5 ms·

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 decad

by mjuarez 6mo ago

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.

SAI_Peregrinus 6mo ago
Lots of amplifier circuits need a bipolar supply: both positive and negative voltages with respect to ground.
yostrovs 6mo ago
Check out older English cars.
bluGill 6mo ago
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?)
servo_sausage 6mo ago
Is that something other than a labelling convention? Is ground actually connected to a earth stake?
- CamperBob2 6mo ago
  Cathodic protection against corrosion was the goal of using -48V, in the telcos' case.
  - myself248 6mo ago
    And the telegraph lines before that.
- magicalhippo 6mo ago
  > Is ground actually connected to a earth stake? Yes, or something similar[1]: A few of the more efficient grounding electrodes for buildings and structures are: - Metal Underground Water Pipe - Metal In-ground Support Structures - Concrete-Encased Electrode (also known as “footer ground” or “Ufer ground”). - Ground Ring As mentioned this is particularly important for telecom and similar systems which have signal wires going literally through the ground. [1]: https://www.nfpa.org/news-blogs-and-articles/blogs/2021/09/27/grounding-understanding-the-essentials-for-building-the-foundation-of-a-structures-electrical-system https://www.nfpa.org/news-blogs-and-articles/blogs/2021/09/2...
jacquesm 6mo ago
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.
aidenn0 6mo ago
RTL and DTL both needed negative-voltage relative to ground, as do many analog circuits.
em3rgent0rdr 6mo ago
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-with-positive-ground https://www.analogisnotdead.com/article26/what-is-going-on-w...
HWR_14 6mo ago
Can you explain why it's -48 VDC as opposed to 48 VDC with the + and - inputs mislabeled?
- jojobas 6mo ago
  In short, ground.
- crote 6mo ago
  Because the chassis is connected to ground (as in, a literal grounding rod hammered into the soil) and by definition your 0V reference point. The crucial difference is the direction in which the current is flowing: is it going "in to", or "out of" a hot wire? This becomes rather important when those wires are leaving the building and are buried underground for miles, where they will inevitably develop minor faults. With +48V corrosion will attack all those individual telephone wires, which will rapidly become a huge maintenance nightmare as you have to chase the precise location of each, dig it up, and patch it. With -48V corrosion will attack the grounding rod at your exchange. Still not ideal, but monitoring it isn't too bad and replacing a corroded grounding rod isn't that difficult. Telephone wires will still develop minor faults, but it'll just cause some additional load rather than inevitably corroding away.
  - tinyhitman 6mo ago
    Do you also happen to why this is not more common? Must be useful for more than just telephone wires.
    - bluGill 6mo ago
      Most large scale systems are AC because transformers are relatively cheap, low maintenance, and efficient. When the system is AC ground makes no difference. With DC systems you generally think about the issues - which is why modern cars are negative ground. However other than cars most people never encounter power systems of any size - inside a computer the voltages and distances are usually small enough that it doesn't matter what ground is. Not to mention most computers don't even have a chassis ground plane (there are circuit board ground planes but they conceptually different), and with non-conductive (plastic) cases ground doesn't even make sense.
fecal_henge 6mo ago
I am STILL designing hardware for -48v telco standard. The first thing we do is convert -48 -> 48v. That's 4 square inches of PCB space we waste.
- eqvinox 6mo ago
  What do you need +48V for?
  - pluies 6mo ago
    Well if it's negative 48V the electricty flows out of your circuit and back to the grid, so you need to make it positive to have the electricity come in.
  - djmips 6mo ago
    Likely as a basis for converting to other useful DC voltages.
  - fecal_henge 6mo ago
    We go -48 -> 48 -> 12 -> 3v3,1v8 etc etc. If you went 48 straight to POL voltages then you would have horrific converter performance.
    - eqvinox 6mo ago
      > If you went 48 straight to POL voltages then you would have horrific converter performance. What's horrific converter performance in numbers? An isolated flyback (to 12V) should be able to hit >92% and doesn't care if it's fed -48V or +48V or ±24V. TI webench gives me 95% though I'd only believe that if I'd built and measured it. What's the performance of your -48V → +48V? [with the caveat that these frequently require custom transformers... not an issue with large runs, but finding something that can be done with an existing part for smaller runs is... meh]
      - fecal_henge 6mo ago
        -48 to 48 claims something like 97% (load dependent of course). It also needs to arbitrate between two input supplies for glitchless redundancy, plus have PM bus and other spec mandated stuff. There is no technical reason why you cant go -48 -> 12 as you state with good efficiceny, but we cant get hold of a part that ticks all the boxes. Horrific performance by my definition would be 48v to say 1v. We only realistically use buck topologies for POL supplies. Such a ratio is really bad for current transients, not to mention issues like minimum on times for the controller.
swed420 6mo ago
> I don't think I've encountered anything else that requires negative voltage relative to ground Automotive collectors can probably still relate to cars from the 1920s-50s having a "positive ground."
- fuzzfactor 6mo ago
  With positive ground the traditional more-sacrificial spark plugs lasted longer.

by mjuarez 6mo ago

SAI_Peregrinus 6mo ago

Lots of amplifier circuits need a bipolar supply: both positive and negative voltages with respect to ground.

yostrovs 6mo ago

Check out older English cars.

bluGill 6mo ago

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?)

servo_sausage 6mo ago

Is that something other than a labelling convention? Is ground actually connected to a earth stake?

CamperBob2 6mo ago
Cathodic protection against corrosion was the goal of using -48V, in the telcos' case.
- myself248 6mo ago
  And the telegraph lines before that.
magicalhippo 6mo ago
> Is ground actually connected to a earth stake? Yes, or something similar[1]: A few of the more efficient grounding electrodes for buildings and structures are: - Metal Underground Water Pipe - Metal In-ground Support Structures - Concrete-Encased Electrode (also known as “footer ground” or “Ufer ground”). - Ground Ring As mentioned this is particularly important for telecom and similar systems which have signal wires going literally through the ground. [1]: https://www.nfpa.org/news-blogs-and-articles/blogs/2021/09/27/grounding-understanding-the-essentials-for-building-the-foundation-of-a-structures-electrical-system https://www.nfpa.org/news-blogs-and-articles/blogs/2021/09/2...

jacquesm 6mo ago

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.

aidenn0 6mo ago

RTL and DTL both needed negative-voltage relative to ground, as do many analog circuits.

em3rgent0rdr 6mo ago

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-with-positive-ground https://www.analogisnotdead.com/article26/what-is-going-on-w...

HWR_14 6mo ago

Can you explain why it's -48 VDC as opposed to 48 VDC with the + and - inputs mislabeled?

jojobas 6mo ago
In short, ground.
crote 6mo ago
Because the chassis is connected to ground (as in, a literal grounding rod hammered into the soil) and by definition your 0V reference point. The crucial difference is the direction in which the current is flowing: is it going "in to", or "out of" a hot wire? This becomes rather important when those wires are leaving the building and are buried underground for miles, where they will inevitably develop minor faults. With +48V corrosion will attack all those individual telephone wires, which will rapidly become a huge maintenance nightmare as you have to chase the precise location of each, dig it up, and patch it. With -48V corrosion will attack the grounding rod at your exchange. Still not ideal, but monitoring it isn't too bad and replacing a corroded grounding rod isn't that difficult. Telephone wires will still develop minor faults, but it'll just cause some additional load rather than inevitably corroding away.
- tinyhitman 6mo ago
  Do you also happen to why this is not more common? Must be useful for more than just telephone wires.
  - bluGill 6mo ago
    Most large scale systems are AC because transformers are relatively cheap, low maintenance, and efficient. When the system is AC ground makes no difference. With DC systems you generally think about the issues - which is why modern cars are negative ground. However other than cars most people never encounter power systems of any size - inside a computer the voltages and distances are usually small enough that it doesn't matter what ground is. Not to mention most computers don't even have a chassis ground plane (there are circuit board ground planes but they conceptually different), and with non-conductive (plastic) cases ground doesn't even make sense.

fecal_henge 6mo ago

I am STILL designing hardware for -48v telco standard. The first thing we do is convert -48 -> 48v. That's 4 square inches of PCB space we waste.

eqvinox 6mo ago
What do you need +48V for?
- pluies 6mo ago
  Well if it's negative 48V the electricty flows out of your circuit and back to the grid, so you need to make it positive to have the electricity come in.
- djmips 6mo ago
  Likely as a basis for converting to other useful DC voltages.
- fecal_henge 6mo ago
  We go -48 -> 48 -> 12 -> 3v3,1v8 etc etc. If you went 48 straight to POL voltages then you would have horrific converter performance.
  - eqvinox 6mo ago
    > If you went 48 straight to POL voltages then you would have horrific converter performance. What's horrific converter performance in numbers? An isolated flyback (to 12V) should be able to hit >92% and doesn't care if it's fed -48V or +48V or ±24V. TI webench gives me 95% though I'd only believe that if I'd built and measured it. What's the performance of your -48V → +48V? [with the caveat that these frequently require custom transformers... not an issue with large runs, but finding something that can be done with an existing part for smaller runs is... meh]
    - fecal_henge 6mo ago
      -48 to 48 claims something like 97% (load dependent of course). It also needs to arbitrate between two input supplies for glitchless redundancy, plus have PM bus and other spec mandated stuff. There is no technical reason why you cant go -48 -> 12 as you state with good efficiceny, but we cant get hold of a part that ticks all the boxes. Horrific performance by my definition would be 48v to say 1v. We only realistically use buck topologies for POL supplies. Such a ratio is really bad for current transients, not to mention issues like minimum on times for the controller.

swed420 6mo ago

> I don't think I've encountered anything else that requires negative voltage relative to ground Automotive collectors can probably still relate to cars from the 1920s-50s having a "positive ground."

fuzzfactor 6mo ago
With positive ground the traditional more-sacrificial spark plugs lasted longer.