On Fri, 11 Jun 2010 13:29:06 -0700, D Yuniskis
wrote:

Hi Greg,

GregS wrote:
In article , D Yuniskis wrote:
I recently had to repair a small LCD TV that was
blowing -- very SLOWLY -- it's DC mains fuse.
I.e., the set was drawing more current than it
was designed to draw. But, not a catastrophic failure
(e.g., nothing *shorted*). In fact, the set would run
for a day or more at a time "perfectly".

The fused supply fed the primary DC-DC converter for the
set. I.e., damn near all of the loads hang off the multiple
outputs of the switching transformer.

After tracking down the problem, it occurred to me just
how hard it is to do such things -- since schematics never
tell you what sorts of *currents* pass through each circuit
node (though you can often find indications of *voltages*).

So, how *should* this problem have been approached (without
risk to the set), out of belated curiosity?

I often use light bulbs on the mains to limit current.

Not an issue with this set. The set ran "normally" (except
blowing the fuse every day or two).

I also think SMPS would make attempts at limiting current
pretty useless (for such small loads)

i don't have one, but the old Textronix Hall current probe
was usefull for tracking shorts.

Again, no shorts here. Everything *worked*.

I also use the hand method of feeling warm paths and parts.
Sometimes an IR temp meter.

I think a PIR imager would have been helpful. Though not
sure if even that would have helped (though it probably would
for some types of failures)

Have you tried putting a meter on the output of the power supply to
see if the current draw is in side the monitor, and how much more than
expected the current is? You also may want to check to see if the
backlight system is what is drawing the excessive current.

Trying to current limit the input of a SMPS is usually not viable. As
input voltage drops, the SMPS simply tries to comphensate, until it
reaches an unstable point then (often bad) unexpected things happen
(or it simply shuts down).