I have a microcontroller that is feeding an H-bridge in order to control the on/off and direction of a small 6VDC motor. I have two LEDs connected back to back across the H-bridge output in order to have a visual indication of the output polarity from the H-bridge. Everything works well until I connect the motor, at which time the motor buzzes but doesn't turn and both the LEDs light as if the polarity is rapidly switching. I suspect the uC is being interfered with by electrical noise from the motor. I have a .1 uF cap across power legs of the uC, but I need to do more.

Suggestions are welcome.

Thanks.

 

I have a microcontroller that is feeding an H-bridge in order to control the on/off and direction of a small 6VDC motor. I have two LEDs connected back to back across the H-bridge output in order to have a visual indication of the output polarity from the H-bridge. Everything works well until I connect the motor, at which time the motor buzzes but doesn't turn and both the LEDs light as if the polarity is rapidly switching. I suspect the uC is being interfered with by electrical noise from the motor. I have a .1 uF cap across power legs of the uC, but I need to do more.

Suggestions are welcome.

Thanks.

Could be anything - and without more info, we'll never know.

 

Isn't this why they make "motor shields"? A type of breakout board to isolate the motor from the uC.

 

I added .1uF caps from each motor terminal to the case and another from one motor terminal to the other. Still no joy.

 

Do you have a star ground system so that motor current doesn't affect signal current paths?
Are the power leads to the motor a shielded twisted pair?

 

Do you have a star ground system so that motor current doesn't affect signal current paths?
Are the power leads to the motor a shielded twisted pair?

Yes

and No

I will unsolder the leads on the control circuit and move them so that they run straight away from the uC.

 

Does the circuit use the same power source as the motor?

 

Yes

and No

I will unsolder the leads on the control circuit and move them so that they run straight away from the uC.

We could use some more information:
Schematic, power supply, etc.
Some possibilities:
If there is no dead time the switches on the same side of the H can be on for a short time causing the supply to drop. Some dead time (leisure time) between direction changes and a big cap, say 220 Ufd. from the top of the H to the ground for the H.
Flyback diodes?
0.1Ufd. may be a little large for the brush noise caps. - maybe .01.
Return the ground and the supply for the H directly to where the supply voltage enters the board and place a large cap there as well. Then power the micro from the same point.
Pay attention to the micro reset pin.

 

I don't have the actual motor that the circuit will ultimately be used with, so describing the motor I used wouldn't help. Tomorrow morning, I am picking up a duplicate of the "real" motor and will repeat my tests with it. I will post a schematic then as well. Thanks for the suggestions.

 

Does the circuit use the same power source as the motor?

Yes...but. In its final application, the circuit and the motor will both run from a set of four alkaline C cells. The motor will run directly on the 6V, but the control circuitry will run on 3.3V derived from the 6V via a linear regulator. Rather than batteries, I have been using a 7.5V regulated wall wart.

 

Here is the schematic drawing. I rerouted the leads from the H-bridge away from the uC, but no change. I also eliminated the wall wart and used a battery for the 6V supply, but no change.

I have never used this H-bridge IC before; I will pick up the actual motor to be used in a few hours and try again.

 

Here is the schematic drawing. I rerouted the leads from the H-bridge away from the uC, but no change. I also eliminated the wall wart and used a battery for the 6V supply, but no change.

I have never used this H-bridge IC before; I will pick up the actual motor to be used in a few hours and try again.

View attachment 101608

Try putting isolating the motor power by putting a diode between the power (battery pack/adapter) and the H-bridge. Add a good sized electrolytic capacitor (470 to 1000uF) from diode's cathode to ground. That way, back EMF from motor is eliminated for logic level circuitry.

Could you post a schematic of the rest of your project - it may not be a noise issue.

 

Could you post a schematic of the rest of your project - it may not be a noise issue.

Thanks for the input. The schematic shows the whole circuit; its purpose is simply to control the motor based on the operation of the five switches.

ETA: Tried the diode and cap you suggested; no change. I am off to get the "real" motor.

 

@tracecom
Now I see the schematic.

The motor driver datasheet has a recommendation about in-rush current limiting


Also, at 3.3V supply to pic-axe, outputs are near 2.5V. On the low end of I/O for your motor driver chip. You could try running the PICAxe on 5V to see if that reduces noise.

Finally, You could try supplying the motor-control Chip's Vdd with a separate logic-level voltage (see optional Diode isolation - circled in Red) and Vm pin (highlited in yellow) for the motor power input. Motor noise may be screwing up the Vref pin signaling.

 

@tracecom
Now I see the schematic.

The motor driver datasheet has a recommendation about in-rush current limiting
View attachment 101627

Also, at 3.3V supply to pic-axe, outputs are near 2.5V. On the low end of I/O for your motor driver chip. You could try running the PICAxe on 5V to see if that reduces noise.

Finally, You could try supplying the motor-control Chip's Vdd with a separate logic-level voltage (see optional Diode isolation - circled in Red) and Vm pin (highlited in yellow) for the motor power input. Motor noise may be screwing up the Vref pin signaling.


View attachment 101628

OK, I am back with the "real" motor and it works properly with my original circuit. That's good, but it still leaves me in a quandary: knowing that there is some problem, I don't know whether to assume it's exclusively with the motor that doesn't work or not. The circuit is not for production or even field trials; it's just for a couple of demo units.

With regard to your markings on the datasheet page above, I am not using a voltage divider to control the speed...just a 10k to Vdd. I have a 10uF cap between Vm and ground. The datasheet wasn't too clear on where to connect Vm, but I found that it worked if I connected it to Vdd.

 

OK, I am back with the "real" motor and it works properly with my original circuit. That's good, but it still leaves me in a quandary: knowing that there is some problem, I don't know whether to assume it's exclusively with the motor that doesn't work or not. The circuit is not for production or even field trials; it's just for a couple of demo units.

With regard to your markings on the datasheet page above, I am not using a voltage divider to control the speed...just a 10k to Vdd. I have a 10uF cap between Vm and ground. The datasheet wasn't too clear on where to connect Vm, but I found that it worked if I connected it to Vdd.

Based on other motor controller chips I've used, there is usually a Vdd for logic level (up to 18 V if using CD4000 series, for example) but should be at what ever will produce the right switching for the two inputs. The datasheet does not show that the logic level switching thresholds changes with change in Vdd. I am confused, too.