measuring current

Any ideas on how to measure current?
chris in napa

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The basic method is to use a shunt resistor and measure the voltage drop across the shunt resistor. Using ohms law will allow you to detemine the current flow through the circuit. The shunt resistor isn't normally a large value. It is typically less than 1 ohm resistance. At high current levels having a shunt resistor of .001 ohms or less is common. Usually these low resistance shunts are a copper or brass strip that is narrowed in the center, as they have to pass a lot of current. Since you know the resistance and the voltage drop across the shunt you can determine the current through the circuit.
For high voltage and high current situations, they usually use an inductive pickup and measure the voltage of the inductive pickup and apply various formulas for reactiance and such to determine the current flow though the cable. It is sort of like using a small transformer next to the power cable. The voltage coming out of the transformer can be used to detemine the current flow.

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Earl Bollinger wrote:

And assuming this isn't for real-time measurement by a microcontroller or computer, we shouldn't forget that a lot of the better meters have internal shunt resistors and can be used to measure current. I like to keep my older analog meter around, because it can measure up to 10 amps. My digital meters are limited to around 200 mA. Weenies.
-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza
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Actually it was for a microcontroller. I wanted a stall sensor to tell when my robot's motors were stalled. I've been racking my brain for something as simple as an RC circuit, but I'm a microcontroller guy not an electronics guy. :-( It looks like I'll have to get a specialized chip. There are some examples on the EME website.
chris in napa
Gordon McComb wrote:

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I sorta thought it was for something like that. What you need is a shunt resistor and a ADC to read the voltage drop across the resistor. If your MCU already has a ADC then your in business. Another method is to use a comparator to measure the voltage drop, like a LM339, and adjust the other differential input (your choice + or -), for the trigger point. Then the comparator generates a high or low signal you can key on. Some MCU's have a built in comparator that you could use as well. The resistance of the shunt resistor might be pretty small, thus a op amp may be needed to amplify the voltage level. But that depends on what your trying to measure, the MCU, et cetera. Some of the h-bridge motor controllers have a stall high current monitor pin as well. You might be able to take advantage of it. Some of the h-bridge controllers have a Error pin of some sort that can be easily monitored when you do this too. A number of the sumo bots use some of these methods to monitor stall conditions. Unfortunately a simple RC circuit won't work. The comparator method is probably the easiest.

detemine
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If you know what ranges of current to expect or if it changes significantly btwn stall/running you can still use a shunt resistor and measure the voltage with your uC A/D converter, no?

detemine
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How much current do you need to sense? Allegro has a new current sensor based on Hall effect device and will send free samples. http://www.allegromicro.com/sf/0750/ Craig
chris wrote:

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chris wrote:

Ah, well, then the resistor technique is the common method, but remember to watch the current draw through the resistor. The shunt resistors have to be selected to match the worse-case current draw, and the amps can get high with even small motors. Use Ohm's Law to figure out the proper wattage based on the voltage and current. Many folks advise against using a wire-wound resistor for this application.
Also, note that the voltage across the resistor is dependent on the motor voltage. As the batteries drain the "stall voltage" of the motor will change. Too, motors tend to pull lots of current when they are first turned on. Have the microcontroller test for stall over a period of time, say at least 1/4 second, and test under a variety of battery conditions.
The ideal stalled motor detector is an encoder of some type -- it doesn't have to be fancy. The encoder approach avoids the above.
-- Gordon Author: Constructing Robot Bases, Robot Builder's Sourcebook, Robot Builder's Bonanza
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Paddlewheel? ;-)
Peter in Surrey
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chris wrote:

I think the original method was to place a magnetic compass near the wire carrying the current and note the defection of the compass needle.
Bill K7NOM
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