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

• From: zhangkun 29902 <zhang_kun@xxxxxxxxxx>
• To: a.ingraham@xxxxxxxx
• Date: Thu, 01 Dec 2005 07:19:01 +0800

```Dear Andy

Thank you for you help.

As I know, the voltage of multimeter is about 12V. 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? In another word, the power net
is of 1.5V. When I measure the dcresistance, I should adjust the voltage to
1.5V. Therefore, I will get the correct dc resistance.

According to my experience, it is because item 3 that I didnot get the correct
result. However, I could not understand that, if there is SRC-like circuit, the
dc resistance should also be very large.

Best Regards

Zhangkun
2005.12.1

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----- Original Message -----
From: Andrew Ingraham <a.ingraham@xxxxxxxx>
Date: Wednesday, November 30, 2005 11:29 pm
Subject: [SI-LIST] Re: DC resistance of the Power Supply on PCB

> > I measure the dc resistance between Power and Ground. Normally, the
> > resistance is about dozens of ohms.
>
> At least three things can make the measured resistance less than
> "normal":
> (1) If you've connected the meter leads so that the applied DC
> voltage is
> opposite normal, it can forward-bias junctions that are normally
> reverse-biased.  Often this doesn't make much difference
> (depending on the
> meter and the selected range, often the voltage is less than a
> diode drop),
> but sometimes it can.  Some electrolytic capacitors also behave
> like diodes.
>
> (2) If there are large electrolytic capacitors that take a long
> time to
> charge up, given the low ohmmeter currents from the multimeter,
> and you
> didn't wait that long.
>
> (3) Active circuits can behave in unexpected ways in the absence
> of normal
> voltages; sometimes there are parasitic devices and SCR-like
> structures that
> turn on.  Abnormal currents should be expected when supply
> voltages aren't
> in the normal range.
>
> Regards,
> Andy
>
>
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