I have a UNL2003 hooked up in the classic configuration to a stepper motor. My VCC here is 12V. It is a motor that drives a scanner so I would suspect that I should at least be able to get some reasonable speed out of it but it just moves at a snails pace. I can't seem to do any better than 3ms delay between steps. what is odd is that if I disconnect the common pin (pin 9) from VCC and just connect VCC straight to both center connectors on my winding the speed picks up significantly with out any change in the delay between steps.

Why would letting common float cause the motor speed to increase so much. Why can't I make the motor move any faster with common connected, with less delay between steps? I know steppers tend to be slow but this slow?

I have
posted videos below with common connected and disconnected top give an idea of speeds.




Note: I have unused inputs grounded.


First with common connected. This is the fastest I can make it go. And no I did not plan the music but it seems to fit.

Now with common floating

 

That common goes to the free-wheeling diodes which protect the output transistor from the motor inductive spikes.
Removing that connection could cause zapping of the transistors.

The motor slows because the diodes carry the inductive motor current when the transistor goes off until the inductive energy is dissipated in the motor and diode resistance, so it takes longer for the motor current to stop, thus increasing the time before you can apply reverse current to the other winding.
If you want the motor to operate faster and still protect the transistors then you can connect a zener between the common (cathode) and +12V (anode).
The zener will rapidly dissipate the inductive energy and stop the motor current much faster than with just the internal diodes.
The zener voltage should be no more than about 30V to keep the maximum transistor voltage below it's maximum 50V rating.

 

That common goes to the free-wheeling diodes which protect the output transistor from the motor inductive spikes.
Removing that connection could cause zapping of the transistors.

The motor slows because the diodes carry the inductive motor current when the transistor goes off until the inductive energy is dissipated in the motor and diode resistance, so it takes longer for the motor current to stop, thus increasing the time before you can apply reverse current to the other winding.
If you want the motor to operate faster and still protect the transistors then you can connect a zener between the common (cathode) and +12V (anode).
The zener will rapidly dissipate the inductive energy and stop the motor current much faster than with just the internal diodes.
The zener voltage should be no more than about 30V to keep the maximum transistor voltage below it's maximum 50V rating.

If I understand correctly, still connect pin 9 of the UNL2003?

 

If I understand correctly, still connect pin 9 of the UNL2003?

Yes, that's where you connect the cathode of the Zener.

 

So those free-wheeling diodes sort of act like dynamic braking?

 

So those free-wheeling diodes sort of act like dynamic braking?

Sort of.
But their purpose is to protect the transistors, not brake the motor.

 

If I understand correctly, still connect pin 9 of the UNL2003?

You don't need a diode, the datasheet says you can just connect pin 9 to Vcc to prevent spikes on inductive loads.

http://www.ti.com/lit/ds/symlink/ulq2003a.pdf

See figures 25 and 28

 

You don't need a diode, the datasheet says you can just connect pin 9 to Vcc to prevent spikes on inductive loads.

In the first post, spinnaker stated he was doing that and it slowed the stepper motor.
That's why I suggested adding the Zener diode.

 

@spinnaker , it depends on how you plan to configure your etch tank but directly connecting to pin 9 will give you more "detent torque" - resistance to turn the motor when it is not energized (kind of like dynamic breaking mentioned by @ElectricSpidey above.

On the other hand, you can use "holding torque" to energize one ore more coils simultaneously to hold the motor in a single position (important for heavy loads that you want to move a non-symmetric load slowly (like a cam against a spring-loaded rocker arm) or hold in a single position if you are fighting gravity - not important if you are simply spinning a wheel that will not recoil if you stop applying power (a symmetric load)).

 

I am wondering if I damaged this motor somehow. Now for some reason. I can barely get it to move. I need to kick it to get it to move. Is it possible that I could have shorted a winding somehow that would cause intermittent operation? I also lengthened the leads so perhaps I pulled a wire loose inside the motor???

 

I am wondering if I damaged this motor somehow. Now for some reason. I can barely get it to move. I need to kick it to get it to move. Is it possible that I could have shorted a winding somehow that would cause intermittent operation? I also lengthened the leads so perhaps I pulled a wire loose inside the motor???

you could have the step sequence out of sorts - does it vibrate or seem to resist turning?

 

The motor is not damaged. Your circuit is wrong or since I read this motor was from a scanner, there might be a protocol initiated normally to control the speed, while you use a custom circuit which does not control the motor properly. You should read the datasheet of the motor and search for the answer. There should be a basic circuit in it.
The zener diodes are always needed and it will cost you nothing to try! Unless you really want to burn your transistors and maybe set the house on fire!

 

I am wondering if I damaged this motor somehow. Now for some reason. I can barely get it to move. I need to kick it to get it to move. Is it possible that I could have shorted a winding somehow that would cause intermittent operation? I also lengthened the leads so perhaps I pulled a wire loose inside the motor???

As you have been running it with the diodes disconnected it is more likely that the ULN2003 is damaged.
[EDIT] You can check the motor by measuring the resistance of the windings.

 

As you have been running it with the diodes disconnected it is more likely that the ULN2003 is damaged.
[EDIT] You can check the motor by measuring the resistance of the windings.

Already changed the ULN2003 out.

Well everything was ohming out ok but now I see an open winding . I bet I pulled something loose inside. Looks like there is no getting in to this thing either. I should not have messed with cutting the wires to extend them rather just build an extension connector.

 

I must have an intermittent open. When I manually spin the motor on the "problem" winding. I get temporary continuity.