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Measure 500V DC with digital scope and 100:1 probe

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Elerion

Member
Hello everyone.
As this is a safety issue, and I'm not sure, I would like to ask instead of experimenting.

Most digital scopes state a maximum input voltage of 300 V RMS.
I want to make some quick measurements on a live tube amp circuit and its power supply, which may go a little bit over that (500 V DC and, to be on the safe side, probably a bit over 300 VAC RMS, superimposed on a DC bias level).
Could I use a pasive basic 100:1 probe like this one? :
http://uk.rs-online.com/web/p/sondas-para-osciloscopios/7296655/


For the sake of completeness, the manual of my Tek TDS2004 scope states:
---
Maximum Voltage Between Signal and Common at input BNC: 300V RMS (CATI/CATII)
Installation Category II; derate at 20dB/decade above 100kHz to 13V peak AC at 3MHz and above.

For non-sinusoidal waveforms,peak value must be less than 450V.
Excursion above 300V should be less than 100ms duration.
---

I suppose that's for 1:1 probe? The Tek 10:1 probes that came with the scope are rated at 300 VAC RMS when 10:1, and 150 VAC RMS when 1:1.

Thank you.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Perfectly fine, that's what the probe is designed for.

However, I commonly used normal x10 probes on TV's with no ill effects, including scoping the 2Kv pulses on the LOPT - but I use a scope (Grundig) that goes to 50V/cm - which Tek don't do.
 

Elerion

Member
Perfectly fine, that's what the probe is designed for.
Thank you!

How can I then relate it to the scope max rating?
How can I know when the scope could be damaged?
Does a 500 VDC, measured with a 100:1 probe generates a 5V DV at BNC input?
If that is the case, theoretically, a 10:1 probe (rated at working voltage; mine are just 300V) could be used too, as 500 VDC would just be 50 VDC at scope input.

I read the full manual, but this isn't clear.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thank you!

How can I then relate it to the scope max rating?
How can I know when the scope could be damaged?
Does a 500 VDC, measured with a 100:1 probe generates a 5V DV at BNC input?
Yes it does.

If that is the case, theoretically, a 10:1 probe (rated at working voltage; mine are just 300V) could be used too, as 500 VDC would just be 50 VDC at scope input.
I wouldn't have even considered there 'might' be a problem, but you've certainly got a much better safety margin with a x100 probe.
 

ronsimpson

Well-Known Member
Most Helpful Member
The data sheet, post #1, clearly shows the probe can with stand 2000 or 1500 volts.
The output will be 1/100 of the input voltage.

So 500V input will put 5V on the scope.
Most digital scopes state a maximum input voltage of 300 V RMS.
Most scopes say that, BUT many scopes have a max range of 10V/division. You can not see all of 300V on the scope. Typically you can only see about 50V on the screen. (0 to 50) or (-50 to 0) or (50 to 100) or (60 to 110)

You are right to use a 100:1 probe at this voltage. (500V)
 

Elerion

Member
Most scopes say that, BUT many scopes have a max range of 10V/division. You can not see all of 300V on the scope.
I know. So I thought of using the 100:1 probe but setting the scope's config to 10:1, hoping that I see voltages divided by ten on the screen, and so I can see several hundred volts at once.
 

Colin

Active Member
The reading you get will depend on the impedance of the 500v and the "sensitivity" of the probe.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Please, could you explain yourself a little bit; I'd like to understand what you wanted to say.
He appears to have no idea of what he's on about? (as is usual), the impedance of the circuit is obviously not a problem (as it's a simple valve amplifier), and the impedance of the probe is blindingly obvious as it's a x100 probe, so consists of a 99Meg resistor, which is in series with the scopes 1Meg input impedance giving 100Meg in total.
 

Elerion

Member
Would it be safer to work on the live amplifier using an isolation transformer?

If the amp is plugged to mains, any short (live wire touching the chassis, which is grounded) will make the RCD break the house electric supply. If the amp is plugged to an isolation transformer, the chassis is no more connected to safety ground. Can this be considered as safer?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Would it be safer to work on the live amplifier using an isolation transformer?

If the amp is plugged to mains, any short (live wire touching the chassis, which is grounded) will make the RCD break the house electric supply. If the amp is plugged to an isolation transformer, the chassis is no more connected to safety ground. Can this be considered as safer?
It shouldn't make any difference - a valve amplifier should already be isolated via it's own mains transformer, and should have an earthed chassis. Unlike a switch-mode PSU there's no need to ever have to use a scope on the primary side.

There's nothing about earthing, or mains isolation, that makes an item intrinsically 'safe' though - this is a common misconception. All such methods tend to make them safer in some ways, and less safe in others - what keeps you 'safe' is understanding the specific dangers and working accordingly.
 

Elerion

Member
It shouldn't make any difference - a valve amplifier should already be isolated via it's own mains transformer
I thought that one of the advanteges of an isolation transformer (with no common earth connection) is that, if you are gounded, and touch any hot/live wire, you don't get any shock.

Like seen in this image:
http://dt7v1i9vyp3mf.cloudfront.net/styles/news_preview/s3/imagelibrary/c/crossfig2.jpg

In the amplifier, even though it has its own transformer, if you are grounded, you'll get a shock when touching any hot/live wire.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The ONLY 'live' wire in that scenario is the incoming mains, the mains switch, the mains fuse, and the wire to the transformer. These are all VERY minor (and unlikely) items to touch during repair work.

An isolation transformer only provides a small degree of protection under certain very specific circumstances - use one by all means if you wish, but don't imagine it makes you 'safe'.

We use isolated benches at work, but I've never considered doing so at home - even though I have had access to free isolation transformers.
 

Elerion

Member
The ONLY 'live' wire in that scenario is the incoming mains, the mains switch, the mains fuse, and the wire to the transformer. These are all VERY minor (and unlikely) items to touch during repair work.
You mean in the scenario of the amplifier?
As far as I know, the amplifier transformer's secondary center tap is attached to the chassis, which is connected to safety ground (third outlet prong). So, any contact with any high voltage wire/point in the amplifier itself supposes a shock hazard. Wouldn't it?
 
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