On 24 Feb 2004 11:48:17 -0800,
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


Try Maxim,

http://www.maxim-ic.com/appnotes.cfm/appnote_number/2015/ln/en

They are usually quite happy to send a sample of an IC. Or you can order
them from the usual suspects like Mouser. Note that the appnote above,
also cuts off the load in the event of overcurrent, but you can cut that
part out and ignore it.

If you'd rather roll your own, you'll need a (couple of) precision low
value resistor(s) about 0.01 Ohm or so, then rig up a voltage drop
measuring circuit around that, adjust for expected current levels. Most
of the circuit cookbooks will have examples you can copy. With 0.01 Ohm,
passing 10A through it, results in 0.1V drop, which is a bit small for
some circuits to reliably measure, but only dissapates 100mw, and if the
current goes to 20A, your resistors are only dissapating 200mw, which is
probably ok for most things. Although it's a couple of amp hours a day
if it runs that long.



You might be able to get away with using a magneto-resistive system, but
with DC, the results are likely to vary widely depending on what other
magnetic fields are around, and are likely to change when you start or
stop the engine, or other unrelated magnetic sources.

--
Jim Richardson http://www.eskimo.com/~warlock
If you can modify it via software, then it doesn't count as hardware.
-- Lionel, in the Monastery