Current Measuring

[gunayburak]

Hello everyone ;

I need a simple solution to measure the current of a DC motor of which current changes between 0A - 1A ... Can you suggest me such solutions ? DC motor works with 12V and its RPM is controlled through PWM ...

[mpgmike]

The ACS715 measures up to 20 amps of current and converts it into a 0-5 volt signal, 185 mV per amp. It can be buffered with a capacitor to smooth out the a/c element. It is only available as a surface mount SOIC, though:

http://www.digikey.com/product-detail/en/ACS715ELCTR-20A-T/620-1261-1-ND/1955903

I use them for one of my gizmos.

Mike

[Demon]

From another forum:


Microchip has several application guides on their website regarding using the ADC; you should read them.

Have a look at AN513 and ADN007 for some ideas.

I remember seeing a design that changed current to voltage using discrete parts, then read that using a PIC.

Can't remember where I saw that.

Robert


Edit: http://www.electronics-lab.com/projects/mcu/008/index.html

0-33V, 0-3A.

[Charlie]

It's called a resistor, Robert :) Teasing aside, you simply choose a value such that there is a volt drop you can measure across the shunt resistor that is still small enough to not impact your circuit when the maximum current is flowing.

Unfortunately with a PWM it's more complicated to figure out what that voltage equivalence really is - first because it's AC, but worse it is a square wave and might be changing a bit fast for the PIC ADC can follow. I'd think along the lines of a shunt resistor with a differential amplifier (op-amp) sensing across it, that charges or discharges a capacitor through a resistor, which is then measured with the ADC. Choose values based on response time needed; fast enough to limit the time the answer is wrong, but slow enough the PIC can follow it. The voltage will be directly proportional to the average current after about 5 time constants.

[SUNFLOWER]

Many HBridge drivers have motor current sensing (0-5 V) pin outputs --

' Dual VNH5019 Motor Driver HBridge http://www.pololu.com/docs/0J49/all

' Dual MC33926 Motor Driver HBridge http://www.pololu.com/catalog/product/1213

[gunayburak]

Thanks guys .. I've taken care of the problem with ACS715- 05 ... It's got an output of 185 mV/A ... It took care of the problem with a tough kind of ADC reading technique ... Like 500 times of ADC readings for one cycle ... Because it is bloody sensitive ...

Regards ..