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

"Norm Freedman" wrote in message
om...
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.

The most simple approach is to use a reed contact. You simply wind the power
wire to the motor around the reed contact (a glass tube and the current
though the coiled wire will produc a magnetic field that closes the reed
contact. The number of turns needed depends on the sensitivity and size of
the reed contact but can be determined by experiment. The goal is to find
the ideal number of turns that does not close the contact at 8 amps, but
does so at 15 amps. This solution gives no voltage drop or heat loss. And
the obvious: you switch the LED with the reed contact :-)

Meindert

"Norm Freedman" wrote in message
om...
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.

The most simple approach is to use a reed contact. You simply wind the power
wire to the motor around the reed contact (a glass tube and the current
though the coiled wire will produc a magnetic field that closes the reed
contact. The number of turns needed depends on the sensitivity and size of
the reed contact but can be determined by experiment. The goal is to find
the ideal number of turns that does not close the contact at 8 amps, but
does so at 15 amps. This solution gives no voltage drop or heat loss. And
the obvious: you switch the LED with the reed contact :-)

Meindert

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




Don W wrote:

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,

Of course, it's not! Near any old 49 cent op amp comparator would be
fine. Of what comparator are you thinking, a bare light emitting diode?

so you'll need to add an op amp to
amplify the voltage and then drive the amplified voltage into the
comparator.

The indicator, you mean, like a d'arsonval ammeter, or digital
readout i.c, or an led with ballast resistor?

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.

Do you need bidirectional current sensing, or just one way?

A hall effect dc current sensor meters current through a hall effect
sensor affected by the the magnetic field of a nearby current
conductor. One 'turn' of conductor seems unlikely to provide flux
enough to accurately measure the current, though it has been a while
since I reviewed the latest hall effect element characteristics.
Sensors must be arranged near the conductor, and could require the
sensor coil and element to be polled by metering pulses, and
monitored for 'kickback' magnetic current restriction hysteresis
quantum decay jumps to indicate the strength of the magnetic field
near the sensor in question. It's complicated. It sounds like it
probably needs to be sized to the current, conductor and insulation
thickness in use.

The way it feels to my experienced old thumb rule is that a .001 ohm
shunt resistor (six inches of copper pipe?) and a gain 1000 non
lockup op amp with a few resistors could be wired to a much more
simple +12 volt supply than the + and - 15 v required for this hall
effect probe thing, a technological miracle of the latest unknown
pedigree, to be sure. This configuration gives 1 volt per amp
indication, and won't waste a lot of power or money.

If this is the depth of knowledge sold to dumbed down electrical
sales clerks by NASA, it's no wonder the space program needs mucho
more bucks to cure martian sand fleas of malaria.

How come this site's highlights haven't been updated since June, 2003?

Can we spell pork barrel? Let's get those NASA techs at work on
medical research and treatment technology! You know, MASH units with
water purification sets for earthquakes, waldo robot internet
surgeon kits, remote heart monitors, recombinant drug research,
remote control de-miners, mobile cat scan units, etc. Health and
drug research and treatments are too important to be left to private
or corporate business. Health services should be like a public
utility, available for all. Nations could compete to provide mobile
international health and disaster services at lowest cost for hire
to the UN.

Oh, all right, leave a few at NASA for doomsday asteroid warfare
research, the only kind of warfare we want in space. Just think,
space tugs bringing home the solid gold or 10 ton solid diamond
asteroids remnants of cold grey dwarf stars smashed by collisions,
space mining technology in support of asteroid defense! What alien
tech might we find in the rubble of the planet that used to be where
the asteroids are? What destroyed that planet, a comet? Homebrew
fusion experiments?

Space should be a holiday spot. Just think, four hours of zero gee
in a private family balloon ballroom with outside window, between
shuttle flights. To save fuel and lower the fare to a real space
amusement hotel, passengers would fly nearly naked, with no luggage
except possibly an excreta / barf bag, a sandwich, a 2 litre bottle
of drinks and a cheap monogrammed paper towel / garment, which they
could keep. Water / urine / exhaled vapor could be recycled using
solar cells into oxygen and hydrogen gases for breathing, energy,
reaction mass, or recombined for drinking. I would expect
passengers to be required to cut their fingernails short before they
go up. No spectacles, contacts only. No sharps / anal appliances or
other sports equipment not approved for hire aboard the mothership
in orbit. Talk about gate security!

What can you do in a space amusement park you can't do on the
ground? How about the contra rotating windmill "docking rendezvous?"

Air ballet in a big enclosure, sky diving extraordinaire, flight
flipper "angel wings?"

The Japanese will open the first space sex motel, if the Chinese
don't beat them to it. By then, Americans may not be able to afford it.

Dave Rutan is working on one civilian shuttle, now, among others.
This is not really a technology question, it's a business question.
How high the fare? How large the market? How cheap the space
plane? Launched from a modified 747, with recyclable solid boosters?
Banned over Boston? Pilotless, ultralight alblative silicon foamed
microballoon bog inflatable gyrocopter parachute re-entry vehicle
landing? A gyrocopter is a sort of sailing vessel, no?

They're working on it!

We're not. Talk about missing a business opportunity.

Terry K

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




Don W wrote:

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,

Of course, it's not! Near any old 49 cent op amp comparator would be
fine. Of what comparator are you thinking, a bare light emitting diode?

so you'll need to add an op amp to
amplify the voltage and then drive the amplified voltage into the
comparator.

The indicator, you mean, like a d'arsonval ammeter, or digital
readout i.c, or an led with ballast resistor?

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.

Do you need bidirectional current sensing, or just one way?

A hall effect dc current sensor meters current through a hall effect
sensor affected by the the magnetic field of a nearby current
conductor. One 'turn' of conductor seems unlikely to provide flux
enough to accurately measure the current, though it has been a while
since I reviewed the latest hall effect element characteristics.
Sensors must be arranged near the conductor, and could require the
sensor coil and element to be polled by metering pulses, and
monitored for 'kickback' magnetic current restriction hysteresis
quantum decay jumps to indicate the strength of the magnetic field
near the sensor in question. It's complicated. It sounds like it
probably needs to be sized to the current, conductor and insulation
thickness in use.

The way it feels to my experienced old thumb rule is that a .001 ohm
shunt resistor (six inches of copper pipe?) and a gain 1000 non
lockup op amp with a few resistors could be wired to a much more
simple +12 volt supply than the + and - 15 v required for this hall
effect probe thing, a technological miracle of the latest unknown
pedigree, to be sure. This configuration gives 1 volt per amp
indication, and won't waste a lot of power or money.

If this is the depth of knowledge sold to dumbed down electrical
sales clerks by NASA, it's no wonder the space program needs mucho
more bucks to cure martian sand fleas of malaria.

How come this site's highlights haven't been updated since June, 2003?

Can we spell pork barrel? Let's get those NASA techs at work on
medical research and treatment technology! You know, MASH units with
water purification sets for earthquakes, waldo robot internet
surgeon kits, remote heart monitors, recombinant drug research,
remote control de-miners, mobile cat scan units, etc. Health and
drug research and treatments are too important to be left to private
or corporate business. Health services should be like a public
utility, available for all. Nations could compete to provide mobile
international health and disaster services at lowest cost for hire
to the UN.

Oh, all right, leave a few at NASA for doomsday asteroid warfare
research, the only kind of warfare we want in space. Just think,
space tugs bringing home the solid gold or 10 ton solid diamond
asteroids remnants of cold grey dwarf stars smashed by collisions,
space mining technology in support of asteroid defense! What alien
tech might we find in the rubble of the planet that used to be where
the asteroids are? What destroyed that planet, a comet? Homebrew
fusion experiments?

Space should be a holiday spot. Just think, four hours of zero gee
in a private family balloon ballroom with outside window, between
shuttle flights. To save fuel and lower the fare to a real space
amusement hotel, passengers would fly nearly naked, with no luggage
except possibly an excreta / barf bag, a sandwich, a 2 litre bottle
of drinks and a cheap monogrammed paper towel / garment, which they
could keep. Water / urine / exhaled vapor could be recycled using
solar cells into oxygen and hydrogen gases for breathing, energy,
reaction mass, or recombined for drinking. I would expect
passengers to be required to cut their fingernails short before they
go up. No spectacles, contacts only. No sharps / anal appliances or
other sports equipment not approved for hire aboard the mothership
in orbit. Talk about gate security!

What can you do in a space amusement park you can't do on the
ground? How about the contra rotating windmill "docking rendezvous?"

Air ballet in a big enclosure, sky diving extraordinaire, flight
flipper "angel wings?"

The Japanese will open the first space sex motel, if the Chinese
don't beat them to it. By then, Americans may not be able to afford it.

Dave Rutan is working on one civilian shuttle, now, among others.
This is not really a technology question, it's a business question.
How high the fare? How large the market? How cheap the space
plane? Launched from a modified 747, with recyclable solid boosters?
Banned over Boston? Pilotless, ultralight alblative silicon foamed
microballoon bog inflatable gyrocopter parachute re-entry vehicle
landing? A gyrocopter is a sort of sailing vessel, no?

They're working on it!

We're not. Talk about missing a business opportunity.

Terry K