[SI-LIST] Re: DC resistance of the Power Supply on PCB

Hi Zhangkun,

> As I know, the voltage of multimeter is about 12V.

Maybe it takes a 12V battery, or runs on 12V external power ... but that
definitely does not mean that it applies 12V to the device being measured
for resistance.  The meter may cut it down to something much smaller.
Modern DVMs may use a voltage much less than 0.5V on the resistance ranges,
and something over a volt on the diode-test ranges.

Anyway, these are the open-circuit ohmmeter voltages.  They drop
substantially when you connect any reasonable resistance.  No good
multimeter would apply a fixed 12V when measuring a resistance of 1 ohm, or
in the dozens of ohms.

>  If I want to measure
> the correct dc resistance between the power and ground, should I adjust
> the voltage of the multimeter to that of measured power net?

Well, that depends on why you want to measure the resistance.

Active circuits can have very nonlinear characteristics, which can cause the
resistance to change by orders of magnitude.  If you want to know the
resistance under operating conditions, you do need to use the normal supply
voltage.

To do that, don't measure resistance with a multimeter.  Use the normal
power supply for the board (or a lab supply with extra current limiting just
in case) and an ammeter in one of its leads.

If you are checking for faults, it may not matter if you use the same
voltage.  Although the reading might not correlate with the resistance at
the normal supply voltage, it should be good enough for a go/no-go test (if
you remember about observing polarity and charging bulk storage capacitors).

Regards,
Andy


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