Tracking down "excessive current"

Hi,

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?

Thx,

--don

Reply to
D Yuniskis
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I often use light bulbs on the mains to limit current. i don't have one, but the old Textronix Hall current probe was usefull for tracking shorts. I also use the hand method of feeling warm paths and parts. Sometimes an IR temp meter.

greg

Reply to
GregS

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)

Again, no shorts here. Everything *worked*.

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)

Reply to
D Yuniskis

Perhaps if you said what the problem turned out to be, we could judge if there might be any 'easy' way to track such a problem current. For example, it would be almost impossible to locate excess leakage current in the PS output filter cap since it is 'inboard' of any circuits that might have voltage/resistance given such that proper current could be calculated from. It would also be difficult to use heating of the cap as an indicator unless the 'normal' temperature was known since such a cap can heat from ripple current/ESR [I^2*R], leakage dissipation or just high ESR. Where practical, I substitute a bench power supply for the unit's built in supply so I can monitor the current drawn, but where the unit being tested has a multi-output supply such as you describe, that may not be feasable.

Neil S.

Reply to
nesesu

Ah, but that's not fair! "Monday morning quarterback". The point of my question is, knowing *exactly* what I knew when I sat down with this problem, what's the *best* way to proceed.

It's sort of like asking *how* to locate your (lost) car keys and being asked "Where did you *find* them?" :>

That's true of all components! I.e., you can look at a schematic and not (easily) determine what the current flowing through a particular node will be (unless it is a simple passive network). Or how warm a component would be, etc.

Reply to
D Yuniskis

"D Yuniskis"

** So this fuse was not being stressed all the time - ie bending or faintly glowing ?
** So ONE of these loads was intermittently drawing high current ?
** 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

Reply to
Phil Allison

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?
Reply to
N_Cook

hotspot

output

and

at

across

as

1.5

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

Reply to
N_Cook

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).

Reply to
PeterD

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")

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").

See above.

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")
Reply to
D Yuniskis

this is an interesting practice- logging such info.

Reply to
Cydrome Leader

(...)

That project is done. And it is cheap too!

formatting link

--Winston

Reply to
Winston

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'?"

Reply to
D Yuniskis

"Cydrome Leader"

** 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

Reply to
Phil Allison

"D Yuniskis"

** 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

Reply to
Phil Allison

Pardon me but if everything "worked" fuse would Not be blowing.

My first thought to a 2-3 day fuse is an "intermittent" problem.

Some ways I handle the intermittent is by close up observation of the boards, heating and or tapping components out.

how hard it is to do such things.

We all know if it is not a loose connection Intermittent are not easy.

risk to the set),

As much as I hate saying/doing this - Sometimes risk is the only way to the solution....

But since it is already fixed I think the best answer to your question would be if we knew how you fix it.

Gabe

Reply to
Alex Simpson

"Alex Simpson"

But since it is already fixed I think the best answer to your question would be if we knew how you fix it.

** Of course, once the nature of the INTERMITTENT fault scenario is known one can usually determine a faster way of discovering it.

But the OP is playing a childish game with us and LYING about what really went on.

.... Phil

Reply to
Phil Allison

Sure they can! All the fuse blowing means is the load was drawing *nominally* more power than the fuse was rated. E.g., a typical 3AG fuse will carry 110% of its rated load for several hours. Given that this fuse lasted *days*, means it was operating near its rated capacity.

I.e., nothing *severe*.

Since there is no way of knowing (from the service documents) what the *intended* load was, it could be the system was just drawing 10-20% more than expected (though I doubt that since proper derating for ambient would typically have the fuse operating at ~60-70% of it's rated current).

No. An intermittent wouldn't take (roughly) the same amount of time to show up each time the fuse blew.

You're missing the point of my question.

You are suggesting that, in fact, the way to "solve" such problems is to ask someone who solved a similar problem previously! So, when you come up with a problem that someone else *hasn't* solved, you're SOL.

My comments regarding the lack of information necessary to track down this sort of problem suggests that service documents should carry additional information. And, possibly, minor changes to circuit topologies to make this sort of troubleshooting easier.

E.g., if you have a (removable) fuse, you have an *easy* way to monitor current through that branch of a circuit. Adding a note on the drawing set indicating what this current should be, nominally, goes a long way to sorting out what problems may exist "in the field".

Likewise, *adding* disposable components to circuit paths to facilitate opening that path to monitor current can be a big win (e.g., FB's or 0 ohm shunts in the secondaries of the switching transformer in my example... easy to replace)

Of course, the conditions for all measurements have to be codified else low line, high line, component variations, etc. can make them meaningless (if the primary increases by 10%, current decreases inverse proportionately, etc.).

Likewise, scope traces with real times/duty cycles noted gives you an idea if things are operating at (or near) their correct operating point.

Hoping to see repeat failures of the same sort is a losing strategy (as it relies on design flaws instead of component failures)

Reply to
D Yuniskis

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