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polarity of power cord - gateway fpd1520

Discussion in 'Electronic Repair' started by zirath, Mar 28, 2008.

  1. zirath

    zirath Guest

    We recently got a gateway fpd1520 (15" lcd flatscreen) monitor from
    ebay. It didn't have a power cord. It says it's a 12v dc 2.5a but it
    doesn't give the polarity. A person from gateway said he thought it
    wouldn't hurt the monitor if it got plugged in backwards but I'd rather
    not try it.

    Would appreciate any help.
  2. Arfa Daily

    Arfa Daily Guest

    If it has got any external metal parts such as RCA (phono) connector outers,
    BNC connector outers, 'D' connector surround etc, or possibly screws for a
    stand, as these often go right through the plastic case, and into the
    internal chassis, then try measuring from any such metal to each of the DC
    power input connector's terminals in turn, using an ohm meter. Chances are
    you'll find a direct connection, and that will be your DC ground ( "-" )
    connection. The other will then be the "+". Assuming that it's a 'standard'
    co-axial DC connector, on most modern equipment, 'pin' is "+" and side
    contact is "-" although that's not cast in stone. Be aware when you are
    obtaining a replacement PSU, that the plug is often a slightly abnormal
    size, being a little larger than those you typically find on 'general' power
    supplies. Also, make sure that you get one well rated for the job, as these
    monitors do draw quite a lot of current, and may well surge up close to the
    quoted 2.5 amps at startup, as the LCD backlights first fire up before
    settling to their run current.

    As to whether it would be safe to reverse connect it, I wouldn't like to
    say. Some equipment is perfectly well protected against such 'consumer
    antics', but it is by no means guaranteed, and if it is not adequately
    protected, the result is often an item that's fried beyond repair, for no
    other reason than unobtainable power supply devices, as many previous posts
    on this subject over the years, will attest ...

  3. Arfa Daily

    Arfa Daily Guest

    I think you'll find that on 'most' modern - and I did say "modern" in my
    original reply - equipment, this has been pretty much standardised such that
    DC "-" *is* common ground. Sony kit that I have seen in recent years has all
    obeyed this 'convention', so I'm willing to bet that any Sony items that
    follow the opposite 'convention', are not "modern". Pin = "-" used to be the
    'convention', but for all mainstream manufacturers whose equipment I work
    on, this has not been the case for many years. It was only usually the
    Japanese manufacturers that followed this anyway, as I recall.

    As for a transistor radio that uses PNP transistors, I haven't seen one that
    uses transistors at all for many years, let alone PNP ones, so I think you
    might be struggling to fit that into my "modern" category, also.

    This is, of course, the very best way, if the owner wants the trouble of
    taking it all to bits, and identifying a suitable electrolytic to use as his
    reference. However, I would put my name on the line that the method I quoted
    before, would 99.5% yield the same result, with any 'modern' item using a
    coaxial DC socket. Perhaps someone out there with a Gateway monitor could
    confirm which way round it is, then neither of us will be applying guesswork
    to experience and coming up with sage advice ... d;~}

  4. BillW50

    BillW50 Guest

    In Arfa Daily typed on Fri, 28 Mar 2008 12:52:58 GMT:
    Well Arfa... they still use transistors (both NPN and PNP types) in
    modern equipment. The reason you don't see them anymore is do to the
    magic of minturization. But they are still there, just neatly packaged
    into what is known today as the intergrated circuit (IC chip). :D
  5. Arfa Daily

    Arfa Daily Guest

    And I though I was pedantic !! Yes, of course ICs contain transistors, and
    yes, I would accept that some of them may be PNP types, depending on block
    function within the IC, but I don't think, with the best will in the world,
    that this is the level of transistor existence that William was referring to
    with his "transistor radio using PNP transistors" scenario. In any event, in
    the case of an IC taking a single polarity rail, it is irrelevant whether
    the transistors inside are NPN or PNP or FETs or whatever. PNP transistors
    are just used 'upside down', as are discrete PNP transistors when used in
    any piece of single polarity rail equipment. The ground is still (typically
    for //modern// equipment) the "-" side of the power supply / battery.

    Anyway, this is getting out of hand. The OP aked a simple question, and I
    gave a simple answer. This afternoon, I was in a friend's computer repair
    shop. Bear in mind that he deals with monitors of all types and makes on a
    daily basis. I asked him how he would go about determining the polarity of
    such a monitor, and he said that he would stick one side of his ohm meter on
    one of the D connector locking screws, and the other on each pin of the DC
    connector. When he found the pin that read short to the connector locking
    screw, it was his contention that he would have identified the "-" side of
    the power supply. So that's pretty much exactly what I said. He also frowned
    and shook his head, and said that he couldn't remember how many years it had
    been since he had seen a DC connector that had the pin as the "-".

    Which is also pretty much what I said ...

  6. Arfa Daily

    Arfa Daily Guest


  7. James Sweet

    James Sweet Guest

    I've certainly seen plenty of them that didn't though.
  8. zirath

    zirath Guest

    Thanks to everyone for your help.
  9. BillW50

    BillW50 Guest

    In Arfa Daily typed on Fri, 28 Mar 2008 16:07:57 GMT:
    That would be okay if it were a negative ground system. But like what
    William Sommerwerck mentioned, we don't know that. The way I would do it
    is to ohm the the power in. And the lower resistance would be the
    correct polarity. Although you would need another meter to read the
    polarity of the ohm meter. As they are not standardized on multimeters.
    You could also use a diode (or LED) to learn of the polarity of the
    meter as well.
  10. Arfa Daily

    Arfa Daily Guest

    JHC !!! Do you not understand the word "modern" ? Do you not understand the
    phrase "... deals with monitors of all types on a daily basis" ? I repair
    this stuff all day every day for a living. I have done for over 35 years. I
    cannot remember the last time I saw a piece of kit of any description, which
    employed a positive ground. My friend, who owns a computer repair shop, and
    has done for many years, cannot remember the last time - if ever - that he
    saw a monitor with an external power supply, that was not negative ground
    with the connector sleeve as the negative connection.

    With so much interconnectivity between household items now, there has had to
    be a degree of standardisation on this issue, and it has evolved through a
    general concensus amongst manufacturers, that negative ground will be the

    As for your method of determining polarity, it makes no sense at all, unless
    you are assuming a series diode, which is quite unlikely in most modern kit,
    as it represents a waste of power due to its forward voltage drop. It may
    even have a shunt protection diode, in which case, your 'test' will ensure
    that the polarity is determined INcorrectly. Even if the device did have a
    series diode, depending on where the supply first goes, there is still no
    guarantee that there will be any reading at all on a standard multimeter on
    ohms. If there is not any diode - series or shunt - any reading of ohms
    obtained across the input socket, is unlikely to reveal anything meaningful.
    What is your experience in fault-finding, I wonder, to have caused you to
    have formulated such a bizarre method, and believe that it would
    uncategorically give you a correct result ?

  11. BillW50

    BillW50 Guest

    In Arfa Daily typed on Sun, 30 Mar 2008 14:23:08 GMT:
    Actually being an electrical engineer for 35 years, I could careless how
    long your friend has been repairing computers. And the reason why the
    ohm meter works is because all of the curcuits are in parallel with the
    supply. Thus you will get a lower reading when the polarity is correct.
    And you will get a higher reading when it is not correct. Thus as all of
    the circuits are reversed biased.

    Whether or not all manufactures use negative ground or not, I have no
    idea. Although in all of my experience, I have learned to never assume
    anything. And I have seen many strange designs. One of them had an OP
    amp's output connected directly to ground. I was confused about that one
    until I chatted with the designer. Then it all made sense. :)
  12. James Sweet

    James Sweet Guest

    I've been working on this stuff for years as well, not as long as you, but I
    haven't been alive as long as you've been at it either. I've never seen a
    positive ground either, it would make no sense to do it that way. It's just
    standard that this stuff is negative ground, and that metal parts of the
    chassis are grounded for shielding, I've never once seen a case where this
    wasn't true so it's good enough for me. If one is still in doubt, pop the
    cover off and check the polarity of the filter lytics.
  13. Arfa Daily

    Arfa Daily Guest

    That is unmitigated nonsense. If there is a shunt protection diode, it will
    be FORWARD biased when the polarity is WRONG. Also, the fact that my friend
    repairs this stuff all day, and as an electrical engineer, you clearly do
    not, that makes him an expert, compared to you ...
    Well, as I repair this stuff all day as well, I *do* have an idea, so that
    clearly also makes me more of an expert on this particular subject, than
    you ...
    In general, I would agree with you not to assume anything, but some things
    are a matter of convention, and in recent years, based on my direct
    experience of such things, I would stick my neck on the line, and say that
    this is one, and that all modern kit, manufactured for the domestic market,
    employs circuitry with a negative ground, to which (most) external metalwork
    is firmly bonded.

  14. BillW50

    BillW50 Guest

    In Michael A. Terrell typed on Sun, 30 Mar 2008 11:48:38 -0400:
    Yes we know Michael. And thanks again. :)
    Why do you say that? aioe is based in Italy.
  15. BillW50

    BillW50 Guest

    In Arfa Daily typed on Sun, 30 Mar 2008 16:24:25 GMT:
    If there was a shunt protection diode, then both resistance measurements
    would be forward biased. Thus you would know that since there was no
    high resistance reading. But let's assume and use your plan for a
    minute. There are plenty of examples where they don't ground the shield
    but just let it float. Yes it sounds stupid I know, but it has been done
    from time to time. And I worry about everything seemingly coming from
    China nowadays. Which IMHO is only going to make things worse. And some
    of this stuff from China, isn't even UL or FCC approved.
    That is indeed possible. I always said and believed that we can learn a
    lot even from a child. :)
    I also have seen grounds that wasn't really ground either. Shields that
    were not connected to anything. Lots of weird stuff goes on in consumer
    grade equipment. Most of it IMHO is done to save a buck. Some of it is
    just ingenious! And some of it is just sloppy engineering. And sometimes
    it was done as a last minute bandaid just to pass FCC radio emissions.
  16. James Sweet

    James Sweet Guest

    It has nothing to do with where it's based. Apparently you missed the very
    long thread about this a while back and the load of impostors trying to
    raise hell spamming from aioe. Most of us blocked the server entirely.
  17. BillW50

    BillW50 Guest

    In James Sweet typed on Sun, 30 Mar 2008 21:44:11 GMT:
    Hi James! Oh yes I did miss that one. Thanks for the heads up.
  18. Arfa Daily

    Arfa Daily Guest

    Michael. I'm not quite sure exactly what you are saying here. I do not doubt
    that you encounter equipment with a negative pin connection on the coaxial
    DC connector. I have not disputed this during this thread. In fact, I
    actually said in my original reply to the OP, that although these days, pin
    = "+" is the common convention, it is by no means cast in stone. I'm sure
    that even though you do have dealings with negative pin equipment, you would
    concede that positive pin is by far the more common at this point in time,
    and has been for some years. Irrespective of which pole of the connector is
    the positive one, you seem to accept that ground being negative is the "...
    given in most instances", which is what most of the controversy generated
    within the thread, has been about. So as far as I can see, we are both 'on
    the same page'.

    I don't understand what you are saying about the word "modern". It is quite
    a well defined word, and fits well, in this context, with the dictionary

    particularly with the entry that refers to it being something that is "not
    obsolete". Whilst you are correct that it is a subjective word, in the case
    of electronic equipment, I would contend that most electronic engineers
    would infer something of the order of 8 -10 years to be meant, when calling
    electronic equipment "modern".

    And Bill.

    I will now explain why your contention that your method will work under all
    circumstances, is not valid. You are quite wrong with your assumption that
    all of the circuits in a piece of equipment are stacked up in parallel
    across the DC input socket. Whilst this might have been the case some years
    back, the DC connector on "modern" (infer whatever period you like from that
    word) equipment, usually connects straight into some form of internal
    ancilliary power supply, or a regulator or regulators, which are often
    switching types.

    The reasons for this are manifold, but include the fact that most modern
    equipment does not contain circuitry that runs just from 12v, which is a
    typical 'standard' value for external power unit equipment, and also
    efficiency, which dictates the regulators typically being switchers. The
    various circuits contained within the equipment, are connected to the back
    end of these regulators, and are thus not connected to the DC power socket
    in any way.

    Often, the input to this regulator circuitry, contained within special
    purpose ICs, will not produce any meaningful ohms reading, when subjected to
    the low test voltage from a multimeter. So, you are just as likely to read a
    virtual open circuit across the socket, irrespective of which way round you
    have your meter. This, in itself, will not help you to determine the
    polarity. But worse. If the unit employs a shunt protection diode, when your
    meter is connected //backwards// to the correct polarity, you will get a
    reading of 700 ohms or so, but when it is connected the //correct// way
    round, you may well read infinity or near. By your definition of how your
    system works to determine polarity, that would give you a clear indication
    of which was the correct polarity, but would actually yield the *INcorrect*

    As for external metalwork not being connected to the internal common ground,
    these days, that is rare. I do come across the situation sometimes, on AV
    amps, where the RCA socket sleeves are floating with respect to the chassis,
    but it is the exception rather than the rule, and is done to help alleviate
    potential ground loop issues when connecting to other equipment.

    As far as the polarity of the ground goes, I am prepared to say that in my
    considerable experience, on modern equipment designed for the consumer
    market, it is always negative. And that really is about as cast in stone as
    anything in electronic design ever is. I could of course be wrong on this,
    but if anyone wants to correct me with specific examples - remembering
    "modern" and "domestic", I'm listening, and willing to modify my position on

    I say again, that the OP asked a simple question, to which there was a
    simple answer. I don't really believe that there was any need to muddy the
    waters to the extent of all of this silly stuff that has been put forward,
    but hey - ho. I guess it all makes for an interesting life ... d;~}

  19. BillW50

    BillW50 Guest

    In Arfa Daily typed on Mon, 31 Mar 2008 00:37:00 GMT:
    Sorry if you got that impression, but that isn't what I meant.
    Yes I remember.
    Yes but the regulators are.
    Yes... so if you have a high/low resistance readings, this is very
    useful. If you have a low/low or a high/high, checking the resistance
    isn't useful under these conditions.
    True, but it will not hurt anything. As the shunt will only allow about
    a negative 0.7v to the rest of the unit. So unless the shunt blows, it
    shouldn't ever hurt it. You can possibly damage the adapter, but it
    should be protected against such anyway. But it is cheaper than the unit
    it is powering anyway. ;-)
    Yes I agree. But thinking that way, you may fry every positive ground
    you come across. If that is an acceptable risk for you and others...
    well what can I say? While some risks are acceptable to me, frying
    something because you had the polarity wrong just isn't one of them. LOL
    I believe having enough information to make a wise choice is far better
    than bozos telling you that you have to do it this way and there are no
    becauses. :D
  20. Arfa Daily

    Arfa Daily Guest

    Well, I guess we could go on dancing around this one for ever ...

    With the circuit loads being connected to the back end of regulators that
    are likely to have high input resistances with respect to the low test
    voltage supplied by a multimeter, you really are unlikely to read anything
    meaningful across the DC input socket. Trust me. I do this (very
    successfully) for a living. Reading high-low, low-low, high-high, is
    fundamentally useless to determining input polarity, unless you have a
    schematic for the equipment to know what you are looking at. If you had a
    schematic, you would not, of course, be trying to determine the polarity in
    the first place ...

    It is also not necessarily true that you will damage nothing if you do
    arrive at a wrong conclusion as a result of applying your ohm-meter test to
    an equipment which employs a shunt protection diode. Remember that the power
    supplies for LCD TV sets and monitors, when these are external types, are
    capable of supplying typically 2 to 4 amps. This is plenty enough to destroy
    a typical 1 amp shunt diode, or to blow a pico or surface mount fuse, as is
    typically found in such devices, or even to take out print, which is
    sometimes deliberately 'necked' to provide a fuse function. Shunt protection
    diodes seldom survive a reverse connection. Ask anyone who repairs CB
    radios, or PMR radios, or plain old car entertainment radios.

    Even if no shunt diode is used, there is still no guarantee that any
    regulator device which has reverse polarity applied to it, will survive. I
    have seen plenty that haven't.

    I honestly don't believe that I am going to fry *any* positive grounds that
    I come across, for the simple reason that on modern equipment, I just don't
    come across them. They died out pretty much with germanium PNP transistors.
    Obviously, if I was trying to determine the polarity of a piece of 30 year
    old kit, I would take the trouble to employ different methods to do so, on
    the off-chance that it might have a positive ground, but again, trust me,
    positive grounds simply *aren't* encountered on modern equipment.

    I hope when you refer to "bozos", you are not including me in that, as it
    would cause me to take extreme offence at you. Having enough information to
    make a wise choice is indeed a laudable objective, but discounting the
    advice of someone who has more than 35 years declared experience in a field,
    borders on stupidity. I certainly would not tell anyone that they *must* do
    it this way, but if I believe, based on my considerable experience in the
    repair field, that a particular method is likely to yield a correct answer
    with a better than 95% certainty, then I am going to advise them of this,
    which I believe is the way I approached the OP's original question, in the
    first place.

    Whilst there are always "becauses" as you put it, in this particular case,
    their validity is negligible, for all of the reasons that I have
    (painstakingly) explained over and over.

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