Tracking down "excessive current"
in [Repair]
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From: D Yuniskis on 11 Jun 2010 14:56 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
From: GregS on 11 Jun 2010 15:46 In article <huu0rv$68h$1(a)speranza.aioe.org>, D Yuniskis <not.going.to.be(a)seen.com> wrote: >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 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
From: D Yuniskis on 11 Jun 2010 16:29 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)
From: nesesu on 11 Jun 2010 18:34 On Jun 11, 1:29 pm, D Yuniskis <not.going.to...(a)seen.com> wrote: > Hi Greg, > > > > > > GregS wrote: > > In article <huu0rv$68...(a)speranza.aioe.org>, D Yuniskis <not.going.to....(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)- Hide quoted text - > > - Show quoted text - 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.
From: D Yuniskis on 11 Jun 2010 18:42
Hi Neil, nesesu wrote: > 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 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?" :> > 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 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. > 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. |