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The reason that you can often get away with using the wrong "brick" is
because most of the monitor's internal circuitry requires voltages other
that 12 (usually 5 and 3.3 among others). The monitor has DC-DC converters
inside which will maintain a constant output voltage over a wide range of
input voltages. When using DC-Dc converters, the higher the input voltage,
the less current will be drawn from the supply. I'd be careful going too
high though, as there will be some devices requiring 12V (audio op-amps
spring to mind). Also, there are probably some electrolytic capacitors
across the 12V rail rated at either 16V, 25V, or 35V. If 16V - 19V "brick"
will kill them. With 25V - 19V will be close, but probably ok. If the caps
are 35V rated, then 19V won't harm them.

When choosing a replacement PSU (brick), the voltage is usually the most
important point of specification. If the original was 12V 4A, then the
replacement should be 12V (or close, eg. 11-14ish) and at least 4A. If the
monitor requires 4 amps, a 2 amp PSU won't be able to cope. A 12V 10A PSU
will work fine. A 20V 4A supply is not recommended.

As for repairing the old "brick", in my experience, the most common
cause of
failure of switch-mode PSUs is electrolytic capacitors 'drying out'.
This is
often because the PSUs are built to a pathetic budget, meaning the
cheapest
parts are used, not the most appropriate. Switch-mode PSUs operate at high
frequencies (usually in the region of 100KHz). With current flowing in and
out of the bulk electrolytic capacitors at these rates, ESR of the
capacitors becomes crucial in order to keep ripple on the output (and
instability as a result) low. When capacitors with high ESR are used, the
result is that the capacitors heat up while in use. This often causes the
top of the can to bulge until the weak point in the middle breaks. At this
point, the electrolyte will evaporate. As this happens, the effective
capacitance of the capacitor reduces to almost nothing. The ripple of the
PSU will increase and eventually, the PSU will no longer even start.

In short, when I am presented with a faulty piece of equipment which I
suspect may be the SMPS, I immediately visually inspect the
electrolytic for
signs of bulging. If there are any with even the slightest of bulges, I
replace them without question. 9 times out of 10, this has worked.
Just last
week, I repaired a PC which would power on and then power down straight
away. I took the cover off and there were 7 caps on the MoBo with bulging
tops. I replaced them and the PC now works fine.

Hope this is insightful for some of you. Please tell me if you think I
missed the point, or have just wasted a lot of your time reading this.

Thanks

Pipester

From: Electronics_101@...
<mailto:Electronics_101%40yahoogroups.com>
[mailto:Electronics_101@...
<mailto:Electronics_101%40yahoogroups.com>] On Behalf Of Roland F.
Harriston
Sent: 24 January 2011 15:41
To: Electronics_101@...
<mailto:Electronics_101%40yahoogroups.com>
Subject: Re: [Electronics_101] LCD monitor power supply

An excelllent source for a variety of power supplies is Marlin P. Jones
Associates, Inc., located in
Lake Park, Florida (www. mpja.com). Prices are very reasonable, and a
wide variety of linear and
switch-mode power supplies are available. MPJ is always my first choice
when I need a power
supply.

Another thought: "Wall Warts" are not (generally) NOT regulated. A
"nominal" voltage is usually
specified on the unit, but since they are not regulated, the actual
voltage delivered to the device
being powered can vary a great deal, proportionate to the V/I demand of
the device being powered.

Switch-mode power supplies are almost always regulated, and these are
they types of supplies
that are used with computer monitors, etc.

IMHO, using an unregulated "wall wart" to powrer some devices can lead
to drastic disaster
at worst, and poor and erratic operation at best.

MPJ usually has in stock, a variety of switch-mode, regulated "bricks"
that are reasonably priced.
I never attempt to repair a failed switch-mode brick..............it
just not economically worth the
effort........that is.........if you can ever actually repair the
blasted thing and get back to spec.

For example: MPJ shows several 12VDC bricks with 1 Amp, up to 4 Amp
capability at prices
around 10 to 15 bucks.

Can't beat MJP's prices, variety, and fast shipment.

Spend your time designing something that's fun and useful.

Roland F. Harriston, P.D.
****

Roland F. Harriston

Andrew Villeneuve wrote:

I've actually had this exact same problem ( 19" LCD with a 12V/4A power
brick that burnt out ), and since I had a pair of 12V/2A wall-warts

laying

around, I ganged them together in parallel. Seems to work great.

Barring that, my standard approach to these problems would just be

to hook

up a spare ATX power supply - you can usually find them for free in
old PCs
that people throw away because they're underpowered they don't know

how to

clean up their virus collection.

-Andrew

On Wed, Jan 19, 2011 at 11:27 AM, Reese <reeza@...

<mailto:reeza%40reeza.com>
<mailto:reeza%40reeza.com>

<mailto:reeza%40reeza.com>> wrote:

I have a bad news story. The 100-240 v input, 12v 4A output power

brick

that sends power to my 19" LCD monitor apparently took a dump

overnight.

An LED on the brick does not light up properly, I can't measure

anything

on the output side. I've already looked in my spare parts piles, I do
not have any power bricks or wallwarts that come close enough.

I'm in the process of opening the plastic case, to see if anything is
obviously wrong and can easily be fixed. It's glued (no screws) so it
is slow going with a hobbyist slitting saw:

<

600-h/413S586EGEL__SL500_AA280_.jpg

On the chance that the brick is properly toasted and cannot be fixed,
what are some other options for 12vdc @ 4a to power the monitor?

Yes, I can Google for something, but I thought I'd ask here first. If
something quick and easy makes for a good project build, why not do
that instead of rushing of to purchase a turnkey item?

Reese