90VDC motor current sense

Thread Starter

cmartinez

Joined Jan 17, 2007
8,220
I want to add a current sense circuit to a PWM motor controller that I was helped to design and build in a previous thread.

Based on a previous circuit that was generously designed by @ronv, I came up with this:

upload_2017-10-26_16-40-33.png
The motor data in the sim is arbitrary. That includes its series resistance and inductance. I put in those values because they were the ones whose results made sense for a 1/10 hp (74.6W) 90 VDC motor.

The motor is being fed a fully rectified (but unregulated) waveform from a 120VAC rms supply. PWM is applied to the motor after the rectifier, although that is not shown in this diagram

C2 is there to help smooth its output. The output's waveform is very similar to the input if that cap is omitted. Ideally, I'd like the output to be a smooth straight line.

I intend to connect its output to an ADC interfaced to an MCU

Questions:
  • Is this circuit properly protected against the commutation surges of a DC motor?
  • Are the values of the resistors used in the opamp more or less the right ones for this purpose?
  • Is C2 the best option for smoothing its output?
  • Is there a way to isolate this circuit from either the intended ADC, or the MCU?

Any other observations would be thoroughly appreciated.
 

Attachments

AlbertHall

Joined Jun 4, 2014
12,345
If this is a brushed motor there will be a huge amount of hash generated into your circuit.
I speak with recent experience of trying to switch a 300W 48V motor (note: only switch it on and off, no PWM involved). This circuit defeated all my efforts to stop it blowing up the MOSFET despite a fast diode across the motor, a combination of ceramic capacitors directly across the motor, and inductors in series with each motor lead. The hash managed to make the drain voltage change fast enough that the charge coupled back to the gate exceeded the permissible gate voltage converting the MOSFET into a three terminal short circuit.
 

Alec_t

Joined Sep 17, 2013
14,280
I played with the sim by adding motor switching and smoothing the non-inverting input. Looks like commutation per se won't be a problem in theory, but Albert's experience says practice is somewhat different :).
 

Attachments

Thread Starter

cmartinez

Joined Jan 17, 2007
8,220
If this is a brushed motor there will be a huge amount of hash generated into your circuit.
I speak with recent experience of trying to switch a 300W 48V motor (note: only switch it on and off, no PWM involved). This circuit defeated all my efforts to stop it blowing up the MOSFET despite a fast diode across the motor, a combination of ceramic capacitors directly across the motor, and inductors in series with each motor lead. The hash managed to make the drain voltage change fast enough that the charge coupled back to the gate exceeded the permissible gate voltage converting the MOSFET into a three terminal short circuit.
Did you try using fast acting bidirectional tvs diodes accross the mosfets?
 

Thread Starter

cmartinez

Joined Jan 17, 2007
8,220
I plan to incorporate this circuit into the PWM controller that I previously designed and built. And after some tweaking and filtering, the thing has been working like a charm for months now. Of course, it's nowhere near the amount of power you tried to control.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
8,220
I played with the sim by adding motor switching and smoothing the non-inverting input. Looks like commutation per se won't be a problem in theory, but Albert's experience says practice is somewhat different :).
Very interesting. You removed the zeners, do you think they're unnecessary? Also, is C3 supposed to mitigate commutation surges?
Why is C3 connected between the positive input and ground? And not between the positive and negative inputs?
 

ronv

Joined Nov 12, 2008
3,770
I want to add a current sense circuit to a PWM motor controller that I was helped to design and build in a previous thread.

Based on a previous circuit that was generously designed by @ronv, I came up with this:
The motor data in the sim is arbitrary. That includes its series resistance and inductance. I put in those values because they were the ones whose results made sense for a 1/10 hp (74.6W) 90 VDC motor.

The motor is being fed a fully rectified (but unregulated) waveform from a 120VAC rms supply. PWM is applied to the motor after the rectifier, although that is not shown in this diagram

C2 is there to help smooth its output. The output's waveform is very similar to the input if that cap is omitted. Ideally, I'd like the output to be a smooth straight line.

I intend to connect its output to an ADC interfaced to an MCU

Questions:
  • Is this circuit properly protected against the commutation surges of a DC motor?
  • Are the values of the resistors used in the opamp more or less the right ones for this purpose?
  • Is C2 the best option for smoothing its output?
  • Is there a way to isolate this circuit from either the intended ADC, or the MCU?

Any other observations would be thoroughly appreciated.
Maybe hall effect?
https://www.digikey.com/products/en...t=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25
 
Top