New Project -- Isolated Scope Probe

[Luhan Monat]

One of those projects I never got to (perhaps in 2010):

Build a battery operated probe that captures via a highspeed ADC the
waveform and stores it. The probe has high-speed optocoupler syncronization
with the probe no. 2, so that a level trigger on one of the probes will
start the acquisition at both probes at precisely the same time. When done -
transfer the signal digitally to a PC for processing (sum, difference or
multiplication of the two channels)

Downside is that it is not realtime :-(

Cheers

Klaus

That gives me an idea. Mount a 433mhz transmitter in a battery operated
probe; connected a 433mhz receiver to the scope. This is good to over
5khz with the cheap units. Range can be up to 100 feet so you can make
measurements around the lab without hauling the scope around.

 

[Luhan Monat]

Postings like this do not make you look good-

Sorry, I just don't like to waste my time on theoretical discussions of
of something that I already have working. I have gotton little useful
information from this guy, only sarcasm and technically dubious comments.

 

[Joe Legris]

Postings like this do not make you look good-

Now we're both soiled.

 

[John Fields]

Sorry, I just don't like to waste my time on theoretical discussions of
of something that I already have working. I have gotton little useful
information from this guy, only sarcasm and technically dubious comments.

---
Translation: "I don't want to have to admit that I made a mistake, so
I'll pretend that it's working, regardless of what anyone says."

You post something which is obviously flawed and then expect not to have
it criticized? Dream on...

As for the "technically dubious comments", if you have anything specific
in mind and can back it up let's hear about it.

 

[John Fields]

That gives me an idea. Mount a 433mhz transmitter in a battery operated
probe; connected a 433mhz receiver to the scope. This is good to over
5khz with the cheap units. Range can be up to 100 feet so you can make
measurements around the lab without hauling the scope around.

 

[John Woodgate]

I read in sci.electronics.design that Luhan Monat

That gives me an idea. Mount a 433mhz transmitter in a battery operated probe;
connected a 433mhz receiver to the scope. This is good to over 5khz with the
cheap units. Range can be up to 100 feet so you can make measurements around
the lab without hauling the scope around.

You need a telescope to see the screen. (;-)

 

[Frank Miles]

[snip]

There are integrated linear optoisolators good for 25 MHz or something
like that.
[snip]

John

Perhaps my accuracy/drift requirements are too great, but what parts are
you thinking about? The linear optoisolators that I've used or tried
(IL300, HCNR201,...) aren't much good above a few hundred kHz at best
if you need good drift specs. Haven't been tempted by trying a split-path
scheme (yet).

-frank
--

 

[James Meyer]

Hi,

Check this out if you need to make scope measurements on 'phone line' or
other floating circuits.....

http://members.cox.net/berniekm/isoprobe.html

Here's my entry into the contest for the most simple isolated probe.
One resistor, one capacitor, one DPDT relay, and a square wave source to match
the relay's coil voltage for power. It's in LTSpice format so you can actually
watch it work.

................

Version 4
SHEET 1 1044 712
WIRE 128 144 208 144
WIRE 128 336 208 336
WIRE 208 208 208 144
WIRE 208 144 288 144
WIRE 208 272 208 336
WIRE 208 336 288 336
WIRE 368 144 480 144
WIRE 576 144 576 208
WIRE 576 288 576 336
WIRE 368 336 448 336
WIRE 576 336 576 400
WIRE 480 144 480 192
WIRE 480 144 560 144
WIRE 480 256 480 336
WIRE 480 336 576 336
WIRE -32 144 48 144
WIRE -112 144 -208 144
WIRE -208 144 -208 192
WIRE -208 272 -208 336
WIRE -208 336 48 336
WIRE 560 144 576 144
WIRE 64 96 64 0
WIRE 112 96 112 32
WIRE 304 96 304 0
WIRE 352 96 352 32
WIRE 64 384 64 592
WIRE 112 384 112 448
WIRE 304 384 304 592
WIRE 352 384 352 448
WIRE 576 336 640 336
WIRE 640 336 640 480
WIRE 640 560 640 592
WIRE 640 592 304 592
WIRE 304 592 64 592
WIRE 112 448 352 448
WIRE 448 448 448 336
WIRE 448 336 480 336
WIRE 352 448 448 448
WIRE 64 0 304 0
WIRE 704 0 704 592
WIRE 704 592 640 592
WIRE 304 0 704 0
WIRE 112 32 352 32
WIRE 672 32 672 336
WIRE 672 336 640 336
WIRE 352 32 672 32
FLAG 576 400 0
FLAG 560 144 out
SYMBOL sw 144 336 M270
SYMATTR InstName S1
SYMBOL sw 272 336 R270
SYMATTR InstName S2
SYMBOL sw 144 144 R90
SYMATTR InstName S3
SYMBOL sw 272 144 M90
SYMATTR InstName S4
SYMBOL cap 192 208 R0
SYMATTR InstName C1
SYMATTR Value .1µ
SYMBOL res -128 160 R270
WINDOW 0 32 56 VTop 0
WINDOW 3 0 56 VBottom 0
SYMATTR InstName R1
SYMATTR Value 1K
SYMBOL voltage -208 176 R0
WINDOW 123 24 132 Left 0
WINDOW 39 24 132 Left 0
SYMATTR SpiceLine Rser=50
SYMATTR InstName V1
SYMATTR Value SINE(5 10 50)
SYMBOL res 560 192 R0
SYMATTR InstName Rscope
SYMATTR Value 1MEG
SYMBOL cap 464 192 R0
SYMATTR InstName Cscope
SYMATTR Value 50p
SYMBOL voltage 640 576 R180
WINDOW 0 24 104 Left 0
WINDOW 3 -266 -57 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(-10 10 0 1N 1N 1.2M 2.4M)
TEXT 768 88 Left 0 !;op
TEXT -280 440 Left 0 !.MODEL SW SW(Vt=0 Roff=1g)
TEXT 768 48 Left 0 !.tran .1
TEXT -280 632 Left 0 ;The four switches, S1, 2, 3 and 4, are a model of a single
DPDT relay.

..................

Jim

 

[Luhan Monat]

I read in sci.electronics.design that Luhan Monat
about 'New Project -- Isolated Scope Probe', on Mon, 29 Dec 2003:




You need a telescope to see the screen. (;-)

Or, clip the probe to the remote circuit and go back to the scope.
Could be useful for testing in hazardous or other situations.

Also, to get a predictable signal, I would use a PIC12F675 on the probe
and send the data digitally. Then decode the data with another PIC and
feed a d/a to the scope. 5 khz data rate would give about 400hz
bandwidth using cheap ASK RF units.

 

[John Larkin]

That gives me an idea. Mount a 433mhz transmitter in a battery operated
probe; connected a 433mhz receiver to the scope. This is good to over
5khz with the cheap units. Range can be up to 100 feet so you can make
measurements around the lab without hauling the scope around.

For serious HF CMRR, how about a small battery-powered probe whose
cable is pure fiber optic? You could float that a megavolt off ground.

John

 

[Luhan Monat]

Or, clip the probe to the remote circuit and go back to the scope. Could
be useful for testing in hazardous or other situations.

Also, to get a predictable signal, I would use a PIC12F675 on the probe
and send the data digitally. Then decode the data with another PIC and
feed a d/a to the scope. 5 khz data rate would give about 400hz
bandwidth using cheap ASK RF units.

Ooooops. Before someone charges me with violation of the Nyquist
constant. That would be 400 samples per second or under 200hz.

 

[Fred Bloggs]

Here's my entry into the contest for the most simple isolated probe.
One resistor, one capacitor, one DPDT relay, and a square wave source to match
the relay's coil voltage for power. It's in LTSpice format so you can actually
watch it work.

This is the classic "flying capacitor"-

[..snip..]

 

[John Woodgate]

I read in sci.electronics.design that Luhan Monat

Or, clip the probe to the remote circuit and go back to the scope. Could be
useful for testing in hazardous or other situations.

Nah! Murphy always makes it fall off just as you get back to the scope.

 

[James Meyer]

Here's my entry into the contest for the most simple isolated probe.
One resistor, one capacitor, one DPDT relay, and a square wave source to match
the relay's coil voltage for power. It's in LTSpice format so you can actually
watch it work.

This is the classic "flying capacitor"-

[..snip..]

Of course it is. I only copy the best ideas I can find.

Jim

 

[Spehro Pefhany]

Here's my entry into the contest for the most simple isolated probe.
One resistor, one capacitor, one DPDT relay, and a square wave source to match
the relay's coil voltage for power. It's in LTSpice format so you can actually
watch it work.

This is the classic "flying capacitor"-

[..snip..]

Of course it is. I only copy the best ideas I can find.

Jim

The simplest isolated probe is the time-honored method of snipping
the ground pin off of the scope's power cord...

Best regards,
Spehro Pefhany

 

[Jim Yanik]

James Meyer wrote:

Here's my entry into the contest for the most simple isolated
probe.
One resistor, one capacitor, one DPDT relay, and a square wave
source to match the relay's coil voltage for power. It's in
LTSpice format so you can actually watch it work.

This is the classic "flying capacitor"-

[..snip..]

Of course it is. I only copy the best ideas I can find.

Jim

The simplest isolated probe is the time-honored method of snipping
the ground pin off of the scope's power cord...

Best regards,
Spehro Pefhany

A Dangerous Practice,it allows the scope chassis and "ground" to float to
unknown potentials.Even if the user recognizes it,other persons may
not.Also against many workplace rules.

Safer to invert Channel 2 and use the ADD function for a crude differential
measurement.

 

[Spehro Pefhany]

A Dangerous Practice,it allows the scope chassis and "ground" to float to
unknown potentials.

Presumably the engineer knows the potentials and when to be careful,
eh?

Even if the user recognizes it,other persons may not.

They shouldn't be touching my scope.

Also against many workplace rules.

Definitely. I'm certainly not recommending that anyone do it.

Safer to invert Channel 2 and use the ADD function for a crude differential
measurement.

Crude is right. Lousy CMRR. The live scope method (which I don't
recommend) puts a lot of capacitive loading on the "Lo" input and can
expose the scope user to possible fatal shocks, but other than that it
works very well. With the availability of battery-powered scopes that
have little exposed metal it is proabably more practical these days.

Best regards,
Spehro Pefhany

 

[Jim Yanik]

Presumably the engineer knows the potentials and when to be careful,
eh?

There's a lot of newbies on this NG who appear to NOT know such things.I've
seen a fair share of test gear for repair that had their grounds connected
to 'hot' points;from reputable companies,too.

They shouldn't be touching my scope.

They could inadvertently touch it.It's happened,IIRC.
People borrowing stuff,managers doing inventory,whatever.They have every
reason to expect that a item's chassis would be grounded,standard practice.

Some workplaces require special practices when floating test equipment,such
as signage,roping off the area,red flags,if it's allowed at all.
It might even void your insurance to defeat safety items.

 

[Keith R. Williams]

Presumably the engineer knows the potentials and when to be careful,
eh?

S/He may, but what of the PHB walking through the lab?

They shouldn't be touching my scope.

Key word: "shouldn't". I don't like it when the PHBs walk through the
lab either, but they're paying the bills so it doesn't matter much
*what* I like. ;-)

Definitely. I'm certainly not recommending that anyone do it.

Our workplace rules (at least long ago-far away where we did such
nonsense) require an "isolation box", which is basically a 3-prong to
2-prong adapter about the size of a lunchbox with a red light on top.
That was considered fair warning.

Interestingly the cleaning crew is required to use ungrounded
receptacles (they're the red ones in the hallways) for their vacuum
cleaners or one of the "isolation boxes" (with no light, for some
reason). They aren't allowed to touch electrical ground.

Crude is right. Lousy CMRR. The live scope method (which I don't
recommend) puts a lot of capacitive loading on the "Lo" input and can
expose the scope user to possible fatal shocks, but other than that it
works very well. With the availability of battery-powered scopes that
have little exposed metal it is proabably more practical these days.

It works, but a differential vertical amplifier plugin works better,
depending on how high the CM voltage is.

 

[Jim Yanik]

Interesting tidbit;at Tektronix,it was mandatory that any scope in for
repair with a cut-off ground pin had to have the power connector or power
cord replaced. Probably driven by liability potential.

When I encountered a scope with a "ground-buster" adapter,I removed
it,taped it securely to a copy of TEK's pamphlet on scope isolation methods
and grounding safety,and returned it to the customer in that manner.
(my personal way of dealing with this;customer education.)