AC mains to scope input

** 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 capacitance to ground at the gate of the FET - typically a 10nF cap is used and this too must be rated for at least 400 volts.

In short - whenever scoping the AC supply make damn sure to use a 10:1 probe and be DAMN certain the switch is the right position.

... Phil

Reply to
Phil Allison
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"Phil Allison"

** Just to be clear, the switch is the one on the side of the probe that has 3 positions: x1, X10 & GND.

... Phil

Reply to
Phil Allison

0:1

Thanks for that...once again things are more complex than I imagined.

Reply to
fungus

10:1

That was good advice in any event; you should ALWAYS be in the habit of spe= nding a good deal more time setting up the measurement than actually making= it. "Grab a probe and stick it on" is a recipe for getting bad measuremen= ts and/or frying lots of nice, expensive equipment.

Bob M.

Reply to
Bob Myers

That was good advice in any event; you should ALWAYS be in the habit of spending a good deal more time setting up the measurement than actually making it. "Grab a probe and stick it on" is a recipe for getting bad measurements and/or frying lots of nice, expensive equipment.

** When trouble shooting electronics, probes are often flying thick and fast and mistakes are bound to be made.

The arrival of affordable, auto-ranging DMMs in the 80s was a real boon - particularly so as the ohms ranges were immune from damage up to 240VAC on most. One gets tired of replacing the low ohms resistor in an analogue multimeter every few weeks.

But one thing I have learnt to be very careful with is *current* easurement - forgetting to put the red probe back in the correct hole for volts, ohms etc is a recipe for disaster.

The Fluke 70 series 2 that I mainly use has no current ranges and this is a good thing !!

Also, wherever possible, I find a resistor ( or even a length of wire) and measure the voltage drop across it on a DMM that can resolve 0.1mV DC or AC.

.... Phil

Reply to
Phil Allison

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Cool, do they use the CB junction of a transistor?

Thanks Phil, George H.

(none of the x10 probes I use have a switch! I hate that thing.)

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Reply to
George Herold

"George Herold" "Phil Allison"

Cool, do they use the CB junction of a transistor?

** Not low leakage enough and low breakdown voltage.

Something like this is the go:

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Max 1nA at room temp and 125V.

Some scope makers use the gate junction of a JFET - cos that has extremely low leakage.

(none of the x10 probes I use have a switch! I hate that thing.)

** I hate swapping probes...

... Phil

Reply to
Phil Allison

My reasoning was that a 'scope *has* to have a very high impedance input. You don't want it sucking current out of delicate little circuits. Even at 10k ohms (quite low) you'll only get 24mA from 240V A/C mains and that's not enough to fry much.

But ... most 'scopes have some other components before you get to the 10k, I hadn't counted on that.

Reply to
fungus

le for

Yep. That one still catches me occasionally.

If I was richer (or electronics was more than a hobby) I'd have two devices - one for current and one for everything else.

Reply to
fungus

"fungus"

My reasoning was that a 'scope *has* to have a very high impedance input. You don't want it sucking current out of delicate little circuits. Even at 10k ohms (quite low) you'll only get 24mA from 240V A/C mains and that's not enough to fry much.

** 240VAC across 10kohms is nearly 6 watts !!!

Enough to fry plenty.

Get real.

..... Phil

Reply to
Phil Allison

Duh, I hadn't calculated the watts...

Reply to
fungus

...but the '10k ohms' was just my bad example.

What I orignally meant to say was: I'd expect the current a 'scope sucks out of a circuit to be tiny, in the microamps range. Certainly less then 1mA. Anything more could be enough to give bad measurements.

A quick google says that typical 'scopes have an input impedance of 1 megaohm so I was in the right ballpark.

At one megahom the power from 240V A/C is only 0.06 watts, which wouldn't fry much.

Reply to
fungus

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OK, I've heard that the CB leakage of transistors is spec'ed at ~10 nA. But if you measure a few it's much better. I got something like

10 pA for a 2n3904 at 10 volts. 'course the reverse voltage will not be enough for a 'scope input.

George H.

Reply to
George Herold

Me neither. How many times do you need a x1 probe?

After enduring the fragility of Tek probes for years, I settled on German made PMK ones. 500MHz with readout switching, and a slew of nice accessories included. Better than half the price of a comparable Tek probe.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
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Reply to
Fred Abse

Not all scopes have a high impedance. Many have a 50 ohm input impedance. You don't want to be hooking them to the 240V mains. ;-)

Reply to
krw

The real answer is DONT DO IT until you have enough knowledge to understnd all the ramifications, which the OP really does not. >:| Theories about how many watts etc will only lead to lots of smoke and maybe worse. :-?

--
John G.
Reply to
John G

ce. You

It'd at least blow out the 50 ohm termination. (unless you'd used the x10 probe :^) But personally I never liked 'scopes with switchable 50 ohm inputs. It was just another setting I had to check. What's wrong with a Tee and 50 ohm load... or inline terminator?

George H.

Reply to
George Herold

"George Herold" " Phil Allison "

OK, I've heard that the CB leakage of transistors is spec'ed at ~10 nA. But if you measure a few it's much better. I got something like

10 pA for a 2n3904 at 10 volts. 'course the reverse voltage will not be enough for a 'scope input. ** See above, the diode is nowhere near the input.

The reverse voltage is only that between the gate of the JFET and each DC supply rail - often only 6 to 12 volts.

The peak signal level at this point rarely exceeds 100mV.

... Phil

Reply to
Phil Allison

For low-level signals. Or for using with a DVM!

John

Reply to
John Larkin

:-)

Luckily for the world I mostly work with batteries and stuff that comes out of small power supplies.

My A/C career so far has been limited to hooking up the occasional light bulb.

Reply to
fungus

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