No this is not about green energy or climate change. It’s about DMM repair, of course. I found it interesting for two reasons, the fact that a very simple fault almost escaped detection, and that the root cause was, while totally obvious to the naked eye, possible to dismiss as just a cosmetic problem.
The victim in this case is a fairly recent Ideal-branded multimeter, a model 61-481 (from Taiwanese maker APPA Technology Corp.) It was the subject of a teardown post over at the EEVblog forum, and then passed by my workbench on the way to someone else. The DMM was described as basically working, but acting a bit funny and not reading voltage correctly in some ranges.
My usual method of testing is to connect the meter to a variable DC power supply, in parallel with another trusted DMM, and see if the readings agree as the voltage is varied from zero on up to the supply maximum. This one agreed within a few counts of the reference DMM all the way from zero to 60VDC. Even the small 5.000V reference board that I use produced a reading of 4.99V, which is fine. [Note for future reference that the TI REF5050 chip used on that device is capable of sourcing 10mA.]
Living Life By the Drop
A clarification on the reported symptoms revealed that when the unit was deemed faulty, it was tested with AA and 9V batteries, and read low on the 9V battery, while reading the AA correctly as compared to a known-good DMM. The difference between those two batteries? Effective internal resistance. A 9V battery has a higher internal resistance than an AA battery, even more so if it has some age on it and is a little weak. A sufficient amount of current draw could lead to a significant voltage drop.
A normal, modern DMM has a high input impedance, usually 10 Megohms, which means it should not draw enough current from a voltage source, even a weak battery, that would cause any appreciable voltage drop. (This is why multimeters do not make the best battery testers.)
An ideal 10V source with a 10 Megohm load should produce a current of 1 μA. (10V/10,000,000Ω = 0.000001A) For a controlled diagnostic test, let’s connect the the suspect unit to a 10V source through another DMM, configured to measure micro-amps. The ammeter’s 100Ω shunt will serve to represent the internal resistance of a battery, as well as allowing us to measure the current. This test shows the Ideal to be drawing 3708μA! That’s 3.7mA, definitely enough to bring down a weak battery. And indeed we can see that 0.37V is being dropped across the 100Ω shunt, yielding a 9.63V reading on the faulty DMM. Ironically, this reading is completely accurate, but due to an exaggerated observer effect, is also completely wrong.
Opening the meter up, there are no problems immediately visible. But after removing the switch rotor, it’s clear there has been a spectacular “event” of some sort. It appears that the meter was subjected to an over-voltage input, causing an arc to flash over from one PCB track to another. It would be easy to dismiss this as “just a black mark” on the PCB, but remember that a printed-circuit board is comprised of an organic resin and glass fibers. When organics get hot enough to combust, the hydrogen reacts with oxygen, leaving the carbon behind. And in addition to causing atmospheric problems, carbon conducts!
Probing with an ohmmeter between the middle of the blackened area and one of the adjacent copper tracks yields an unsteady reading of 150 to 2000 ohms, depending on how the probe is held. This is forming a shunt path between unit’s input jacks, causing the lowered input impedance and higher current draw. Simple repair: remove the carbon residue.
The moral(s) of the story? DC power supplies do not make a complete DMM test. Always look under the switch rotors. And remember that organic materials that start out as insulators, can become very good conductors after being subjected to enough heat to cause combustion. Proving once again, there’s magic smoke in everything.