AC mains to scope input

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.

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.

On Dec 30 2011, 2:06 am, "Phil Allison"
wrote:

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.


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

wrote:

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.


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

** 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.

** 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

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.

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.

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

   Enough to fry plenty.

   Get real.

On Jan 3, 10:30 am, "Phil Allison" <phi...@tpg.com.au> wrote:

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

   Enough to fry plenty.

   Get real.

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

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

"George Herold"
 "Phil Allison"







** 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.

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

** Not low leakage enough and low breakdown voltage.

Something like this is the go:

http://www.centralsemi.com/PDFs/products/1n3595.pdf

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- Hide quoted text -

- Show quoted text -

On Jan 2, 9:43 pm, Bob Myers <bobmyer...@gmail.com> wrote:

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.


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.