[SI-LIST] Re: DC resistance of the Power Supply on PCB
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 30 Nov 2005 10:29:20 -0500
> 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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