Voltage & Current..

From: Steve Thatcher <melamy_at_earthlink.net>
Date: Mon Jan 26 16:30:28 2004

I am not sure who started this thread, but why not talk about a solution
rather than what is wrong with the world of wall warts. If the end in site
is too "know" when the AC input goes below a certain threshold, then you
need to know two things to make a determination of "low AC line" when
utilizing a wall wart as your power source and "line voltage sensor". You
need to know the peak voltage you are getting from the wall wart under load
conditions and you also need to know the load the wall wart is experiencing
at the moment.

The peak voltage can be gotten "cheaply" by using a diode off the positive
side to a capacitor that has a resistor in parallel with it to ground. The
cap will "charge" to the peak level regardless of ripple, etc. The resistor
is chosen to bleed off the cap's charge fast enough for your sensing
requirements, but slow enough not to "see" the ripple from the wall wart.

The load current can be gotten from a "small" series resistor (0.2-1.0
ohms) in line with the positive side of the wall wart. 5ma to 850ma was
mentioned in the discussions here. Don't worry about small currents back
rather be interested in higher currents will cause the wall wart voltage to
degrade. The differential voltage on the resistor will give the load that
is being "presented" to the wall wart.

Depending on the wall wart, the "brown out" curve can be experimentally
determined with a variac on the AC side feeding the wall wart. The software
can then look for terminal points on the load and the peak voltage to
determine when to cut over to battery supply.

best regards, Steve Thatcher
Received on Mon Jan 26 2004 - 16:30:28 GMT