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Products => Test Equipment => Topic started by: GlennSprigg on April 04, 2020, 12:54:33 pm

Title: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: GlennSprigg on April 04, 2020, 12:54:33 pm
Firstly, this is not a re-run of the 2014 post in this forum about measuring 240v A/C on a scope!!  I understand about the scopes limitations, and the applied voltages peak-to-peak values, where 100x would be better than 10x etc. etc. I'm asking about the PROBES themselves!  :)

I use/have only switchable probes, (1x & 10x), and if one is measuring say up to 50v using a 10x probe then that's all sweet!  But just as Dave knocks cheap <$20 Multimeters, (rightly so!), when measuring say 240v to 500v as there is little protection, and the Meter may end up like "a Dingo's guts" etc., I can't help thinking about the PROBES internals as well, as this is rarely mentioned?   :(

If you have say 200v to 500v, (and not a low-power signal!), would/could cheaper probes break down internally, destroying not only the Probe itself, but then what it may pass on to the actual Scope too!!  I've never looked inside such Probes to see what they do??

AS A BIG SIDE NOTE !!  hahaha...
A "Mr Carlson's Lab" video from 2018, was discussing a very old 1930 'National' Oscilloscope. I couldn't believe the absolutely minimalist circuitry, and lack of components!!  Vertical scaling/voltage? no problem!! Hard wired input straight to the vertical deflection plate? Horizontal Oscillator not even a sawtooth, but deflectors almost directly connected to a low voltage A/C tap on the main transformer!   Wow!!  Check out the Schematic below, starting at 26:44 in the video...   8)
https://www.youtube.com/watch?v=NKbE5jcLF3U (https://www.youtube.com/watch?v=NKbE5jcLF3U)
Title: Re: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: GlennSprigg on April 04, 2020, 01:34:33 pm
Hmmm.  I posted this accidentally in 'TestGear', but meant to post it in 'GeneralChat'.
I moved it, but was moved back(?) by gnif. I had a reason for wanting it in 'General'  :)
Title: Re: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: blueskull on April 04, 2020, 01:44:15 pm
Probes are generally rated at very low CAT ratings. If a 100x probe is rated 1kV, chances are that is CAT I 1kv, meaning no overvoltage protection capability at all.
If you tap a CAT II 240V circuit, under the slightest chance, it can blow up. FYI, CAT II 300V class is rated for up to 2.5kV transient.

Also, always RTFM when probing high voltage. Probes have maximum capacitance dissipation limit, and their marketing material might not be accurate.
For instance, Keysight 10076C (my usual choice for single ended HV probing) is marketed as a 4kV probe, but it is only rated for 3.7kVpk, and CAT II 1kVpk.
At DC, it withstands up to 3.7kVpk, and at 1MHz, it withstands only 1kVpk. This is a thermal limit, and if you only view a few pulses, you can ignore it (AFAIK, and AFAID).

When in doubt, go with a differential probe. If you have loads of money, go for an isolated probe or isolated scope or at least a portable floating scope.
And lastly, stay away from Keysight U1600 series. The isolation is only galvanic for LF, and for HF is uses differential caps, making it horribly unreliable for measuring high dv/dt signals.
Title: Re: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: Grandchuck on April 04, 2020, 01:46:34 pm
This reminds me of an oscilloscope built by my father in the early 1950's.  It used a motor, loudspeaker and a light source.  Similar to this:  https://youtu.be/oeGdQRQBeiw

He used a 16 mm movie projector for the light source.
Title: Re: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: 2N3055 on April 04, 2020, 01:58:44 pm
You should read "ABC of probes" by Tektronix, and also "Tektronix Oscilloscope Probe Circuits Concepts".
Title: Re: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: vk6zgo on April 04, 2020, 02:27:26 pm
Firstly, this is not a re-run of the 2014 post in this forum about measuring 240v A/C on a scope!!  I understand about the scopes limitations, and the applied voltages peak-to-peak values, where 100x would be better than 10x etc. etc. I'm asking about the PROBES themselves!  :)

I use/have only switchable probes, (1x & 10x), and if one is measuring say up to 50v using a 10x probe then that's all sweet!  But just as Dave knocks cheap <$20 Multimeters, (rightly so!), when measuring say 240v to 500v as there is little protection, and the Meter may end up like "a Dingo's guts" etc., I can't help thinking about the PROBES internals as well, as this is rarely mentioned?   :(

If you have say 200v to 500v, (and not a low-power signal!), would/could cheaper probes break down internally, destroying not only the Probe itself, but then what it may pass on to the actual Scope too!!  I've never looked inside such Probes to see what they do??

AS A BIG SIDE NOTE !!  hahaha...
A "Mr Carlson's Lab" video from 2018, was discussing a very old 1930 'National' Oscilloscope. I couldn't believe the absolutely minimalist circuitry, and lack of components!!  Vertical scaling/voltage? no problem!! Hard wired input straight to the vertical deflection plate? Horizontal Oscillator not even a sawtooth, but deflectors almost directly connected to a low voltage A/C tap on the main transformer!   Wow!!  Check out the Schematic below, starting at 26:44 in the video...   8)
https://www.youtube.com/watch?v=NKbE5jcLF3U (https://www.youtube.com/watch?v=NKbE5jcLF3U)

There are high voltages & high voltages!

At about 2:30 in the morning, we were trying to find out where the drive signal from the previous stage prior to the PA had disappeared to on a MF AM Broadcast transmitter.

I reasoned that, if I wound the RF drive right back, I should be able to see the drive at the grid connection to the PA, using a 'scope & a x10 probe.

Setting it up, I turned the Tx on,

"Kerblam!"& the transmitter shut down!
Opened the cabinet, & everything past the finger guard on the probe had vaporised.
Oops!----- maybe winding the RF drive "right back"didn't quite do what I thought!

Doh!
In fairness, though, we both had finished a normal full days work, driven 300+ km, & it was 2:30am ("Stupid O'clock", as another workmate called it).

With great trepidation, I approached the input circuitry of the 'scope (a Tek 453 from memory) only to find no damage------ The probe had "given its all " & protected the 'scope!

Re the sidenote:-

I remember many years back, the old Perth Tech College had an "open day"where they had a lot of weird stuff up & running, one of which was a "homebrew" version of just such an Oscilloscope, built in a perspex box, so you could see the guts.
(Now that was a real "Tech School", with lots of real stuff--not like this "TAFE" nonsense!)

My 1976 ARRL Handbook has a schematic & a description of the same sort of device. although it wasn't presented as a "project".

Apparently, the idea is that the mid portion of a sine wave is "quasi-linear", so it can be used for horizontal deflection.
Title: Re: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: rstofer on April 04, 2020, 03:54:18 pm
I remember many years back, the old Perth Tech College had an "open day"where they had a lot of weird stuff up & running, one of which was a "homebrew" version of just such an Oscilloscope, built in a perspex box, so you could see the guts.
(Now that was a real "Tech School", with lots of real stuff--not like this "TAFE" nonsense!)

My 1976 ARRL Handbook has a schematic & a description of the same sort of device. although it wasn't presented as a "project".

Apparently, the idea is that the mid portion of a sine wave is "quasi-linear", so it can be used for horizontal deflection.

A scope was in the ARRL Handbook far earlier than that!  I built such a scope somewhere around 1958 (could have been a couple of years later, give or take) and it was a terrific project for a kid.
Title: Re: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: joeqsmith on April 04, 2020, 06:07:52 pm
Quote
If you have say 200v to 500v, (and not a low-power signal!), would/could cheaper probes break down internally, destroying not only the Probe itself, but then what it may pass on to the actual Scope too!!  I've never looked inside such Probes to see what they do??

I had posted the internals of a very low cost 100X probe.  You may find that here:  https://youtu.be/v1jJ1Z_fgFU?t=518    I'm not sure what "not low-power" means.  I'm not aware of any of these smaller probes having any sort of safety rating.  I doubt you will find safety rating on some of the larger probes you would typically use for electronics.   


Quote from: rstofer link=topic=7=236122.msg2998752#msg2998752 date=1586010446
I remember many years back, the old Perth Tech College had an "open day"where they had a lot of weird stuff up & running, one of which was a "homebrew" version of just such an Oscilloscope, built in a perspex box, so you could see the guts.
(Now that was a real "Tech School", with lots of real stuff--not like this "TAFE" nonsense!)

My 1976 ARRL Handbook has a schematic & a description of the same sort of device. although it wasn't presented as a "project".

Apparently, the idea is that the mid portion of a sine wave is "quasi-linear", so it can be used for horizontal deflection.

A scope was in the ARRL Handbook far earlier than that!  I built such a scope somewhere around 1958 (could have been a couple of years later, give or take) and it was a terrific project for a kid.
[/quote]

Enjoy some old pictures....
Title: Re: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: tggzzz on April 04, 2020, 08:37:49 pm
Always look at how a probe's maximum voltage depends on frequency. Get it wrong and the probe melts :)
Title: Re: 1x, 10x, 100x Scope Probe ITSELF quality/safety??
Post by: GlennSprigg on April 05, 2020, 06:47:54 am
You should read "ABC of probes" by Tektronix, and also "Tektronix Oscilloscope Probe Circuits Concepts".

Have now downloaded both those, as .PDF documents. Thanks !!  :-+