HV scope probe question

[Oznog]

I have a fluke 87 scope and I need to measure voltage across a 450v cap.

The scope has a 300V rating on the inputs. The probes, however, have a 600V rating.

These are 10:1 probes though. So doesn't this mean that the scope will see only 45V, thus not exceeding the ratings for either the probe or scope?

I just want to know if I'm missing something, that's a nice expensive scope and I'd hate to toast it due to any misunderstandings.

 

[Dean Huster]

You're good to go, Oznog. The probes will handle the voltage and (along with the 1M ohm scope input impedance) will knock the cap voltage down by a factor of 10, plenty safe for the scope. That's one of the reasons for using the attenuator probes. Others are higher input resistance for lower resistive circuit loading; lower capacitance for lower reactive circuit loading; resultant higher bandwidth capabilities for the scope. The only disadvantage of an attenuator probe is the loss of system sensitivity, and that's not usually a big problem in most instances.

Dean

 

[toddbailey]

Can we take this question a step further?

Assuming we can safely attach a scope probe to the cap, and I'm assuming it's part of a high voltage DC power supply. What are we looking for?


Reason I'm asking is a have a Tek 7904 scope and a tube amp with power supply issue. While I don't expect a pure dc output, I'm wondering how much ac in the ps is going to be a problem, if you have 450 volts dc how much ac is ok.

scope has ac and dc coupling, with a max vert input setting of 5v/cm or 50 volts 250 v max input w/ 10 x probe

So like above post, do I have a problem?
And what setting do I want to start with?

 

[Nigel Goodwin]

Using a x10 probe you should be fine, I've regularly used one on line output stages with over 1000V pulses on it - although the probes do VERY occasionally expire under the abuse.

We don't use Tek scopes though, I had one for evaluation and it had very limited voltage ranges, too low for our purposes, so we evaluated (and purchased) a Grundig scope, to replace the older Philips one (which also had more sensible voltage ranges).

But for scoping a valve HT rail, particularly if using AC coupling, you should be fine.

I don't know how much use you will find it though, I suspect you'll be horrified how much ripple there is

 

[toddbailey]

Thanks for the info.

I'm expanding my exposure to include tube audio, and I just acquired a 45 years old McIntosh amp and it worked quite well for a few days,
All the voltages check ok, but now a very loud ac buzz in the left and very quiet in the right channels. I just had readjusted tube bias, but still couldn't reach the spec's. I hope I didn't cause the damage, but manual said .7 and all I could get was .35

Time to put all this expensive test gear back to work and figure out what's broken but don't want to take out a scope amp in the process.
All the electrolytics are being replaced. Time to take the gloves off and do it right.

 

[HiTech]

Reason I'm asking is a have a Tek 7904 scope and a tube amp with power supply issue. While I don't expect a pure dc output, I'm wondering how much ac in the ps is going to be a problem, if you have 450 volts dc how much ac is ok.

If you are measuring the AC component of the signal, an acceptable figure would be 5-6% AC ripple or less for a well designed PSU. However it's not uncommon to measure 10% and higher on some supplies. A lot of it depends what the PSU's application is for. A medical, life support device would require very low AC ripple of near 1-2% or less. A high ripple supply for a cheapo clock/radio on the otherhand isn't as critical.

 

[kchriste]

I have a fluke 87 scope and I need to measure voltage across a 450v cap.

Last time I checked, a fluke 87 was a multimeter with a 10MΩ input impedance. You x10 scope probes are designed to work with a scope which has a 1MΩ input impedance. x10 scope probes are just a 10MΩ resistor in series with the signal lead & shunted with a frequency compensation cap.
If the "scope" has a standard 1MΩ input impedance, then yes, the probes will be fine for this application.
EDIT: ARRGH! Stupid me. The OP is from 2005!