🚀 go-pugleaf

"Rob Paisley" <rpaisley4@cogeco.ca> wrote in message
news:bb39a577.0304291800.392d720b@posting.google.com...
> Here is some Comparator information.
>
> http://home.cogeco.ca/~rpaisley4/Comparators.html
>
>   You could adapt one of the time delay circuits about 1/2 way down
> the page to eliminate any jitter but it should not be a problem with
> such a large spread between the high and low current levels.
>
>   At 725 milliamps the voltage  drop across 0.3 ohms should be about
> 0.22 Volts
>
>   Use a threshold voltage of about 0.1 Volts.  This will give a
> detection level of about 330 milliamps.
>
>   This is assuming the resistor is in the common side of the circuit.
> If it is in the supply side a simple comparator will not work.
>
> Rob.
>
>
> "Werner" <Werner@NoSpam.com> wrote in message
news:<Cfsra.66831$t_2.6142@afrodite.telenet-ops.be>...
> > Hi fellow electronics enthusiasts,
> >
> > I'm trying to build a on/off detection circuit, which is able to check
if
> > the connected LCD monitor TV is in standby mode (31 mAmps/12 volts) or
> > active mode (725 mAmps/12 volts). I have put a 0.3 Ohm resistance in
series
> > with the 12 Volts DC power supply, so I have a small voltage drop across
my
> > 0.3 Ohm resistance, maybe enough to measure this drop using an LM311
> > comparator IC.
> >
> > But I'm afraid that the LM311 might go into an oscillation state as the
> > voltage drop is small ... so I've thought about adding an LM318 (diff.
> > amplifier) in front of the LM311 to boost the voltage drop signal ...
but I
> > don't seem to be able to get this to work correctly. Both LM's I would
like
> > to power off the 12 Volts DC supply of the LCD monitor.
> >
> > Can anybody suggest? How to wire it up?
> >
> > Thanks a million,
> >
> >
> > Werner

Won't work that way either, because the LM311 isn't a rail-rail chip.   The
input has to be at least 1 to 1.25 V less than the positive rail and greater
than 0.2 to 3 V above the negative rail.  My advice is to either use a
differential gain stage between the sense resistor and the comparator or
find a comparator that can handle rail-rail inputs.  Better yet is to get a
high-side sense amp such as the MAX471 or Max472.  The MAX471 has the sense
resistor built-in, and it's only 0.035 ohms, making a lot less voltage drop
at the load.  Check the data sheet and app notes for these chips at
http://www.maxim-ic.com/PowerSupplies.cfm.

Tweetldee