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From: Cydrome Leader on 12 Jun 2010 12:05
Phil Allison <phil_a(a)tpg.com.au> wrote:
>
> "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.

this is an interesting practice- logging such info.
From: Winston on 12 Jun 2010 13:52
On 6/12/2010 12:18 AM, N_Cook wrote:

(...)


> I happened to be playing with the innards of a domestic PIR unit for hotspot
> "divining".

That project is done. And it is cheap too!

http://www.harborfreight.com/non-contact-laser-thermometer-96451.html

--Winston
From: D Yuniskis on 12 Jun 2010 14:14
N_Cook wrote:
> D Yuniskis <not.going.to.be(a)seen.com> wrote in message
>> 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

Sorry, I meant *imager* (not just a "thermometer").
I.e., look at large areas and see different temperature
ranges in pseudo-colors.

"Gee, why is this section so 'red'?"

> 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: Phil Allison on 12 Jun 2010 22:00

"Cydrome Leader"
> Phil Allison
>>
>> ** 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.
>
> this is an interesting practice- logging such info.


** Keep in all in my " little red book" - aka a telephone index.

Brand and model name/number, followed by idle current and typical on-load
current if any - makes it a doddle to check adjust bias settings on most
tube and SS amps.

I also list the primary ohms for transformer PSU items in the same index.



..... Phil





From: Phil Allison on 12 Jun 2010 22:30

"D Yuniskis"

> The nameplate will tell you what
> the nominal power requirements are (from which you can deduce
> the nominal current).


** Only very rarely is that true.

The amp ratings shown on the back of things like CRT monitors and audio
amplifiers give almost no clue as to the actual current draw when in use.

Eg; My 17 inch CRT monitor has a back panel rating of 1.5 amps but in fact
draws only 345mA rms from the 240 volt AC supply - the figure of 1.5 amps
appears to come from the fuse size used which must cope with inrush surges.




..... Phil


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