Hallo,
Searching the forum I found this post: https://forum.allaboutcircuits.com/threads/current-limiting-for-a-dc-motor.126217/page-2
that shows a nice circuit design to limit the current flowing into a small DC motor (i'm pasting it here):

I'm writing because I would like to adapt this design to my needs: I have 3,3V (3,7V Lithium Battery plus linear regulator) to supply motor and circuit, motor should be turned off at 60/70 mA; would it be possible to change the design above to meet such requirements?
thanks a lot!!

D

 

I think if you change R1, you can adjust the current limit. But just remember, a motor starts with a high current when stationary so this sort of circuit may not do what you want.

 

I think if you change R1, you can adjust the current limit. But just remember, a motor starts with a high current when stationary so this sort of circuit may not do what you want.

Thank You!! I admit I did not think about the peak current... load on motor is almost zero but of course it might not work properly!
settin R1 at 10 Ohm should lead to about 60mA shutdown current.
what about feeding 3,3V instead of 12V in the rest of circuit? would it require some component to be changed?

 

would it require some component to be changed?

R11 would need to be reduced, but the other components should be ok as is.

 

R11 would need to be reduced, but the other components should be ok as is.

Thanks!!

 

I've just noticed that in the above schematic the value for R13 is missing, got any idea of how to calculate it?

 

10k should be just fine.

 

The circuit does not include any current regulator, only and overcurrent shutoff. There may be a small amount of negative feedback due to the emitter resistor of the motor drive transistor, but the circuit is not really a current regulator in the linear sense.

 

I found an IC that i think it might replace the hole circuite above: INA300 by Texas Instruments, here the datasheet:

http://www.ti.com/lit/ds/sbos613b/sbos613b.pdf

I was thinking about using such IC in latching configuration connecting a solid state relay directly to ALERT pin, solid state relay would cut off ground connection of my load (motor) when current rises above threshold.
Do you Guys think it might work? (obviously the part relevant to controller would not be present in circuit)

 

With the circuit posted in #9 you do need to have some code running in that micro controller and somehow that code must be created and loaded and that will take a lot more expense and effort than using the circuit shown in the first post. Certainly that INA300 device will cost more than the parts shown in the first circuit posted. Another option would be a current limiter circuit, but that would not switch the motor off in the event of an overload, it would only limit the current to some preset value. It would also limit the voltage available to the motor for normal running, and that may be a problem.

 

as far as I'm using the INA300 as an OverCurrent Switch I will use no programmable controller, picture in #9 it's just something I grabbed on google to give a hint of the circuit.
here the circuit design I have in mind both for Motor timing and for Overcurrent Switch; idea is to use Alert pin to pilot a solid state relay that would cut off gnd connection of DC motor when overcurrent is detected.
do you think it might work?

 

I do not see where the motor current is sensed, so I may be missing something. Even if P2 is the motor supply I don't see any sensing resistor or diode. Probably some explanation of the actual current path would help.

 

R7 near ina300 is the current sense. P2 is the actual motor.

 

R7 near ina300 is the current sense. P2 is the actual motor.

OK, now I see it, sort of. Splitting the drawing like that is confusing indeed. Very confusing. I would give the design only a 30% chance of being wired correctly just from the drawings.I see that the reset button short circuits the LED in the motor control switch, and that might be a problem if the INA300 does not have current limiting on the "alert" output. I have not studied the data sheet for the INA300 so I can't offer an opinion on how that will work. But at least I do see a current flow path so the motor should be able to run. It does seem a bit inefficient to have two switching devices in series with the motor.That would reduce the voltage drop and extend the battery life.

 

Alert is open drain so I see no issue there. You should have a diode going from pin 1 to 2 of the motor connector, to prevent high voltage spike when you turn the motor off. I would also use just one switching device, preferably a mosfet to control the motor and do the shut off logic elsewhere.

 

OK, now I see it, sort of. Splitting the drawing like that is confusing indeed. Very confusing. I would give the design only a 30% chance of being wired correctly just from the drawings.I see that the reset button short circuits the LED in the motor control switch, and that might be a problem if the INA300 does not have current limiting on the "alert" output. I have not studied the data sheet for the INA300 so I can't offer an opinion on how that will work. But at least I do see a current flow path so the motor should be able to run. It does seem a bit inefficient to have two switching devices in series with the motor.That would reduce the voltage drop and extend the battery life.

I apologize for my drawing! unfortunatelly the INA300 is only available in VSSOP-10 package thus being pretty impossible to be breadboarded, i will go straigh for a prototype PCB professionally printed (allpcb, pcbway, etc...). thanks a lot for your comments because you made me realize that schematic was definetly wrong: in order to reset ALERT output LATCH pin has to be pulled low an not ALERT one!!! so i modified the design as per attached drawing adding 10k pull up resistor to latch pin and a switch to pull it down when resetting is needed.
as per voltage drop I agree it is not an efficient configuration but motor is a 3V 30rpm motor that will be on for 2 seconds and off for 60 second thus a lower voltage drop will not be a problem as far as the motor will be able to provide the proper torque.

 

I apologize for my drawing! unfortunatelly the INA300 is only available in VSSOP-10 package thus being pretty impossible to be breadboarded, i will go straigh for a prototype PCB professionally printed (allpcb, pcbway, etc...).

Various VSSOP to DIP adapters are available for prototyping.