H-Bridge Cirucit, Current measurement

[wakaka]

My motor ratings are 24V 180W. meaning in no load case it draws 7.5A.

Hi all, This is my circuit for my H-bridge. I've use opto to seperate the ground between the H-bridge and my PIC microcontroller. I wanna add one small resistance resistor below the H-bridge, and wan to detect the voltage accross the resistor by performing ADC function. If the voltage larger than certain amount, the controller will off all 4 MOSFETS.I know i need to common the ground between H-bridge and opto for this function.
-If i do like that, will it be useless that i use opto, coz the ground is commoned instead of separated?
-If I common the ground to perform ADC, will the back emf or the current damage my controller although i use opto? WIll the high current somehow flow back to my controller from the ground and damage my circuit?

 

[wakaka]

help plz.......

 

[IanP]

One option is to connect to the Rsense a comparator with OC output (for example LM311) and another optocoupler fed by the comparator's output ..
A variable resistor can be used to setup the limit point ..
Once the limit level on Rsense is riched the comparator activates (through optocoupler) PICs input and PIC shuts down MOSFETS ..
Having this comparator at the motor "side" you will be able to keep optical isolation between the motor's 0V and PIC's GND ..

Regards,
IanP

 

[tl1130]

You can not use this way, the resistor should be serial connected to the motor. Otherwiae, you have only one direction of sensed current.