🚀 go-pugleaf

In sci.electronics.design Werner <Werner@nospam.com> wrote:
> 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.

There should be no problem with oscillation.
Add a bypass capacitor across the resistor to cut down on any problems.

A simple 100 ohm/1.2K divider between 12V and 0V, and the comparator connected
to the TV end of the 0.3 ohm resistor.

-- 
http://inquisitor.i.am/    |  mailto:inquisitor@i.am |             Ian Stirling.
---------------------------+-------------------------+--------------------------
Lord, grant me the serenity to accept that I cannot change, the
courage to change what I can, and the wisdom to hide the bodies
of those I had to kill because they pissed me off.                  -  Random

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

"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

In sci.electronics.design Ian Stirling <root@mauve.demon.co.uk> wrote:
> In sci.electronics.design Werner <Werner@nospam.com> wrote:
>> 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.
> 
> There should be no problem with oscillation.
> Add a bypass capacitor across the resistor to cut down on any problems.
> 
> A simple 100 ohm/1.2K divider between 12V and 0V, and the comparator connected
> to the TV end of the 0.3 ohm resistor.

As others have pointed out, I'm wrong.
Somehow I managed to misremember that the comparator in question works down
to the negative rail, not up to the positive one.

If the monitor takes a composite or UHF input, you might consider moving
the resistor to the negative rail (and dropping it's value to 0.01 ohms or
so) and using the comparator across that).

When bypassed with a capacitor, you won't notice any difference.
(you need to check for any ground connections other than power on the monitor)

-- 
http://inquisitor.i.am/    |  mailto:inquisitor@i.am |             Ian Stirling.
---------------------------+-------------------------+--------------------------
All I want is a warm bed, a kind word and unlimited power -- Ashleigh Brilliant.