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DC Brushed Motor Current Sensing (and Limiting)

Hi,

I have a quick question regarding current sensing in DC motors. I know this topic is beaten to death so I will make this quick.

I need to get the current reading into my microprocessor A/D (0-5V). I want to sense up to 15A. I have heard of many ways, but one seems easiest to me, the MAX4372H current amplifier. It seems that with a properly sized sense resistor, I would be able to sense the current in the fashion that I would like.

I have one question though. Since the motor I am working with reverses direction, will this cause a problem with the current sensing using this chip? I want to make sure that having a negative voltage across the RS+ to RS- will perform the same as a positive one from RS+ to RS- as shown in the typical application here

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The spec sheet is found here
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and describes the internal working of the chip. It seems that it would function properly, but I want to make sure.

One more thing, just to clarify, if I am using this chip with my motor driver, how would it be connected? When looking at the typical application, I am guessing that the 24V battery (it is a 24V motor) will be connected to Vcc, and the "Vin" will be the output of the motor driver (M+), and the "load" will be the motor, and on the other side of the load, where in the typical application it is ground, there will be the M- fromt he motor driver?

As I mentioned, the voltage developed across the RS+ and RS- (sense resistor) will be both positive and negative depending on motor direction, and this is my main concern for this setup, to ensure that this is possible. If not, are there any similar circuits that I could use?

Thanks for any help!

Reply to
Andrew
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Put the sense resitor in series with the power feed instead of the motor itself. This will give the same polarity regardless of which way the motor is running.

This circuit measures only a threshhold, but an op amp connected to the resistor would give a measurable voltage to use to determine current thru the motor.

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--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm
Reply to
Luhan Monat

The only problem with doing this through the power source is that the motor driver is sourcing two motors (in parallel), meaning that I would get the current draw for the total of the two. I really need it to be for each motor. What would happen with the MAX 4372H chip if I put it in series with the motor? When the voltage at the sense resistor swings negative, will this cause a problem?

Reply to
Andrew

Sorry, I'm not familiar with that chip.

--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm
Reply to
Luhan Monat

In message , Andrew writes

(snip)

I take it these are lead-acid batteries you are using. If so, you might find the IC dies from over-voltage. The maximum rated voltage is 28 Volts and a 24 Volt battery can exceed that at times.

My approach would be to amplify the volt drop across a section of the supply cable in a similar way to Luhan Monat's circuit. He is running at much lower power so he has used a 1 Ohm resistor. At 15 Amps a short length of cable should drop a few milliVolts which ought to be usable.

--
Graham
Reply to
graham

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