Right placement of current sensor in motor application

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Hi,

I'm going to use the TLI4970 current sensor to monitor the current on a PMDC motor. I've place it at the drain, as shown in the attachment. However, I've seen that it could also be placed at source lead. Is there any significant difference? Which one could be the best arrangement?

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DickCappels

Joined Aug 21, 2008
7,704
I would have placed it in series with the source lead because it would be electrically quieter than the drain.

Kjeldgaard

Joined Apr 7, 2016
453
Just for the same reason as DickCappels, I would suggest the top of the electric motor.

And that location allows for the shortest track and least induction in the Gate / Source circuit.

ericgibbs

Joined Jan 29, 2010
13,624
hi Xavier,
What is the voltage supply of the PMDC motor.?
E

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
hi Xavier,
What is the voltage supply of the PMDC motor.?
E
It's the rectified and filtered 120VAC mentioned in one of my previous posts.

ericgibbs

Joined Jan 29, 2010
13,624
hi X,
Unless I am mis-reading your diagram The Vbus voltage Motor voltage ~120Vdc, uses the same ground as the MOSFET Source and Gate. GND2.
So if the Motor load was in the Source, what would happen to the Vgs switching voltage.?
E

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
hi X,
Unless I am mis-reading your diagram The Vbus voltage Motor voltage ~120Vdc, uses the same ground as the MOSFET Source and Gate. GND2.
So if the Motor load was in the Source, what would happen to the Vgs switching voltage.?
E
Do you mean high side switching configuration?

ericgibbs

Joined Jan 29, 2010
13,624
hiX,
Your posted diagram shows 'low side' drive, with the motor in the Drain, I was trying to point out, you could not just change the position of the motor to the Source, without making changes to the Gate driving circuit.OK.?
As you say, it would mean converting to a 'high side' Gate drive.
E

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
hiX,
Your posted diagram shows 'low side' drive, with the motor in the Drain, I was trying to point out, you could not just change the position of the motor to the Source, without making changes to the Gate driving circuit.OK.?
As you say, it would mean converting to a 'high side' Gate drive.
E
Got it. Thank you for pointing it out. Now I know a new thing . I wasn't aware of the changes to be made for converting a N-Channel low side to a high side. Even though I won't convert this circuit to high side.

Back to the current monitor, as others said, it would be suitable to place it at source lead. However, I read that putting it at the FET source will just add the much smaller IGS to the IDS current I'm are already monitoring. But I'm not sure about that concept.

There is also the ACS712 on ebay for a couple of 's.