Control 24v stepper with motor shield

[bigone5500]

I have a seeed motor shield which states that it can output 5 to 15 volts. I want to control a 24 volt stepper but am unsure what to do at this point. I am thinking I can use some FETs along with optocouplers to do this.

What are your thoughts?

 

[bigone5500]

After thinking about this a bit more, I am thinking that the optos may not be able to switch fast enough to operate the FETs for smooth rotation. I was thinking of using the optos to put a barrier between my controller and the 24V source.

 

[mofy]

Optos are fast enough. They only need to go to about 50kHz, and you can clean up the output with a schmitt buffer e.g 74HC14.

 

[bigone5500]

So I don't need the FETs? Just the optos?

 

[mofy]

The optos only provide voltage isolation, they cannot drive the stepper motor, you need power devices like FETs or BJTs for that.

 

[hevans1944]

If you have the technical acumen, you might consider hacking the seeed motor shield. It uses a L298 dual H-bridge driver that is specified to run with motor supply voltages up to 40 VDC.

I can't find any schematics for the seeed motor shield, and I don't care to reverse engineer their Eagle files. Nor is there any explanation of why seeed specifies a maximum of 15 VDC. There is a two-pin connector on the shield, identified as "Supply Power Connector," with a jumper across it, identified as "MB_EN.," This was apparently intended to allow an external motor power supply, independent of the lower voltage USB power that is normally applied to the board. Seeed has this to say about that: The purpose of "Supply Power Connector" is using the jumper cap to select USB or external power for driving the motor, But you must insert the jumper cap now. We will later be resolved the problem. Yeah, sure.

I think seeed screwed up their design by not providing for the use of these two pins to provide motor power. It probably has something to do with the voltage limitations of the on-board 5V regulator chip. This should have been three pins, not two! One pin to supply power to the L298 for motor drive; one pin to connect to the external (USB) power; and the third pin to connect to an external motor power supply.

Seems to me that it should be possible to find the motor power Vc terminal on the L298 (check datasheet for pinouts corresponding to whatever physical package seeed used), isolate that pin from whatever else it is connected to, and provide that terminal with up to 40 VDC with respect to "ground" to drive motors with voltage requirements greater than 15 VDC but less than 40 VDC.

Don't take my word for it that this hack would be safe and effective for your 24 VDC stepper. Proceed at your own risk. I am just passing on what it says on the L298 datasheet.. I don't have a seeed motor shield, so I have no way to test the hack.

Hop

 

[bigone5500]

Well I do have two of them. I could try it out. But I don't want to be left with just one. So. I think I will just try to make a breakout board of sorts.

 

[hevans1944]

Well I do have two of them. I could try it out. But I don't want to be left with just one. So. I think I will just try to make a breakout board of sorts.

Good luck with that. Can you post a picture of whatever circuit you come up with that works?

I have a Superior Electric Slo-Syn DC/Synchronous stepper, model MO61-FD08, that I would like to drive. It is rated 1.25 V @ 3.8 A so I think I will need a PWM modulator and maybe 12 to 15 VDC to drive it with full torque. Later I may try to microstep it, but that requires a more sophisticated driver for the H-bridge. The STMicroelectronics web site has some interesting chips and tutorials for that.

 

[bigone5500]

I decided to go ahead and connect higher voltages to my shield. Connected motor is 24v inkjet printer motor. Here is my finding:

USB powered through arduino: very little motion if any at all. Mostly a high pitched squeal.
USB disconnected and two 9v batteries in series connected to the battery terminal: motor turns great.

The MB_EN jumper is if you are wanting to power the whole system using the batteries connected to the shield. If you want to power the arduino and shield separate from the motor, then you will need to remove the jumper.

Worth adding is that it cannot output the full 24v needed to control my stepper. I had the two 9v batteries connected but was not getting the full voltage. I will continue to develop a controller for 24v use.