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Old 21st September 2014, 04:51 PM   #1
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Default scope mod

I have an old scope that has a limit of 350v on the input. I would like to see the ripple AC component on power rails above this level. Is the voltage limit on the AC setting a function of the input cap in the unit? Could I replace the original input cap with something with a 630 volt rating and get away with using the scope on 450 volt rails?
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Old 21st September 2014, 05:01 PM   #2
Mooly is offline Mooly  United Kingdom
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That's a fair assumption... but I would just add a caution to that and say that (scope depending) a higher voltage will result in a higher "charge current" in the cap and so result in a higher transference of energy to the scope side of the cap (the scopes input circuitry) at the instant you connect the probe to a higher voltage. It probably wont be an issue, as long as you understand the problem.

Why not just make a high voltage divider probe. All you need are a nine 1 meg resistors in series assuming your scope has a standard 1 meg input impedance. Ripple is all low frequency and so the performance of the "probe" at hf isn't an issue.

Commercial divide by 10 or 100 probes do have definite voltage limits so check carefully if you were buying one for that purpose.
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Old 21st September 2014, 05:35 PM   #3
jcdrisc is offline jcdrisc  Australia
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Do you have a problem using a 10 : 1 probe ??
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Old 22nd September 2014, 02:44 PM   #4
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Default scope 1 to 10 probe

Thanks for the info Mooly. It is an old scope so no real loss if the charging current is to much for the scope. I will take the safety precautions for myself. JCDRISC; The use of a 1 to 10 probe reduces the amplitude of the AC component riding on top of the DC, which is exactly what I want to see. Want to learn a little about what effects I can have by adding component to the various power supply units in my tube amps. 150, 300, 390, 465 and 645 vdc. The 645 is two units added together so I wanted to check the two that add up to it individually.
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Old 22nd September 2014, 03:20 PM   #5
AndrewT is offline AndrewT  Scotland
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using a 10:1 or using a resistive divider ladder will do the same thing: attenuate the wanted ripple riding on the high voltage bias.

I would use a Y1 or Y2 capacitor as the AC coupling capacitor.
I would also ensure the Probe can insulate YOU from the HV you are touching !

Some DMM probe are now rated with the maximum voltage and the Class.
My fairly new probes are rated as 1000V CAT III and on the other side rated as 600V CAT IV
Whereas the back of the DMM is rated 1000V CAT II 600V CAT III
But probably made in China and means nothing.

Are HV scope probes similarly rated?
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Old 22nd September 2014, 05:21 PM   #6
Mooly is offline Mooly  United Kingdom
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Another option if you want to retain the basic scope sensitivity is to use the cap but in series with a low value resistor (say... 20k or so as a guesstimate) and add a zener clamp on the scope input side using say two 27 volt zeners (or higher if you have them) and two series ordinary diodes such as 1n4007. That would protect the scope from excess input voltage at the instant the probe touched a high DC voltage. The inaccuracy caused by the series resistor being so low is minimal.
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Old 22nd September 2014, 08:35 PM   #7
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Default rating of probe

The probe is rated for 500v so I'll keep it off of the 645. It's a Tektronix type so at least not China made. Thanks for the additional input Mooly, I have a couple 37v zeners(somewhere). The accuracy is no so important as I am just trying to minimize the AC component riding on the DC. Also, while working in this higher voltage arena, I wear a leather glove on the probe hand and keep the other one clear.
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Old 22nd September 2014, 10:04 PM   #8
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Quote:
Originally Posted by optimationman View Post
It's a Tektronix type so at least not China made.
What a fine punch line, and therefore us from Europe we have now to believe that your patriotism will keep you safe just by it self.

And to accept the fact that the Chinese products is reincarnated Evil which serve one and only purpose and that is to kill people.

For your information today if Tektronix has even one active production line, this is located in China.

And regarding the leather glove make me a favor and kick in the **** the one who advised you to act that way.
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Old 23rd September 2014, 02:32 PM   #9
marce is offline marce  United Kingdom
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Leather is not much use....
80%+ of the worlds electronics are made in the east...basically just about everything that is not bespoke or cannot be made there for other reasons such as ITAR restrictions....
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Old 23rd September 2014, 05:11 PM   #10
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Mooly View Post
Another option if you want to retain the basic scope sensitivity is to use the cap but in series with a low value resistor (say... 20k or so as a guesstimate) and add a zener clamp on the scope input side using say two 27 volt zeners (or higher if you have them) and two series ordinary diodes such as 1n4007. That would protect the scope from excess input voltage at the instant the probe touched a high DC voltage. The inaccuracy caused by the series resistor being so low is minimal.
Agree but you don't need the 1N 4007 diodes, just put the Zeners in series, pointing to opposite ways of course.

And as far as gloves, there is an approved Electrician's rubber glove.
Which is 2 layer to better avoid minuscule nicks and cracks.

And Pro Electricians I know add a third , house hold type rubber glove inside.
It doesn't hurt

Last edited by JMFahey; 23rd September 2014 at 05:15 PM.
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