolete]

This product is replaced by N2791A 25 MHz differential probe.

www.keysight.com

...aah heres the previous post i had been seeking prior to my post. For some reason it never surfaced till i actually posted

https://www.edaboard.com/threads/diff-probe-for-offline-flyback-drain-which-is-up-to-850vpk.396721/

--- Updated Apr 8, 2021 ---

The 600V CAT III bit is just referring to the tranient peaks (6kv) that it may go up to.

 

[treez]

Hi,
I have just seen 690Vpk on a offline flyback prototype drain voltage. (Seen with this Agilent N2772A diff probe into a Agilent DSO-x 3014A scope).
Do you think this probe may give a higher reading than there really is, due to the ~4MHz ringing frequency, and the microsecond peak just being too short a time and the N2772A reporting a higher voltage spike than there really is?

I mean, i am going to see how accurate this diff probe is now by probing the raw mains with it.......if i see 339Vpk then that means N2772A is accurate...but does it?...the mains is a much slower, less peaky waveform.

--- Updated Apr 8, 2021 ---

The scope (on dc coupled) with the N2772A on 200:1 (into DSO3014A scope) is reading mains peaks as +325vpk and -332vpk (on AC coupled its +333vpk, -342vpk)
Fluke 70 III DMM reads mains as 237VAC...that means 335vpk.........so the N2772A/DSO3014A is pretty accurate(?), and i therefore believe that the 690Vpk drain voltage spike peak (seen with N2772A/DSO3014A) is pretty accurate...would you agree?

Is this rationale good?

 

[Easy peasy]

It says 700V differential, so higher is out of spec, likely it wouldn't blow up at 850V or a kV - but you never know ...

--- Updated Apr 9, 2021 ---

it's only 20MHz though ...

 

[treez]

Thanks,
theres some doubt
in this post, it had been said it was good to 1kV and beyond

https://www.edaboard.com/threads/diff-probe-for-offline-flyback-drain-which-is-up-to-850vpk.396721/

The datasheet does say its OK to 1kV.
The "600v" thing i assumed was referring to the "600v CAT III" standard, which just means spike voltages can go up to 6kV?

 

[treez]

Woops, no it is literally only rated for 600Vpk....the 1000v was a differential measurement for eg +500V to -500V
Heck, i dont know how i fell for this.
I was looking at peak voltage up to 880Vpk (to ground).....the peak was only for 100ns every 30us, so i hope the probe will not be damaged?. Please confirm?

--- Updated Apr 9, 2021 ---

Although on page 14 of the N2772A diff probe user manual it says for calibration purposes, a +/-1100v dc source is required.......so they obviosly connect it up to 1100v to ground during calibration.....so why not at other times aswell?

N2772A users manual:

N2772A Differential Voltage Probe - PDF Free Download

99 Washington Street Melrose, MA Phone Toll Free Visit us at Back to the Agilent N2772A Product Page User s Guide sa Publication number N

docplayer.net

 

[FvM]

Woops, no it is literally only rated for 600Vpk

Completely wrong. It's 1000V CATII, or 1000Vrms continuously at least.

 

[treez]

Thanks, but isnt the 1000V like +500v to -500v (both voltages related to ground).
The user manual appears to say this.
It says its for use with an scope which has earth gnd for the gnd terminal.

it's only 20MHz though ...

Thanks, yes, please advise on your statement, since the drain node ring here is only 2.7MHz.
Are you referring to the rise time being only 0.35/20MHz = 17.5ns?
...to be honest, the drain node rise time is probably about 17.5ns, and certainly no less, so we should be ok?

--- Updated Apr 9, 2021 ---

Look at the datasheet for this diff probe here (TA041)

https://www.farnell.com/datasheets/2207207.pdf

...Absolutely no doubt its ok up to 1400v...and that to earth......gold star for these guys....leave the punters in no doubt.

 

[FvM]

You need to distinguish between voltage that can be measured without clipping and absolute maximum, which should not damage the probe but can not necessarily be measured correctly. According to datasheets, both probes can measure your 850 V signal. I won't be sure about exact waveform reproduction with either probe.

 

[treez]

According to datasheets, both probes can measure your 850 V signal. I won't be sure about exact waveform reproduction with either probe.

Thanks, do you think they would represent the voltage as being.....
a)...higher than it really is?
b)...lower than it really is?

If (b), then its not so bad

 

[FvM]

I won't expect higher reading with 4 MHz ringing frequency. If you see ringing at 15 MHz and above, you should consider that the real voltage may be higher than indicated.

 

[Easy peasy]

At 20MHz you are already 3dB down - almost all scope probes also have a roll off in freq with voltage - so a 100x probe ( passive ) has a better bandwidth at 1vrms compared with measuring a 700V flyback turn off - in effect it filters the true signal and you don't quite see the full peaks and / or ringing associated - have a good read of a good probe data sheet ....

 

[treez]

Hi,
Just to see how the N2772A diff probe would manage, i probed a drain voltage of a flyback converter with it on 200:1. The ringing voltage at the switching node obviously went up as i increased the output load of the flyback. However, after the drain got up to 800V, then any further increase in the load saw the drain voltage getting distorted and no longer sinusoidal. (as attached) It was almost as if the 650V rated FET was avalanching at 800V. ..but it couldnt have been that as the FET wasnt getting particularly hot. So was it the N2772A's inability to correctly depict waveforms with peak voltages above 800V?

 

[FvM]

If it's begin of avalanching, it won't neccessarily cause much heating.