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Years ago the Seeburg jukebox used a ferrite core memory to keep track of the selections of records to be played. It replaced a lot of electro-mechanic al crap. It was all tube circuitry and the sense lead was was connected to a pulse pulse amplifier in it's own little plugin metal box with a 12AX7, IIRC, a nd a small ferrite looking pulse transformer contained therein. The module plu gged into the main control chassis. It's the only +V grounded circuit I've eve r seen except for some PNP germanium audio amplifiers and PNP computer stuff. I never found the reason for this. I suspect it was done to increase the noise im munity. I will say that the -V wire had quite a layer of dust attached to it. T he later NPN solid state versions used a -V ground with the sense lead went to the base of a SCS, silicon controlled switch. Once it fired it stayed on unti l the current flow was interrupted. Other than convention, I never cared which end of the power supply was connected to ground. I never touch bare terminals to check their potential to ground. I don't care what color they are.

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I think you have a poor understanding of basic current flow theory and li kely very little practical experience. This may be a good place to start. So y our job is connecting equipment to other equipment and you have no idea how to do this?

itive and the other negative. Unless there is an internal connection to gr ound (like the power line safety ground - which there shouldn't be) it does n't make any real sense to call it a -48 volt supply.

Sure it does. It's intended for use in a SYSTEM that uses -48V as the power source.

s a -48 volt supply or just 48 volts? Or is the -48 volts just what you se e in references to the supply elsewhere?

He clearly stated that one terminal is marked -48V, the other return.

Why is it confusing? One terminal is the negative polarity, the other the positive. On your supply, one terminal is marked -48V, the other return. I would also make the -48V black, the return one red. but even with just -48V on one and return on the other, it's clear which is negative, which is positive. If you had a 9V battery and one terminal was marked -9V, or 'neg", would you be confused about the polarity?

Woah there Pilgrim! Are you saying on that -48V supply has a RED terminal or wire for the -48V and BLACK for return? Now that I would agree would be confusing. I would expect black to be on the -48V, red on the return.

It's either one polarity or the other, positive is never called negative. What is called the "hot", the "source" or the "return" could be either depending on the system polarity.

With one voltage, two wires like you have, it can't.

But it does make sense. That power supply is designed to be used in a positive ground system. So the supply terminal is labeled -48V.

So what color is the -48V terminal? Is there a color? I would expect it to be either black or no color identification and the return to be either red or no color.

- output.

• output.

Why are you now adding neutral to this? Neutral is on the AC side.

Sure, if it's a positive ground system, that would be correct. The negative side of the supply is viewed as the source, the hot.

Imagine you are designing a new car with a 12V battery. Since the car is made of metal, instead of using two wires to every light bulb, fan, solenoid, you can use one "hot" wire to each and ground the other side of the load to the metal chassis, using that as the return path to the battery. So, one side of the battery gets connected to "ground"/chassis. If it's the negative, like in all cars today, then it's a negative ground system. Measuring between the hot side anywhere in the system to ground you'd have +12V. Put the battery in the other way and you'd have a positive ground system and you'd measure -12V.

Did he say that? I hope not.

With your -48V supply, the -48V terminal would be called the "hot" the "supply", but not the positive.

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ositive and the other negative. Unless there is an internal connection to ground (like the power line safety ground - which there shouldn't be) it do esn't make any real sense to call it a -48 volt supply.

is a -48 volt supply or just 48 volts? Or is the -48 volts just what you see in references to the supply elsewhere?

As usual, you are wrong again, always wrong.

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Please don't confuse the guy by not reading what he writes.

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  Rick C. 

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That is the plan. I've not had the opportunity to do so yet.

Okay.

Fair enough. I've not had the exposure to (positive) 48 volt power.

I wasn't meaning to focus on the color code. Allow me to modify my statement slightly by removing the colors from it.

I believe that if I hook the positive lead of a voltmeter to the positive lead of a power supply, and the negative lead of the volt meter to the negative lead of a power supply, I expect the following voltages:

Standard bench top 12 VDC power supply: + 12 VDC Negative 48 VDC telephone power supply: - 48 VDC

Okay.

I believe it was a permutation of "hot" -> "red" -> "positive" in a system that has negative voltage.

Okay.

I had a few minutes to look at the -48 VDC power supply today.

connected to earth ground.

The implication of polarity with volt meter leads doesn't help this thread. But I generally agree with your statement.

I believe the red lead does show a negative voltage.

I still need to measure, but I believe the positive lead has 0 volts to ground.

Agreed.

I agree that ground and neutral (in typical 120/240 VAC 1? wiring) are separate conductors and should not be used interchangeably. However, ground and neutral are bonded together in the main electrical panel. (At least every main panel that I've ever looked in. Sub-panels are decidedly different with ground and neutral isolated from each other.) So, there is a conductive path between ground and neutral.

Ground is quite prevalent in networking equipment and servers. Some vendors refuse to honor warranty if the equipment is not properly grounded. Static is a (big) concern. So ground is used to dissipate static before it builds up and causes problems.

After looking at the -48 VDC power supply earlier today, I am quite convinced that the positive terminal has electrical conductivity with the building's earth ground. (Positive terminal strapped to chassis which is bare metal connecting to the rack it mounts in (also bare metal) which is earth grounded.)

--
Grant. . . . 
unix || die

I /thought/ I had an acceptable understanding, particularly for a server / network administrator.

That is a bit ambiguous.

My job is working with server and network equipment, including racking / stacking / cabling (data and power). AC power is quite simple and to be honest, hard to mess up. DC power is much more rare and requires being more careful. Hence this thread.

Copper data connections can be tricky, particularly if equipment is powered from different legs / phases and something about the power is not correct. That's a great way to burn out ports. Sneak current's are a bitch. Ground loops can be a problem if you're not careful. Optical cables are safer to work with.

So, "connecting" is a bit ambiguous and can involve a number of different things.

--
Grant. . . . 
unix || die

Having looked at the equipment earlier today, you'd be wrong.

Negative had the red wires. Positive / ground had the black wires.

The confusion is understanding why things are wired the way that they are.

In a negative ground system, no, I would not be confused.

In a positive ground system, I apparently am.

Yes!!!

That is EXACTLY what I'm saying. (See above.)

Hence this thread.

Nope.

I don't know what I would have expected / guessed this time last week. I'm loath to guess, lest I confuse myself more this week.

I'm slowly coming to the conclusion that I have naively assumed that red ? positive. Which is obviously wrong for positive ground systems.

I think I need to retrain myself that red = hot and that what is hot is dependent on the type of system it is; negative ground or positive ground.

I think that I need to reword my statement to be "a hot wire has a negative voltage ? in a positive ground system".

I checked, and one terminal is labeled "negative" and the other terminal is labeled "positive".

There is no color (save for polished / chrome steal).

The red wire was connected to negative. The black wire was connected to positive.

I've not had the opportunity to measure the voltage yet. I'm planing on doing that tomorrow.

I'm fairly certain that neutral and ground are bonded together somewhere upstream. Thus I typed "neutral / ground" as "neutral or ground". Because, from an electrical conductivity standpoint, they are at the same potential and in some ways the same.

--
Grant. . . . 
unix || die

I'll try to make a point to do so tomorrow.

Okay.

Unless anyone objects, and provides an explanation of why they object, I'm going to try to make a point to use the term "hot".

I'm learning that.

Previously, I had assumed that ground / common also made reference to the negative side. I'm having to unlearn that assumption as I go through my Negative 48 Volt DC journey.

I want to agree, but I'm reticent because of the word "positive" in there.

I agree that hot will be positive in a negative ground system.

I'm learning that hot will be negative in a positive ground system.

Thank you.

Agreed.

I think chassis is another similar thing that can be used as a reference, which may also be distinct from ground (protective earth).

Agreed.

Okay.

Fair.

It's starting to.

--
Grant. . . . 
unix || die

Positive and negative are electrical terms of physics. It has nothing to d o with convention of colors or which polarity of common you use. Connect t he positive meter probe to the positive power line and you will always meas ure a positive voltage no matter what.

Let's forget color. If it is a -48 volt system the positive wire will be t he common, but it will still be positive.

Ok, so they are using the convention that red is "hot" and black is "common ". They are also grounding their common to earth ground which is likely co nnected to a protective earth wire.

I was talking about the voltmeter leads. But whatever.

You just said above the positive power lead is common and connected to a gr ounded chassis. You can't get any more zero than that. Of course you have to use common or ground as your reference (e.g. connect the black meter le ad to common).

re

And that is the hazard. If the neutral connection to the panel is lost, th ere is no return path for the power circuit and anything touching neutral w ill be hot. The bond between protective earth and neutral must be connecte d in a way that no failure can cause the protective earth to be hot.

That would imply the equipment cabinet be grounded and the equipment connec ted to that. The power source can be floating and the equipment should not care. Do the equipment vendors define "properly grounded", that is, any s pecific instructions?

Is this strap at the power supply or the equipment being powered? Braided copper strap or just a regular wire? Just curious. As I've said, I've not worked with this sort of equipment.

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  Rick C. 

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Yes, that is the only real difference.

I'm talking about negative ground systems which I consider "typical".

Yes.

Yes.

No, any exposed metal is always connected to protective earth. That way no matter what happens with the power circuit (AC or DC) the exposed metal parts won't become "hot". Either a fuse will blow or the power source will blow up. lol

That may also be connected to the common power rail, but the point is it MUST be protective earth.

Yeah, it's not hard, but takes a bit of work to get the mental connections correct.

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  Rick C. 

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Presuming the negative meter probe is connected to negative power line, and I agree.

The thing that I've learned is that there is no correlation between "hot" / "common" / "return" / "ground" with positive or negative like I have naively thought for the last 30 years.

Yes. The better documentation calls out very specific directions on how to bond ground to the equipment. They are quite specific about earth / safety ground.

It is part of part of the power supply.

It is a piece of solid (what looks to be) steal that fits over the bolts of the positive terminal where wires get bolted on top, thus sandwiching the steal between the terminals and the restraining bolts. The other end is bolted to the metal chassis.

--
Grant. . . . 
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*nod*

Fair enough.

I agree, negative ground has been about 99% of what I've seen and worked around.

Sorry I mistyped, I was thinking where common and hot were floating inside of and insulated from the chassis and the chassis itself is grounded to earth. Thus chassis and and common can be at two different potentials.

*nod*

Yep. Now that I've bifurcated ground / common from the negative polarity, along with some better understanding of terms, I think I'm starting to understand.

--
Grant. . . . 
unix || die

That's because positive ground systems are the exception rather than the no rm. Telephony related gear is an exception... when powered from 48 volts, but not always even then. I have seen gear that didn't have an internal co nnection to common and so could be used as positive ground or negative grou nd.

That surprises me a bit. A AC/DC power supply output is typically isolated from the input power line, so common is whichever terminal you connect to. .. common. Having a connection internally to the AC protective earth limit s the applications with no added advantage.

That sounds very external then. That makes sense. That's not really part of the power supply. They just provide the strap and you likely can use it on either output. But since the positive terminal is labeled "common", ma ybe not.

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tation ran on -48VDC.

time

There were plastic cases with Mobile CB radios mounded inside for Truckers, back in the '70s. They had a 6A or 25A full wave bridge on the DC input, a nd a mag mount antenna with a thicker than normal insulating pad on the mag mount, or a 1:! RF transformer to isolate the output for Mirror mount ante nnas. They were popular with Fleet truckers who never knew if the truck wou ld have positive or negative ground. They were a pain in the ass on either grounding system, but truckers bought them.

tion ran on -48VDC.

They used some COTS -48VDC items, so the entire station was designed to ope rate off the batteries and solar cells at that voltage.

The equipment that we supplied used a pair of Vicor 120/240 input supplies. One +5VDC and one +12/-12V dual output supply. It was a simple modificatio n to replace them with a single, triple output,-48VDC input power supply. T he biggest problem was that we only had one power supply at the factory tha t could power it, so it had to be moved from station to station through fin al test with the electronics.

Michael Kellett wrote in news:e6SdnVob snipped-for-privacy@giganews.com:

Lead acid batteries used in system UPS's are 12V. Series 4 together and get 48 Volts. Batteries are isolated sources, so either node can be 'fashioned' as 'ground'. The power supplies used to charge such battery back up systems had to make more than 48 Volts to make a charge condition occur on the bank.

Whether they had individual battery watchdog circuits I do not know, but having a bad cell in a 4 battery 48 Volt system causes less of a droop than having a bad cell in a single battery 12 Volt system. So the supply could be a bit down in voltage yet still do the needed job. Whereas a single battery or parallel bank may not be as happy with a burned cell in play.

So my guess is that the voltage was chosen to match the current storage cell technology of the times, and it simply stuck even though electrical demands to perform the same jobs reduced as solid state matured.