| ▲ | bluGill 7 hours ago |
| 48vdc was common in phone exchanges. They filled the basement with lead-acid batteries and to could run without the grid for a couple weeks. In turn the phone was 99.999% reliable for decades. |
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| ▲ | mjuarez 6 hours ago | parent | next [-] |
| Not to be _that_ guy, but it was technically -48V DC. Honestly, that was pretty surprising to me when I had to work with some telco equipment a couple of decades ago. To this day, I don't think I've encountered anything else that requires negative voltage relative to ground. |
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| ▲ | jacquesm 6 hours ago | parent | next [-] | | Yes, and that tiny little difference can cost you a lot of expensive gear if you run it off the battery and plug in a serial port or something like that. You'll also learn first hand what arc welding looks like without welding glass. | |
| ▲ | em3rgent0rdr 4 hours ago | parent | prev | next [-] | | Some old guitar effects used -9V DC.[1] And the convention with guitar effects power adapter is the barrel is center negative (which is motivated with facilitating easy wiring of the socket's switch to connect to a 9V battery inside). [1] https://www.analogisnotdead.com/article26/what-is-going-on-w... | |
| ▲ | HWR_14 3 hours ago | parent | prev | next [-] | | Can you explain why it's -48 VDC as opposed to 48 VDC with the + and - inputs mislabeled? | |
| ▲ | SAI_Peregrinus 5 hours ago | parent | prev | next [-] | | Lots of amplifier circuits need a bipolar supply: both positive and negative voltages with respect to ground. | |
| ▲ | aidenn0 4 hours ago | parent | prev | next [-] | | RTL and DTL both needed negative-voltage relative to ground, as do many analog circuits. | |
| ▲ | servo_sausage 6 hours ago | parent | prev | next [-] | | Is that something other than a labelling convention? Is ground actually connected to a earth stake? | | |
| ▲ | CamperBob2 6 hours ago | parent [-] | | Cathodic protection against corrosion was the goal of using -48V, in the telcos' case. | | |
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| ▲ | bluGill 6 hours ago | parent | prev | next [-] | | positive ground used to be in all cars. When they went from 6 volts to 12 the disadvantages became appearant fast and so everyone went negative ground then (mid 1950s). I am not clear why positive ground was bad (maybe corrosion?) | |
| ▲ | yostrovs 6 hours ago | parent | prev [-] | | Check out older English cars. |
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| ▲ | MathMonkeyMan 6 hours ago | parent | prev | next [-] |
| Yeah I always heard that the phone lines carried their own power, and in Florida the phones did keep working when the power went out, but I never knew why. So the grid was always charging up the lead acid batteries, and the phone lines were always draining them? Or was there some kind of power switching going on where when the grid was available the batteries would just get "topped off" occasionally and were only drained when the power went out? |
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| ▲ | pocksuppet 5 hours ago | parent | next [-] | | The phone grid predated the electrical grid. There was no other choice for power. Actually, there was one. Even earlier phones had their own power. A dry-cell battery in each phone, and every 6 months, the phone company would come around with a cart and replace everyone's battery. Central battery was found to be more convenient, since phone company employees didn't have to go around to everyone's site. Central offices could economize scale and have actual generators feeding rechargeable batteries. | |
| ▲ | bluGill 6 hours ago | parent | prev | next [-] | | Grid charging batteries, phone draining them as I understand. Of course there were switches all over the us so I can't make blanket claims but from what I hear that was normal. | |
| ▲ | qingcharles 4 hours ago | parent | prev [-] | | It's a pretty decent chunk of power down a POTS cable too, as it was designed to ring multiple big chunky metal bells in the days of yore. I was wiring in a phone extension for my grandma once as a boy and grabbed the live cable instead of the extension and stripped the wire with my teeth (as you do). I've been electrocuted a great number of times by the mains AC, but getting hit by that juicy DC was the best one yet. Jumped me 6ft across the room :D | | |
| ▲ | elcritch 9 minutes ago | parent | next [-] | | I discovered the same exact thing wiring a second phone line to my bedroom as a teenager. I jumped into a pile of fiberglass insulation! :/ | |
| ▲ | rdtsc 4 hours ago | parent | prev [-] | | The teeth. Yikes! But yeah, I remember having the rotary phone disassembled and touching the wires adjusting something when a ring came. Gave me enough of a jolt to remember. |
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| ▲ | divbzero 6 hours ago | parent | prev | next [-] |
| Interesting, so this is why the phone line still worked when power was out across the whole town. |
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| ▲ | tverbeure 5 hours ago | parent | prev | next [-] |
| -48V! :-) |
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| ▲ | idiotsecant 6 hours ago | parent | prev [-] |
| I still have a bunch of 48vdc comms gear in my powerplant. |
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