Measuring resistance of resistor in PCB with DMM....depends on which way round probes

[treez]

Hello,
We often measure the resistance of a 0603 resistor which is connected to the current setting pin of an offline LED driver chip. (the resistor is soldered into the PCB when we measure it). We obviously do this with the circuit unpowered.
We need to do this as its too time consuming to desolder it, measure it, then solder it back on.
With our cheaper DMM meters, the resistance reading varies depending on which way round the probes are applied to the resistor. This is obviously because the ESD diode of the chip is getting slightly forward biased by the meter in one connection orientation.
Our better quality meters don’t suffer this problem, and either way round with the probes gives the same resistance measurement.
How do we find whether or not a meter has a measurement voltage above the 0.7V or so that it takes to forward bias the circuit diodes? These meters specs don’t say…eg the Tenma ones….

Tenma DMM
https://www.farnell.com/datasheets/...MIlJea_7vw4QIVy73tCh18-QmHEAAYASAAEgLMAfD_BwE

 

[barry]

Set your meter to the ohms scale. Connect the probes to a second meter to measure volts.

Signed,
Captain Obvious

 

[treez]

Thanks, i mean how can i know the result of that before buying the meter.

 

[wwfeldman]

if you want to know the resistance of the resistor, measure it before putting on the circuit board

as you probably know, ohmmeters work by injecting a voltage and measuring the resulting current
the cheaper meter probably doesn't have the current capability if the expensive meter
hence it has to inject more voltage to get the current in a range it can measure.

more voltage, more likely to forward bias the diode
to answer your question, look at the low current capability of the meter.
somewhere between your cheap meter and your expensive meter is the current threshold.

otherwise
specify the polarity when you measure the resistance in circuit, so the diode is reverse biased.

try a kelvin connection right to the resistor

 

[barry]

If you cant find the manufacturer's spec online, CONTACT THEM. If they can't give you an answer, find a different mfr.

And buy a decent meter. They dont cost that much.

 

[FvM]

Different ohms range use different test currents, so it's possible to make a correct measurement with almost any DMM.

 

[treez]

Different ohms range use different test currents, so it's possible to make a correct measurement with almost any DMM.

Thanks, I would assume the test “voltage” needs to be higher, the higher the resistance range(?)
We have one meter which , when on the lowest (200R) range, forward biases the said diode…we would not have bought this meter if it was going to do that.
We can just switch the probes round, yes, but that issue adds confusion for our assembly staff.
The resistor in question is a power setting resistor, and sometimes we get an order in for a different power level, and have to measure the resistor on the board , and then change it…..we have to measure it, because our board stuffer has started putting non marked 0603 resistors on our boards.

 

[FvM]

Thanks, I would assume the test “voltage” needs to be higher, the higher the resistance range(?)

No, why? Most DMMs are switching the test current, using same voltage range (e.g. 2V) for maximum resistance in each range.

 

[wwfeldman]

the board stuffer works for you

assign a different part number to those with a different resistor
loaded board 12345 uses a 10k resistor
loaded board 12346 uses a 4700 ohm
etc
then order the correct loaded board from the stuffer