Thanks for all the suggestions.

This is really a simple application. I have no trim tab indicators.
All I really need to know is if they are fully retracted or extended.
When that happens, the motor current goes from about 8 amps to about
15 amps. Most of the time, the circuit is off. I just thought this
would be a simple way to tell, using a waterproof LED.



Don W wrote in message igy.com...
Norm,

Several people have suggested a low resistance current shunt, and
then amplifying the voltage across the resistor. That idea is workable,
but somewhat problematic.

For example, if you use a .1 ohm resistor in series with your circuit,
it will produce a 1V differential voltage at 10 amps, and a 2V drop
at your max of 20 amps. You could then use a simple comparator with
a potentiometer circuit to adjust the "LED on/off" voltage. The problem
is that at 20 amps, you are dropping your 12V dc down to 10V, and your
sense resistor is dissipating 40 watts!

If you reduce the size of the series resistor to .01 ohms, you only get
.1V at your 10 amps, and .2V at 20 amps. This voltage is a little low
to drive the comparator directly, so you'll need to add an op amp to
amplify the voltage and then drive the amplified voltage into the comparator.
You can still set up the pot circuit to adjust the current at which the
LED is turned on. Unfortunately, your sense resistor is still dissipating
4 watts when the current gets to 20 amps. That is going to be one HOT
power resistor. (Hot enough to burn you if you put your finger on it).

A better solution is to use a Hall Effect current sensor such as a HAW 20-P,
or LA 20-PB. These sensors measure a DC current and put out a voltage
that is proportional to the current. You can purchase these from Digikey
(www.digikey.com) and although they are a little pricey ($22-$38) in
small quantities, they are a much more elegant solution to the problem
you are trying to solve. You can drive the voltage output of the sensor
directly into a comparator and then adjust the voltage at which the
comparator switches with the pot circuit.

For a discussion of hall sensors and battery current sensing, see
http://powerweb.grc.nasa.gov/elecsys/doc/hall.html.

Good luck with it,

Don W.

Norm Freedman wrote:

Any electronics experts out there? Would appreciate help with a simple
circuit:

to light an LED when current in a 12vdc circuit goes above
approximately 10 amps (adjustable from about 8 - 13 amps would be
nice) and drop out when current goes below that set point. Maximum
current is about 20 amps.

Any ideas?

Thanks for the help