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.
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 ...
Arfa, you're usually dead-on, but this is quite incorrect. The side of the connector that's "grounded" is not necessarily negative! A transistor radio using PNP transistors would (presumably) have a positive ground, not negative.
No, it's not. I have Sony equipment where the pin is negative, not positive.
Rather than seeing which side of the power connector is grounded, I would look to see which side of the electrolytic capacitors is grounded.
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;~}
In news:KS5Hj.22783$ email@example.com, 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
Many things are NOT reverse polarity protected. Careful! Usually the center of the plug is + and the side - but you can check that with a meter. On the monitor it should be marked or say something in the manual. You can check that too with a meter if you can get to a board ground contact and ohm out the plug. Careful you don't blow a circuit with the meter battery.
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.
In news:xJ8Hj.20508$ firstname.lastname@example.org, 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.
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 convention.
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 ?
In news:gnNHj.25773$ email@example.com, 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. :)
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.
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.
I have resisted commenting but can do no longer; I don't know about the U.K., but I frequently encounter negative center coaxial power equipment in my work. These are not monitors, but a variety of consumer and industrial portable devices. When the connector is not labeled and I don't have docs, I will physically inspect the internal wiring or the pcb that hosts the power connector and also do resistance measurements between ground planes and the power connector contacts to determine ground. The assumption that ground planes are negative is a given in most instances. As for the subjective label "modern", that is a religious issue that shouldn't be a factor in good electronics practice ;)