Author Topic: Driving a DC motor from a constant current sink  (Read 648 times)

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Offline OM222OTopic starter

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Driving a DC motor from a constant current sink
« on: June 02, 2020, 05:33:49 pm »
For a robotics project I needed to design a "light house" where it spins a light 360 degrees. I ended up making a hexagonal shape with 3 pairs of LED COBs to cover all 6 sides. I then designed a current sink for driving the leds (basic op amp + N fet) and since they don't make op amps in 3 packs, I went with a quad design. That left one empty current sink which accidentally fits the current rating of the DC motor (it's a small geared one). I have tested it for a short while and it seemed to spin the motor without any problems.

Are there any issues for long term use for either the sink circuit or the DC motor? I'm assuming the voltage across the motor goes all the way to VCC when changing poles because the op amp sees nothing for a while and that causes an initial current rush, since the inductance of the motor is quite low (again, small DC motor). Or maybe the back EMF from the motor can end up damaging the fets or the control circuit somehow? I'm just not sure if that's a good idea, but would be a very convenient one.
« Last Edit: June 02, 2020, 05:49:21 pm by OM222O »
 

Offline schmitt trigger

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Re: Driving a DC motor from a constant current sink
« Reply #1 on: June 02, 2020, 05:37:47 pm »
You are talking about a brushed DC motor, correct?

If so, there is always some overlap between the brush and the commutator segments, it never really goes open circuit.
 

Offline OM222OTopic starter

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Re: Driving a DC motor from a constant current sink
« Reply #2 on: June 02, 2020, 05:48:23 pm »
yes it a 6V brushed geared DC motor rated for about 300mA
 

Offline richard.cs

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Re: Driving a DC motor from a constant current sink
« Reply #3 on: June 02, 2020, 06:47:50 pm »
You may find speed control is poor - when run from a current source the thing that's held constant is torque, so the speed then becomes heavily dependent on the load. In your case it sounds like you just want it to turn so it'll probably be fine. It is possible that the inductance of the motor might cause the current sink to go unstable and oscillate, but that shouldn't actually cause damage. You should probably have a diode across the motor to prevent damage to the fets if/when the control circuit decides to reduce the current rapidly.
 
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Offline ahbushnell

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Re: Driving a DC motor from a constant current sink
« Reply #4 on: June 02, 2020, 08:11:45 pm »
You might want to put a freewheeling diode across the motor.  This will prevent voltage spikes when you turn off the motor rapidly.  1n4007.
 


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