"John Rumm" <see.my.s...@nowhere.null> wrote in message
news:sol62h$i4q$1...@dont-email.me...
>> Has anyone come across a SMPSU which has "infinite resistance" (ie open
>> circuit even on multi megohm range) and yet which works fine?
>
> Yup - especially if you are not applying enough voltage to forward bias
> the rectifier diodes.
Ah! I bet that's it. I wonder what voltage a multimeter typically applies to
a load when it is measuring resistance.
>> Do they use capacitors as "dropping resistors" (dropping impedances) on
>> the mains pins?
>
> Not usually unless they are supplies designed for very low loads. Not
> typical for your typical external wall wart.
I remember in Elec Eng at university we were given a project in teams to
design a SMPSU with certain criteria as regards permitted variation of load
voltage as more current was drawn, and permitted range of input voltages
(provided with a Variac for the test).
For some reason that escapes me, a dropping resistor was needed on the input
to the transformer - or maybe it was between bridge rectifier and switching
transistor. (This was an "old fashioned SMPSU" where the transformer was fed
at mains frequency rather than switching frequency as happens nowadays to
keep transformer as small as possible).
One of my team-mates (and I wish it had been me!) had the brilliant idea of
using a series capacitor instead of a series resistor to do this, with a
size based on impedance at the relevant frequency. This was a lossless
solution unlike a resistor, and meant we achieved significantly better
efficiency than any other teams. We won the prize!
(I remember that the switching transistor that we were required to use did
*not* like to be turned on permanently when full load was being drawn. The
feedback circuit limited the M:S ratio to a little less than 100% even with
heavy load. But it only took a loose wire during development, before
everything had been permanently soldered, and the transistor lost its
switching signal and was on permanently. We all did it at one time or
another. There was a noise like rustling autumn leaves for a couple of
seconds, and then the transistor blew up. Ours left a dent in the ceiling!
After that, we were all made to wear safety goggles just in case...)