phone line as an unregulated voltage source - Page 1
Forum section: electronics / beginners

author: ewcc
author_rank: Contributor
timestamp: April 06, 2023, 03:16:56 am
content: Hi,As I understand phone lines, the voltage on them is around 48vDC with some peak voltage hitting more than that when it's on hook and drop down to 6V or so. (Fromhttps://www.quora.com/What-is-the-line-voltage-on-a-telephone-line?share=1).I don't have a bench powersupply (nor do I have space for one); but I was wondering if it's possible to use the phone lines as a voltage supply and still be able to use it as a phone line? I'm guessing I need some sort ofadapter (hand-waving) that has incoming signals going to the telephone as well as signals going tothe thing I'm powering/testing? (with the following caveats)1) Ensure the voltage entering into whatever I'm powering is regulated to a stable voltage level2) Ensure that whatever I am powering *isn't* going to short circuit and cause a backfire andkill the telephone system and/or telephone so I'm guessing some sort of 'fuse' system.While in theory, I think this 'can' be done; but in practice, I'm not too sure. After all, the telephonethat I use is just a bunch of electronic components on a board with a display.Any clarifications appreciated.ewcc

author: retiredfeline
author_rank: Frequent Contributor
timestamp: April 06, 2023, 03:22:10 am
content: A long time ago my dad had a speaker phone that was line powered. It never really worked well, sometimes there wasn't enough power from the line for decent volume. I wouldn't count on getting enough power for anything more than low current gadgets.

author: Swainster
author_rank: Frequent Contributor
timestamp: April 06, 2023, 04:21:26 am
content: 1) You can only draw a few uA before the central office will register a fault, block incoming calls.2) When it rings the AC ring voltage can be over 100Vrms, with a few mA driving the ringer inside the phone. Been zapped by this many times - not much fun.So you can use the power, but only given very specific circumstances - that is literally how those old line powered phones worked.Source: I uses to design POTS (landline) phones. At one point about 50% of households in HK had one of my designs, so depending on your age you have well have used one, but I'm not going to admit which models I'm responsible forAlso, this thread seems relevant:https://www.eevblog.com/forum/microcontrollers/power-from-phone-line/

author: srb1954
author_rank: Super Contributor
timestamp: April 06, 2023, 04:52:44 am
content: Quote from: ewcc on April 06, 2023, 03:16:56 amIf it is a POTS (Plain Old Telephone Service) line the open circuit line voltage will be in the range 48 - 51V. The source resistance will be at least 400and could be up to 2500depending on how far you are from the telephone exchange. The theoretical maximum power you could guarantee getting from such a circuit would be around 230mW, which won't power very much.Trying to continuously draw power from a telephone line creates all sorts of problems and would be frowned upon by your telephone company. If they see a continuous power draw from the line they will assume it is a fault and could do an insulation resistance test on the line, which would apply up to 500V to your equipment. If the continuous power draw persists then the telephone company may disconnect your line.QuoteNot really feasible to simultaneously use as a power source and a telephone. When the phone is taken off-hook it will essentially draw all the power available from the line leaving nothing for your circuit.It will be far easier to buy a wall wart or laptop power supply and add a small step-down voltage regulator PCB, which would be not much larger than a matchbox, so you can get an adjustable O/P voltage. Surely you can find space for something like this.

author: ewcc
author_rank: Contributor
timestamp: April 06, 2023, 05:54:51 am
content: I'm not very sure what kind of line this is; but given what @srb1954 and @Swainster stated (particularly the"draw a few uA..."), I guess this idea of using the phone line to power things will have to die. But since I'vegot a few old HP dc adapters, I guess I can use those. I just need to find the right female socket (or is ita male socket? I've always been confused in how to tell).Thanks for the advice, guys. Appreciate it very much.ewcc

author: gf
author_rank: Super Contributor
timestamp: April 06, 2023, 06:52:48 am
content: If I remember correctly, ISDN NTBSs were powered from the phone line at that time, and the power was supposed to suffice for the NTBA andoneconnectd ISDN telephone (for emergency calls, when there is a mains outage).