Phone Line Ground Loop Hum [RE-wrenches]

R. Sparks Scott sharkey at mrsharkey.com
Sun Dec 3 20:45:32 PST 2000


	Wrenchers;

	Not just to change the subject, and just because it's a little slack on
the list right now (at least is was 45 minutes ago), I thought I'd mention
that I have solved the Bane of all system installers who try to run
answering machines and cordless phones directly from battery power.
	Some weeks ago, I was given a Siemens "Gigaset 2415", integrated answering
machine and 2.4 GHz cordless telephone as a gift. Of course, it came with
the reviled "wall wart" phantom power cube, which I used for one whole day
to test the system before pitching it in the spare parts bin.
	This telephone system is quite versatile, and supports up to four handsets
on one base unit, complete control over the answering machine functions via
remote handset (or remotely via the telephone line), and seems to have an
adequate range, about 400' around my yard (some antenna replacement will
fix that). The handset uses standard 'AA' NiCd or NmH batteries instead of
a proprietary pack of hard-to find cells. All connections to the base unit
are via one RJ-45, 8 conductor modular jack.
	The wall cube supplied with the base unit stated that the output voltages
were 12VDC and 10VAC. Never able to resist looking inside of any new
device, I found that the 12V ran the electronics and the 10VAC was for the
battery charger on the handset, and had no connection to the electronics
inside the base unit.
	OK, obviously the electronics would run directly off of the RE system, and
it did work fine right up until I connected the phone line, and then got
the familiar ground loop buzz and continuous off-hook condition. I ran the
base unit off of a 7AH 12V gel-cell while I figured out what to do about
this condition. The base unit operated perfectly on battery power, drawing
about 50-60 mA during idle, slightly more when the answering machine
(digital) is operating.
	A look in  the Jameco Electronics catalog revealed DC-to-DC converters in
a variety of input and output voltages with output currents up to 1.65
amps. I chose a 9-18VDC input, +12 & -12 VDC @125mA unit, mostly because of
the price ($19.95). Other converters are available with input voltages up
to 36 VDC for higher voltage systems. This converter is spec'd out at 3,000
volts of isolation between the input and output. It seems to work just
fine, no buzz, no hum, just normal operation. The whole thing mounted
inside a RJ-31X surface mount box that I had in the shop, and a standard 8
conductor modular phone cord connects between the base unit and the 'supply
box'
	Oh, the 10VAC for the battery charger? Well, obviously, the handset must
have a rectifier and charge controller built in, so I just used a 16 ohm, 2
watt resistor in series with that input, feeding it from the 12 volt input
fed by the RE system. Bet I've got the only cordless telephone with an
1,800AH supply battery!	
	If anyone is interested in more details, I have photos, can supply
schematics, catalog numbers, additional sources, etc. Seems like the answer
to a lot of sticky phone/RE interconnection problems.

	-S

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