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From: Phil Allison on 11 Jun 2010 22:22

"D Yuniskis"

> 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".

** So this fuse was not being stressed all the time - ie bending or faintly
glowing ?


> 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.


** So ONE of these loads was intermittently drawing high current ?


> 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?


** The good, old fashioned way is to use a larger fuse and wait for
something to get hot or start smoking.

Dunno about you, but I use a current meter in the AC supply for all bench
service jobs - it is sensitive enough ( 1mA resolution) to observe small
changes in the AC current draw and indicate if the current is creeping up.
I also keep a book recording the normal current draws ( of and on load for
amplifiers ) of everything I see.

In your case, the AC draw was slowly or intermittently increasing by a
significant percentage and that means something MUST dissipating all that
additional energy as HEAT.

Normally, there would be other observable symptoms in the performance of the
unit - as whatever it is overheats.



...... Phil



From: N_Cook on 12 Jun 2010 03:18
D Yuniskis <not.going.to.be(a)seen.com> wrote in message
news:huu68u$ile$1(a)speranza.aioe.org...
> Hi Greg,
>
> GregS wrote:
> > In article <huu0rv$68h$1(a)speranza.aioe.org>, D Yuniskis
<not.going.to.be(a)seen.com> 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)


I happened to be playing with the innards of a domestic PIR unit for hotspot
"divining". Not too sure whether it would be useable. Uses RPY96 6 to 14
micron pyro and LM324 (.8 micron is red light) . Monitoring an lm324 output
for a small component 8 degree C over ambient, gave about 50mV swing up and
down of a nominal not too steady quiescent long term level. So if static at
2.3V would swing up to about 2.35 and then down to about 2.25 with a time
constant of a second or so , so again inconvenient. You have to scan across
as not a static process of generation of signal. There is a piezo effect as
well with the sensor so you would have to be very gentle in movement to
monitor 1 or 2 degree C over ambient. I shrouded the TO5 sensor with a 1.5
inch long dense foam rubber tube but matt metal maybe better, any thoughts?


From: N_Cook on 12 Jun 2010 03:59
N_Cook <diverse(a)tcp.co.uk> wrote in message
news:huvcba$rgi$1(a)news.eternal-september.org...
> D Yuniskis <not.going.to.be(a)seen.com> wrote in message
> news:huu68u$ile$1(a)speranza.aioe.org...
> > Hi Greg,
> >
> > GregS wrote:
> > > In article <huu0rv$68h$1(a)speranza.aioe.org>, D Yuniskis
> <not.going.to.be(a)seen.com> 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)
>
>
> I happened to be playing with the innards of a domestic PIR unit for
hotspot
> "divining". Not too sure whether it would be useable. Uses RPY96 6 to 14
> micron pyro and LM324 (.8 micron is red light) . Monitoring an lm324
output
> for a small component 8 degree C over ambient, gave about 50mV swing up
and
> down of a nominal not too steady quiescent long term level. So if static
at
> 2.3V would swing up to about 2.35 and then down to about 2.25 with a time
> constant of a second or so , so again inconvenient. You have to scan
across
> as not a static process of generation of signal. There is a piezo effect
as
> well with the sensor so you would have to be very gentle in movement to
> monitor 1 or 2 degree C over ambient. I shrouded the TO5 sensor with a
1.5
> inch long dense foam rubber tube but matt metal maybe better, any
thoughts?
>
>

A more practicle solution is probably a rotary vane in the viewing path to
avoid the need for manual scanning to get down to the 1 or 2 deg C
resolution level


From: PeterD on 12 Jun 2010 08:10
On Fri, 11 Jun 2010 13:29:06 -0700, D Yuniskis
<not.going.to.be(a)seen.com> wrote:

>Hi Greg,
>
>GregS wrote:
>> In article <huu0rv$68h$1(a)speranza.aioe.org>, D Yuniskis <not.going.to.be(a)seen.com> 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).
From: D Yuniskis on 12 Jun 2010 11:41
Hi Peter,

PeterD wrote:
> On Fri, 11 Jun 2010 13:29:06 -0700, D Yuniskis
> <not.going.to.be(a)seen.com> wrote:
>> GregS wrote:
>>> In article <huu0rv$68h$1(a)speranza.aioe.org>, D Yuniskis <not.going.to.be(a)seen.com> 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,

Set consists of a "brick" and the display itself.
Brick provides three (?) separate supplies to the set:
- inverter power
- audio amp power (?)
- the rest of the set

Inverters are fused separately and individually.
Fuse that was failing was in the "rest of the set" branch.

That fuse feeds the main switcher *inside* the set
(not to be confused with the switcher in the *brick*!).
The internal switcher seems to have five or six outputs
(i.e., all the various supplies needed by "the rest of
the set")



> and how much more than expected the current is?

But that's the point -- you have no way of knowing what
the "expected" current is! The nameplate will tell you what
the nominal power requirements are (from which you can deduce
the nominal current). But, that applies to the entire set!
How much current should flow through each of the fuses
(two for the inverters plus two for the audio + rest_of_set)
isn't something you can know with any certainty.

Except to say that the fuses' ratings should exceed the
nominal current they are each expected to pass! :>

Given that the set runs for a day or more without blowing
the fuse in question suggests the "excess" isn't that
severe (or, the fuse is significantly overrated *and*
the brick is overdesigned to be capable of supplying
that "excess").

> You also may want to check to see if the
> backlight system is what is drawing the excessive current.

See above.

> 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).

Correct. All you do is change duty cycles.

My question regards how you can get this sort of information
from service documents. I.e., at best, you'll see voltages
marked at various test points in the circuit. And *maybe*
some *nominal* waveforms. But, nothing that would clearly
allow you to diagnose this type of problem.

(e.g., drawing enough extra current to blow the fuse implies
the duty cycle of the internal switcher is higher than
intended. But, nothing in the types of documentation
that I just mentioned would tell you what the duty cycle
*should* be "for a typical load")
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