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I am probing two pins on a custom circuit board which I believe have been shorted somewhere and am attempting to measure the resistance between these pins with an Agilent 34401A multimeter. When trying to get a definitive resistance measurement, the readout of the multimeter does not settle on a single value, as it would if measuring a passive resistor. Instead, it jumps around between displaying values of 5ohms, 40 ohms, 25ohms, etc.

When measuring these pins on a unit that does not have the short, the readout is between 2-4kOhms, but also does not home in on a single value and instead jumps around in this range.

Is there any conventional knowledge or perhaps a rule of thumb as to why this is happening, or is it an issue that requires more detailed knowledge of the specific circuitry?

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    \$\begingroup\$ For clarification, you are probing a blank PCB or an assembled PCB? And have you tried turning off the 34401's auto range mode? \$\endgroup\$ – MarkU Nov 12 '15 at 20:53
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I've seen this happen before with my Agilent (and other) DMMs. When you measure a resistor directly, the meter is essentially running a small current through the resistor and watching for the voltage across the leads.

If there are other components involved (like the ones on your assembled PCB) you can get some odd measurements back - particularly when there are capacitors or transistors in circuit that store or switch the energy added to the circuit by your meter.

It might be helpful to turn the auto-ranging function of the meter off.

One of my favorite features on your meter that I often use for finding shorts on a PCB is the four-wire resistance mode. If you can scrounge up four leads you can put the meter in this mode and it can measure the resistance with greater precision. This probably won't help your "jumping around" issue very much, but the extra precision may help guide you as you work to get closer to the short(s) on your board.

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This is to be expected, particularly if there are components (other than resistors) on you PCB. Resistance is just the ratio of voltage and current, given by Ohm's law: R = V/I. Resistors are devices that have constant resistance, other devices have more complicate voltage-current relationships.

For example, when a capacitor is discharged you would measure a low resistance (apply a voltage and you would measure a high current), but as it charges up, it's ability to take on more charge diminishes, so the current will reduce thus its effective resistance will be high. An inductor is the opposite, when you first apply a current the voltage across it will spike (high resistance) but as it reaches steady state the effective resistance will go to 0.

And transistors (the building block for most ICs) have resistances that vary as the voltage (or current depending on the type of transistor) applied to their gate pin varies, which means that the resistance you measure in one part of your circuit can very based on what's happening in another part of the circuit. This is especially true for ICs, the pin resistance you measure will vary as the internal state of the IC changes.

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