In news:Z8PHj.18868$ snipped-for-privacy@newsfe6-win.ntli.net, 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. :(
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.
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 definition
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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* polarity.
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 it.
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;~}
In news:MmWHj.26784$ snipped-for-privacy@newsfe5-win.ntli.net, 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
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.
The ohm meter showed something like a capacitor on the center pin (it went to 0 and then back towards the center). The side showed no resistance.
The monitor went on briefly and then went black when plugged into the power. Seems like it's a dud. Not sure if it's worth trying to repair it or just return it.
Is there no little pic of the connector around where it give the info about power? Usually there is a universal icon used that indicates polarity, kinda like a big C around a dot. Most, but not all, coaxial power connectors have the minus side on the outside, that way if it touches a grounded side, it will not destroy the adapter. If the adaptor cannot start the monitor because it is not powerful enough, it will sometimes just flash the backlight then go off.
Before condemning the monitor, I would feel inclined to just try a different power supply (bearing in mind that the one you are using now is not an 'original'). The reason that I say this is that there can be quite a high initial pulse of current demand, as the backlights strike, and it just might be that the power supply sags a little when hit with this, even though it
Thinking about it again, were you trying it with a signal going in ? LCD monitors, like their CRT counterparts in many cases, don't just 'idle' when there is no signal. They do just like you say - that is power up long enough to check if they can detect an input signal, and if they can't, go back to an inert condition.
I tried it in a few different conditions: with the monitor on 1st and then the computer - it showed the computer booting for a few seconds and then went black - then it came on again after the boot was finished for a few seconds and then went black again; with the computer on and then the monitor; with the monitor disconnected from the computer (the manual says it's supposed to bring up a diagnostic screen in this condition but it didn't). The screen stayed black the whole time in subsequent attempts.
The power led would go from yellow to green when the computer started booting but the screen stayed black.
I also noticed that the screen was not completely flush in the housing - like someone had tried to, or did, open it and didn't close it completely.
The fact that the light stays green, is a good sign. What you next need to do, is to shine a strong desklamp or hand flashlamp at the screen at an angle, after it has gone back to black. If you can then see an image on the screen, that indicates that the backlamp tubes are not staying alight. This could well be because either the inverter is faulty, or that a worn tube is making the inverter shut down. Either is quite a common fault condition.
OK. Well that tells us that the whole monitor part is working, and the failure is in the LCD screen backlighting circuit. It is generally a bit fiddly, but it is repairable if you are that way inclined. Unless you are able (or want to) just throw it back where you got it from.
If the screen comes apart easily, start by examining the backlight tubes. Be careful when extracting them as they are quite fragile. If any are badly blackened at the ends, suspect them. It's hard to test the inverter, without having known good tubes to connect to it. If you have a 'scope, you can get a good idea by looking to see if you have waveform at the primaries of each of the tube high voltage drive transformers. If you do get a momentary burst that then disappears, this could be because the inverter is detecting a bad tube, and shutting down. You might see the tubes initially flash up, and you might see one not do so. You can disconnect the tubes one at a time, and see if that keeps the inverter running, and if it does, you can cross connect the tubes to the opposite inverter outputs, to prove the drive electronics. Inverters and tubes are available from a number of sources. See, for instance,
One thing I started wondering about after you were saying it could be a problem with the adapter--
The picture of the original adapter that came with the monitor says the plug is supposed to be 6.5mm o.d. 4.4mm i.d. and 9.5mm long. (Thanks to Michael Terrell -
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.
The measurements of the plug that came with the adapter I got (from ebay) are : 5.5x2.5 / 9.5mm.
As long as the plug is 'making' ok to the centre pin, it shouldn't be a problem. However, the way round that you have described it sounds wrong. If the original was looking for a plug with an id of 4.4mm, (which is *very* big for this type of plug) that would suggest that the monitor has a 'thick' pin, which I'm surprised that your replacement at just 2.5mm, will fit over.
2.5mm is a pretty 'standard' size.
However, if you can see an image with an external light shone at the screen, that says that the plug *is* making ok, and the whole monitor is working, less the backlights. So no, I don't think that will be the problem.
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