Author Topic: Blown multimeter switches  (Read 650 times)

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Online najrao

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Blown multimeter switches
« on: May 14, 2018, 09:54:33 am »
I have previously dealt with this issue under "u1241b multimeter woes".
In my huge collection of modest meters, I have seen this blown-switch-tracks syndrome on quite a few. Most are not worth doing anything about, but this latest was an 80000 count super duper job (withholding name; was accused of dragging Keysight through mud falsely), so got into it to see what can be done.
The before pic tells all: signal from hot volts input applied to closely spaced tracks at switch has somehow bridged across and chewed up the fibreglass. The usual 1k fusible resistor, ptc and spark gap failed to stop the let-through, and remain unaffected. The energy at the fault has had to be several kJ as evidenced by the gaping cavity created by it. And there was no soot or smudge spread out around either. I was then, and am now, baffled by what sort of pulse  could get through 1k, not fuse it, but reduce some 100mg of epoxy fibreglass to cinder.
I did the root canal surgery and used a Kevlar wire shunt on the back, drilling a 0.5mm hole to get one end in.
The meter works well as new, and has a better intertrack spacing to boot.
Joeqsmith,  sure you would be interested, but can you think of a more plausible explanation?

Offline joeqsmith

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Re: Blown multimeter switches
« Reply #1 on: May 14, 2018, 10:40:07 am »
Not sure but I do like how the wiper contacts have cut through several of the traces mask and I always like them putting the vias right in the middle of the switch contact pads.  From this, I would say the layout is poor at best. 

For the input protection circuitry to do it's function, it requires a well designed PCB.   If you watched that video on the UT181A, they too made some poor choices in the layout.   
How electrically robust is your meter??

Online najrao

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Re: Blown multimeter switches
« Reply #2 on: May 14, 2018, 02:55:00 pm »
I hold no brief for the pcb designer, perhaps there are faults, including via's placement, and certainly track spacing.
But  I have to remain curious, baffled,  about what hit it to cause such damage. Unless the energy came in as a stray packet of high energy gamma rays from deep space, tightly focussed to hit precisely, it was an electrical input at the hot socket. This means high voltage and/or high current,  the product making up the power,  for a time sufficient to deliver the energy needed to burn up the fibreglass. Now, even 5kV will pull only 5A through the input protection, and could not have lasted even a microsecond  since the fusible resistor was unaffected. If we admit to there having been an arc at the place of burn,  this would have reduced the current to even lower, but may have extended the time somewhat. Still, the arc power would be its voltage current product, and the energy delivered would be less than a few joules at most. A long  'arcing' period is possible only at very low current, below the survival rating of the fusible resistor;  indeed it may not be an arc at all, but some kind of surface discharge, with the ability to build up heat enough to a temperature high enough to burn -- slowly.  I called this 'tracking' before; metal particles from contact wear, and any other poorly insulating debris may have accelerated the effect. Note that the blow-out is always at a position of critically low track clearance,  and between tracks subject to high voltages from the input terminal. If my surmise is the likely failure mode, new design rules are required for such meter switches. May be Keysight is listening.
I would appreciate any criticism, and would stand corrected by higher wisdom.

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