current sense resistor location

[gogoman]

Hi  the circuit  uses an H bridge to drive a 15v DC motor. The current sense resistors can be placed in series with the motor or between the bridge and ground of each leg
What are the advantage/disadvantage  of of one location  over the other.

Thanks

[rstofer]

If you put the resistor in the ground side of the H-bridge, you shift slightly the voltage level required to turn the MOSFETs on.

The best place to put the resistor is in the + supply lead to the H-bridge and then use a High Side Current Sense Amplifier.

https://www.maximintegrated.com/en/app-notes/index.mvp/id/746

[T3sl4co1l]

If you put the resistor in the ground side of the H-bridge, you shift slightly the voltage level required to turn the MOSFETs on.

The best place to put the resistor is in the + supply lead to the H-bridge and then use a High Side Current Sense Amplifier.

https://www.maximintegrated.com/en/app-notes/index.mvp/id/746

The difference, in the first case, is minuscule (and, indeed, may have the benefit of providing short-circuit current limiting -- because it is a source degeneration resistor as well), while saving the cost of a current sense amplifier.

Some controllers provide the current sense amp function internally, allowing the resistor to be placed anywhere.  Or it's internal (integrated switch controller).  This is probably more common among SMPS controllers than motor controllers, not sure.

Tim

[rstofer]

If you were to run the current sense voltage into a 5V ADC, wouldn't you need an amplifier either way?  The voltage drop across the resistor probably shouldn't be anywhere near 1V and that just doesn't fill up much of the ADC input range.

ADCs vary, of course, but still, I would think some kind of amplifier is required.  The advantage of the high side amplifiers I have used is that they also include some selectable internal gain.  Then again, so do the low side amplifiers...

When I built a boat with a 24VDC motor, I used a 50 mV shunt in the high side.  It's been a long time but I think it was rated for 50A full scale.  By itself, the 50 mV wouldn't scale well with the ADC input.  50 mV shunts are available in a wide range of full scale ratings.  These are typically used in industrial applications.

Here is a paper that discusses MANY approaches to current monitoring:

http://cds.linear.com/docs/en/application-note/an105fa.pdf