| ??? 10/18/07 04:26 Modified: 10/18/07 04:33 Read: times |
#145873 - Sorry, but I cannot see the evidence Responding to: ???'s previous message |
Richard said:
If you try to grasp what's going on, it helps to know that the ring voltage is an large AC signal centered around the negative 48 volt "battery" that appears on the "on-hook" line. It's also not obvious to the casual observer that the "battery" voltage is negative with respect to nominal earth-GND. Yes, but how could the assumption of a current-loop better explain the negative potentials? I cannot see the evidence. Richard said:
It is, however, clamped to earth, though not necessarily at a point local to the point of observation. The result is that it can wander quite a bit from earth, but not beyond a certain point. Hhm, due to fact, that there's no local earth connection, where the phone lines enter the house, these lines can show dangerously high common mode potentials against earth. The following image shows a typical protection scheme for phone lines: 
The potential against earth is clamped by a surge arrester, while the potential between the lines is additionally clamped by a transzorb. (For the latter also a VDR S10K150 can often be seen.) This indicates, that the interference against earth can also be very high, not limited to any "certain point". Kai |
| Topic | Author | Date |
| Telephone line voltages | 01/01/70 00:00 | |
| Beware! | 01/01/70 00:00 | |
| about that -48 volts ... | 01/01/70 00:00 | |
| i did, OUCH | 01/01/70 00:00 | |
| the key is to remember it\'s a current-loop | 01/01/70 00:00 | |
| The old stuff | 01/01/70 00:00 | |
| OK... YOUR grandparents ... MY parents ... | 01/01/70 00:00 | |
| Great Stuff ! | 01/01/70 00:00 | |
| Pulse dialing | 01/01/70 00:00 | |
| Not really surprising. | 01/01/70 00:00 | |
| Not necessarily | 01/01/70 00:00 | |
| Here, it detects both | 01/01/70 00:00 | |
| Pulse Dialling | 01/01/70 00:00 | |
| Current Loop | 01/01/70 00:00 | |
| Why is that the key?? | 01/01/70 00:00 | |
| It's the key to understanding it | 01/01/70 00:00 | |
| Sorry, but I cannot see the evidence | 01/01/70 00:00 | |
| Your circuit diagram shows it adequately | 01/01/70 00:00 | |
| What do you intend to build? | 01/01/70 00:00 | |
What I want to build | 01/01/70 00:00 |