AC mains to scope input

Phil Allison · 2011-12-30T01:06:39+00:00

** After just a little research - it is not hard to see why connecting a scope directly to the AC mains is such a bad idea.The vast majority of analogue and digital scopes have switched, passive input attenuators using small resistors, trim capacitors and wafer switches employing several banks. All these components have max voltage ratings that do not include supply voltages like 240 volts AC.Possibly the most vulnerable component to high voltages is the tiny trim capacitor connected * directly across the input * to equalise capacitance loading for all ranges - it is crucial that a scope's input present a constant value of C ( usually 15 to 20 pF) in parallel with 1 Mohms on all ranges so that the trimmer on a 10:1 probe can be adjusted once and left.Wafer switches are also not rated for high voltages and lack sufficient insulation and clearances to survive the spike voltages that regularly appear on the AC supply. Insulation breakdown in the trimmer or switch would be sudden and bit spectacular when full AC mains power is available.Many scopes have a max input rating of 400 volts ( DC+AC) peak - which means they are capable of accepting such peaks on a wave being viewed without damage, despite the fact the actual peaks may not be shown. It also means that when AC coupled, the max DC voltage should not exceed 400 volts as that is the rating of the series input capacitor.The worst case of ACCEPTABLE input overload is with 400 volts at the input and the attenuator set to pass the voltage straight through to the FET preamp. Makers typically fit a series resistor of 100kohms to 470kohms between the switch and the gate of the first FET to limit current to a few mA - then add one or more diodes to direct this current safely into the +/- DC supply rails. These diodes must be low leakage types so not to create an DC offset at the gate of the FET.This high value series resistor needs a cap in parallel to pass high frequencies that would otherwise be lost due stray...

Dec 30, 2011 27 Replies

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Michael A. Terrell 14 years ago

Then you didn't work with a lot of RF.

Reflections at high frequencies. :)

One more thing to keep track of, on the bench.

You can't have a sense of humor, if you have no sense.

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Fred Abse 14 years ago

Not that often, for me, then. 10, or even 50 mV per division is mostly enough. I can always pinch a 1:1 probe off a DVM for the odd occasion I don't mind the capacitive loading.

Switched probes, never.

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

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John G 14 years ago

That was my approach about 65 years ago and slowly as I learnt a bit more (College and on the job) I was able to understand the risks and so far have managed to only blow one Multimeter current fuse and never damaged a scope. :-)

John G.

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krw 14 years ago

Speed. The highest speed amplifiers (this goes up with time) have 50 ohm inputs. Active probes are the norm in some applications.

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John Larkin 14 years ago

My Rigol came with some very nice switched probes. All the regular (passive) Tek probes I have here are switchable 1x/10x.

John

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Fred Abse 14 years ago

I've found that Tek probes have a habit of breaking at the tip. The PMK ones I settled on have replaceable tips, choice of rigid or spring-loaded. I don't like the spring-loaded ones much.

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

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krw 14 years ago

You're not supposed to use them as darts. I've broken a lot of probes but I don't think I've ever broken a tip.

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Fred Abse 14 years ago

It's the teeny little molding that holds the tip in that cracks and falls out, complete with tip.

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

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