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Scoping phone line interface circuits

B

Bill

Jan 1, 1970
0
I want to use a scope to check out a circuit that is connected to
the phone line tip & ring (a plain old USA analog home phone line).
A continuity check shows that the scope probe ground clip is tied
to the green-wire ground pin of the power chord.
Am I right in thinking that I need to float the scope for this (ie. power
the scope with a 2-wire extension chord)?

Bill
 
A

Anthony Fremont

Jan 1, 1970
0
"Bill"
I want to use a scope to check out a circuit that is connected to
the phone line tip & ring (a plain old USA analog home phone line).
A continuity check shows that the scope probe ground clip is tied
to the green-wire ground pin of the power chord.
Am I right in thinking that I need to float the scope for this (ie. power
the scope with a 2-wire extension chord)?

Yes.
 
R

Rich Grise

Jan 1, 1970
0
"Bill"


Yes.

Well, it's 'cord', but it might be cheaper to just get a cheater plug.
It's a two-prong plug and a flying green ground lead with a lug, and
a 3-prong socket. We used to float scopes with these all the time -
just cut off or securely tape up the ground lead.

Good Luck!
Rich
 
T

Tim Shoppa

Jan 1, 1970
0
Bill said:
I want to use a scope to check out a circuit that is connected to
the phone line tip & ring (a plain old USA analog home phone line).
A continuity check shows that the scope probe ground clip is tied
to the green-wire ground pin of the power chord.
Am I right in thinking that I need to float the scope for this (ie. power
the scope with a 2-wire extension chord)?

That's one way to do it.

More preferable is to use a differential probe, or two channels in
difference mode (gotta be within the common-mode voltage limits
though), or a battery-powered scope.

Tim .
 
B

Bill

Jan 1, 1970
0
the scope with a 2-wire extension chord)?
Well, it's 'cord', but it might be cheaper to just get a cheater plug.
It's a two-prong plug and a flying green ground lead with a lug, and
a 3-prong socket. We used to float scopes with these all the time -
just cut off or securely tape up the ground lead.

Good Luck!
Rich

Yes, 'cord', :eek:), that struck me after I posted.
(I'm involved with music more than electronics lately...can you tell?)

Thanks to all for responding.
Bill
 
R

Rich Grise

Jan 1, 1970
0
Yes, 'cord', :eek:), that struck me after I posted.
(I'm involved with music more than electronics lately...can you tell?)

Thanks to all for responding.


And, of course, you know all the safety rules arout working around a
floating scope, right?

Good Luck!
Rich
 
M

Meindert Sprang

Jan 1, 1970
0
Bill said:
I want to use a scope to check out a circuit that is connected to
the phone line tip & ring (a plain old USA analog home phone line).
A continuity check shows that the scope probe ground clip is tied
to the green-wire ground pin of the power chord.
Am I right in thinking that I need to float the scope for this (ie. power
the scope with a 2-wire extension chord)?

The only caveat is the internal mains filter which connects the scope ground
to both mains lines though a few capacitors. A better way would be to use an
isolation transformer.

meindert
 
B

Bill

Jan 1, 1970
0
Rich Grise said:
On Thu, 27 Jul 2006 14:26:25 -0400, Bill wrote:

And, of course, you know all the safety rules arout working around a
floating scope, right?

Well, maybe?... it is one of the reasons I decided to post. I mostly do
embedded controller programming, and my hardware design experience has
been primarily digital, low voltage. This is my first experience with
telephone related circuits.

Let me list what I've considered:
1) Since the ground clip of the scope probe is tied to the chassis,
the chassis of a floating scope will be at whatever potential the
probe ground clip is attached to. So, make sure the scope chassis
isn't touching anything else (other than the wood bench top :).
2) Even though the scope controls are mostly plastic, it'd be wise
to go by the "keep one hand in your pocket" rule when making
scope adjustments.
3) Don't connect the telephone line to the circuit until the probe
is securely connected to the test point of interest, and hands
are free, ...then plug the phone line jack in.
4) The phone will be ~50VDC (on-hook), and could be 130Vrms when
ringing.

Now, I did just see the response posted regarding the caps on the
scope's transformer and use of an isolation transformer..., so after
I digest that a bit, the above list may grow. But if there is
something I didn't list that I should be aware of, I'd certainly
appreciate any and all advice.
Thanks,
Bill
 
B

Bill

Jan 1, 1970
0
Meindert Sprang said:
The only caveat is the internal mains filter which connects the scope ground
to both mains lines though a few capacitors. A better way would be to use an
isolation transformer.

meindert

Thanks for pointing that out. I do have the tech manual & schematics
for the scope, packed away in some box due to a recent move. I'll
dig that out and get a better picture of things.

As to the isolation transformer, could a Superior Electric Powerstat
Variable Transformer be used? I just happen to have one..., purchased
~15 years ago for an engineer I was assisting who decided he didn't
need it after I'd ordered it, and I decided to just keep it instead
of returning it. It's never been opened...it'd be nice to finally
put it to good use after lugging it around all these years. :eek:)
I'll have to dig that out too and see if there is a schematic inside,
because I do remember it having a power switch and a 3-prong receptacle.

Thanks,
Bill
 
N

no_one

Jan 1, 1970
0
Bill said:
Thanks for pointing that out. I do have the tech manual & schematics
for the scope, packed away in some box due to a recent move. I'll
dig that out and get a better picture of things.

As to the isolation transformer, could a Superior Electric Powerstat
Variable Transformer be used? I just happen to have one..., purchased
~15 years ago for an engineer I was assisting who decided he didn't
need it after I'd ordered it, and I decided to just keep it instead
of returning it. It's never been opened...it'd be nice to finally
put it to good use after lugging it around all these years. :eek:)
I'll have to dig that out too and see if there is a schematic inside,
because I do remember it having a power switch and a 3-prong receptacle.

Thanks,
Bill
Many variable transformers are auto transformers and therfore not isolating.
 
R

Rich Grise

Jan 1, 1970
0
As to the isolation transformer, could a Superior Electric Powerstat
Variable Transformer be used?

No. Powerstats are Variacs, which are autotransformers with a movable
wiper.
http://www.superiorelectric.com/POWERSTAT.htm

It won't give you any isolation, although it could be useful if you were
troubleshooting the scope power supply. ;-)

Just float the scope with a cheater, and ground your signal ground to
the + side - you're talking POTS, right? Usually, the positive side of
the POTS line is closer to earth ground potential - it has something to
do with galvanic corrosion, or something.

And, of course, remember that a floating scope can electrocute you.

Have Fun!
Rich
 
R

Rich Grise

Jan 1, 1970
0
Well, maybe?... it is one of the reasons I decided to post. I mostly do
embedded controller programming, and my hardware design experience has
been primarily digital, low voltage. This is my first experience with
telephone related circuits.

Let me list what I've considered:
1) Since the ground clip of the scope probe is tied to the chassis,
the chassis of a floating scope will be at whatever potential the
probe ground clip is attached to. So, make sure the scope chassis
isn't touching anything else (other than the wood bench top :).
2) Even though the scope controls are mostly plastic, it'd be wise
to go by the "keep one hand in your pocket" rule when making
scope adjustments.
3) Don't connect the telephone line to the circuit until the probe
is securely connected to the test point of interest, and hands
are free, ...then plug the phone line jack in.
4) The phone will be ~50VDC (on-hook), and could be 130Vrms when
ringing.

Now, I did just see the response posted regarding the caps on the
scope's transformer and use of an isolation transformer..., so after
I digest that a bit, the above list may grow. But if there is
something I didn't list that I should be aware of, I'd certainly
appreciate any and all advice.

Your safety list looks pretty OK for me, especially if all you're doing
is poking around with POTS (plain old telephone service) lines. You might
even try the scope grounded, the probe grounded to some "earth ground",
and see what potential you see on each line relative to earth ground.

If you have a dual-trace scope, you can watch them simultaneously, and
if it has an "A-B" function, you've got your differential scope right
there. :)

Good Luck!
Rich
 
J

jasen

Jan 1, 1970
0
I want to use a scope to check out a circuit that is connected to
the phone line tip & ring (a plain old USA analog home phone line).
A continuity check shows that the scope probe ground clip is tied
to the green-wire ground pin of the power chord.
Am I right in thinking that I need to float the scope for this (ie. power
the scope with a 2-wire extension chord)?

That can be done. At worst there'll be 100V on the phone line (barring acts
of god like thunder storms). your call.
 
B

Bill

Jan 1, 1970
0
Rich Grise said:
No. Powerstats are Variacs, which are autotransformers with a movable
wiper.
http://www.superiorelectric.com/POWERSTAT.htm

It won't give you any isolation, although it could be useful if you were
troubleshooting the scope power supply. ;-)

I had doubts about it, but since I've yet to open it I wasn't sure.
I keep thinking a project might come my way someday that will need
it, but as such things often go, that will probably happen a week
after I decide to sell it on Ebay :).
Just float the scope with a cheater, and ground your signal ground to
the + side - you're talking POTS, right? Usually, the positive side of
the POTS line is closer to earth ground potential - it has something to
do with galvanic corrosion, or something.

And, of course, remember that a floating scope can electrocute you.

Have Fun!
Rich

Thanks for the 'comforting' warning :). Actually it's the mains
filtering thing that bothers me the most, 'cause that would mean
the chassis would have, say, 1/2 the power mains voltage on it
at all times. The service manual schematic for my scope doesn't
show any such caps, and the quality of the manual doesn't give me
any reason to beleive it isn't complete, so it probably doesn't
apply for this scope. But I think if I do decide to float the
scope, I'll see if I can borrow an isolation transformer just to
be safe(r). But first I'm going to try your suggestion of leaving
the scope grounded and just probing the circuit without the ground
clips. It is a dual channel scope, has an ADD button, and one
channel can be inverted. And yes, this is a plain old home phone
line. Also, after digging out the manuals, I was reminded that
this scope has a connection for being powered from a 24V battery,
so that provides another option to ponder if needed.

Thanks again for your help,
Bill
 
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