That's no fun; a SubCycle will make a nice hum, and it's 100% short circuit proof; an elegant piece of magnetics...
-- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
It's heavy and wastes energy. It's big, and the ones I have laying around have nice, sharp corners. Not everyone likes things to hum mindlessly. If it didn't bother anyone, there would be no need for kill filters on usenet. :)
There was one in every 1A2 phone system, and I've junked about 100 of them.
-- You can't fix stupid. You can't even put a band-aid on it, because it's Teflon coated.
I've read all the answers so far looking for mentions of hum and this is the first I've seen. As I recall, there is an earth ground rod at the entrance, but the installer can tie it to either line of the balanced pair to minimize hum.
Ring voltage used to be about 90 volts - can you use a bridge rectifier feeding a voltage comparator to detect it, then some dc circuit to ring.? 48 volts is normal idle, the phones are low enough inpedance to drop it near 0 when off hook, but the ring voltage emulates the old hand-cranked generators with about 90 volts AC out.
Then again, I've never worked for a phone company and haven't tried this stuff since the 70s.
Alan
This is a pretty neat group.
J.B. Wood:
As far as I know the Tip should be to ground (only at the central office's side) and the Ring at -48 VDC. This was due to prevent corrosion and ion migration.
The off-hook voltage should be between 6 and 10 V.
No! If you ground tip or ring anywhere except via the line card, the imbalance will destroy the line.
Both T & R do have a overvoltage protector that shunts to local ground, and that is tied to the building/power ground (MGN - Multi Ground Neutral).
Party line ringing disturbed this balance, but in theory the L-C ringer circuit limited the imbalance.
-- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
There's a ground rod, yes, but neither tip or ring are 'tied' to it on the subscriber end.
I didn't mean that the user could ground it, just that the user shouldn't depend on being grounded. It may be grounded at the installer's discretion. You can't depend on it.
I'm sure he meant the universal 'you' as electronics don't care if the 'you' connecting things is wearing a Telco uniform or not.
The phone line is a balanced pair with neither side, ring or tip, wired to ground on the subscriber end and checking for shorts to ground (insulation failure, water ingress, etc) is one of the first faults they look for.
On the Telco end (or line card) the balanced pair is AC isolated from the DC supply (and ground) through coils (or equivalent) so there is a DC path but no AC path.
As Lesher noted, back in olden days subscriber party line ringers could be connected to ground but the thing that made the bell go ding-a-ling-a-ling was a (solenoid) coil so AC imbalance was minimized
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