Scoping phone line interface circuits

"Bill"

power

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

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 .

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

Thanks to all for responding. Bill

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

Good Luck! Rich

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

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

power

ground

an

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. :o) 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.

No. Powerstats are Variacs, which are autotransformers with a movable wiper.

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

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

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

--

Bye.
   Jasen

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 :).

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