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Negative voltage: practical application

Discussion in 'General Electronics Discussion' started by Pascal666, Aug 20, 2012.

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  1. Pascal666


    Aug 20, 2012
    I am familiar with negative voltages in theory, but I'm a bit confused when trying to actually hook up a new piece of equipment. It wants -48VDC @ 5A. Its power supply connector has three screw terminals labeled -, +, and ground. There is a separate chassis ground lug.

    If I hook up a string of four 12v batteries in series to give 48V nominal, how would I connect them to the equipment? If the equipment wanted 48VDC I would just hook + to + and - to -. I'm tempted to hook + to - and - to + since it wants -48VDC, but that just doesn't seem right to me. Do I need to bond the two grounds to anywhere?

    I have a SOLA SFL6-48-100 I would also like to use. It is rated 48VDC @ 6A. It only has + and - on the DC connector. I am tempted to connect it + to + and - to -, and bond + to the equipment's power supply connector ground and bond the equipment's chassis ground lug to the grounding conductor also connected to the power supply's input side.

    I will have a serial cable running from the equipment to my laptop. Serial cables have a shield grounded on both ends, so it will be connecting together both chassis. Does it matter at all if my laptop is running off of its internal battery or from an external power supply?

    Thank you in advance for your assistance.
  2. alfa88


    Dec 1, 2010
    In my opinion plus should be connected to plus and negative to negative. Do not connect either negative or positive to ground. The purpose of ground is to literally carry current to ground should voltage accidentally, somehow touch the case. I don't see a problem with either laptop configuration, however, battery power would be the safest.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Voltage is the difference in electrical potential between two points. There is no absolute zero potential. Therefore it is an arbitrary decision to be made by the user which point he defines as the reference potential or "0 V".
    In the case of a 12 V battery: It is very common to declare the "-" pole as the reference, thus making the "+" pole having + 12 V.
    One can (and sometimes it is done) declare the "+" pole as the reference. Thus the "-" pole will show -12 V. But it is still the same battery.

    Hooking up the battery's "+" to the equipment's "-" will most certainly not work. If you're lucky, the equipment will simply refuse to operate. If you're not so lucky, it might even be destroyed.

    From what you write I cannnot say for sure but it looks like ground and +48 V might have the same potential, thius making the "-" terminalk receive a supply of -48 V. Can you supply a wiring schematic or link to a datasheet of this particular type of equipment? I'will need more information.

    As to the connection from equipment to laptop: A typical laptop power supply is solated and has no direct ground connection. Therefore - in theory - it should make no difference whether you operate the laptop from the battery or the from the power supply. There is however an almost unavoidable AC connection between laptop and mains (through cpacaitive coupling). If you operate the laptop from mains, it is advisable to check that the ground/shield of the laptop's connector has the same potential as the ground/shield of the equipment's connector. Otherwise a high fault current may flow between laptop and equipment, depending on the details of the installation's wiring.
  4. JMW


    Jan 30, 2012
    Positive ground equipment has been around for awhile. I recently saw a generator/starter combo for a 1916 Cadillac IIRC it was + ground.
    As to your question. Get an ohmmeter and measure the + terminal to ground, it might measure 0 Ohms, then again it might not. If you use your batteries, please do the following, connect 2 headlights in series and use a 6 amp or so fuse. As for the headlights, they have 3 pins Hi, Lo and common. Connect them as follows: Connect the lamps using just the Hi and Lo leaving the common disconnected. This will give you a current limiting circuit for approx 5 amps. Now connect the + batt to the + term and the - batt to the - terminal. If you use your power supply, place your UUT (Unit Under Test) and the power supply on rubber mats, (Auto Floor Mats) are perfect for this. Many power supplies are grounded and if in fact your UUT has a metal case, you could get a bit of a spark. Sounds like this is Telco equipment
    Let us know
  5. Pascal666


    Aug 20, 2012
    Wiring schematic? Datasheet? ROFL. You have no idea how much I would love to have one of those. The only information I can find anywhere on the Internet is a couple pictures of the outside of the unit. Not much help since I have the unit in front of me anyway. I have no information other than the text written on the outside of the unit, and there is not much of that. I have a vague idea from that text what the unit does, but no specifics.

    The front of the unit says "DAX64A Digital Access E1/T1 XBAR Switch" and there is a sticker on the back that says "ECTel Incorp. Digital XBAR Switch DX64A". I love how the model number doesn't even match from front to back.

    Once I figure out power and console port access (via a serial port) then I get to figure out how the eight DB78s are wired. I'm hoping the console port will tell me how many T1s it supports and dividing that into 78 will give me some clue.

    Fairly high res front & back pictures can be found at:$T2eC16RHJGYE9nooh7LsBQIUtn4lsQ~~60_57.JPG$T2eC16NHJGwE9n)ySfVBBQIUu(pbGg~~60_57.JPG

    The unit does have a 5A fuse. Is there any additional advantage to the current limiting circuit JMW detailed?

    My multimeter tells me the ground screw terminal on the power supply connector and the chassis ground are bonded. + and - do not appear to be bonded to anywhere.

    I pulled the cover off and found two DC-DC converters inside. A XWD4805-12 and a XWS4805. Their datasheets (below) appear to indicate these both take 48VDC nominal, with a permissible range of 36 - 72. Neither says -48VDC anywhere.

    So as far as I can tell, they just put "-48VDC" on the back to confuse people. I can't see any reason it wouldn't run off of 48VDC just fine. I guess when it blows up in my face I'll know otherwise.
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    But it does run off 48 V (DC). The only thing is that ground is connected to the "+" pole of the battery (or power supply), thus making the second terminal -48 V. As I said: It is only a matter of definition. From the point of view of the terminals, the potential difference is 48 V. Which point you declare to be 0V (aka ground) is up to you (or the manufacturer, respectively).
  7. donkey


    Feb 26, 2011
    ground is always neutral. consider that 0. + and - are the voltages away from ground.
    if I were using a transformer like you described and it read -48v grnd +48v and I required -48v I would use ground and -48v. if I required +48v I would use grnd and +48v. If I required 96 volts I would try the -48v and the +48v.
    alot of people over think this but it really is simple.
    consider you are half way up a stretch of road. you are at point 0. the end of the road is +48v, the part behind you is -48v. the stretch of road is 98v and you in the middle are what is dividing it.
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