DIYbanter - View Single Post - polarity of power cord

zirath

zirath wrote:
Arfa Daily wrote:
"zirath" wrote in message
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Arfa Daily wrote:
"Arfa Daily" wrote in message
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"zirath" wrote in message
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Tim wrote:
In article xTkLj.3775$bQ1.832@trndny09,
says...
Arfa Daily wrote:
"zirath" wrote in message
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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.
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

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.

- Tim -
The adapter symbol shows center + and side -. It is labeled 12vdc
4a. The monitor is 12vdc 2.5a so the adapter should have enough
power.
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 *says* that it is rated
for 4A ...

Arfa

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.

Arfa
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.

Arfa

I was able to see the image on the screen - but the monitor remained
black the whole time (it didn't go back to black).

(I shined a bright light on the monitor at an angle).