K8055 strange digital output voltage

Greetings once again,

I’ve been using stepper motors with my K8055 card lately.
One is a 12v 25 step motor, the other is a 4volt 200 step motor.

My setup goes like this :

12volts, ground on the GND pin, live wire on the Clamp pin.
Both my stepper motors are 6 wire ones, 2 beeing ground wires and 4 beeing pulse wires.
All four ground wires are going into the Clamp pin while the 8 pulse wires go into channel 1 to 8 (1-4 for first stepper motor and 5-8 for the second one).

Both operated fine untill I made a programming mistake just now.
I make the motors turn by using timers in VB6. At first I was letting the card generate a pulse to the engine, then turning it off to let it wait for the next step to generate a pulse, letting current flow through the pulse for a short period of time.
I also experimented with letting the current flow through the coil untill the next step when current was removed and put on the next coil.

All of this worked fine untill I started programming a new application and I made a critical mistake.
In the first application the current was removed from the last coil after I told the application to stop the engine from turning, because the current was supposed to be on untill the next step, and after telling the application to stop turning, there is no next step.
In the last application, I didn’t program this option and the current stayed on the coil even after telling the motor to stop turning. I didn’t notice this and the 12 volts stayed on the 4 volt motor coil for 30 seconds (wild guess).

After I noticed I quickly turned off the current and touched to motor to see if it was hot, it wasn’t. But the motor did not turn as it should after running my application again.

From there on I started doing several tests.

The 4v motor that had the current on it for 30 seconds was on channel 1 to 4.
I disconnected the motor and saw 12 volts coming through every pin when the switch was on.
So I tought I blew my motor.
I manually touched the 4 wires of the disconnected motor one by one with the ground on the ground of the power supply, and the motor turned without a hitch.
When connected back to pin 1-4 again the voltage on the pins dropped with pin 4 giving 5 volts, pin 3 giving 300 millivolts and pin 1 and 2 beeing dead.

The second (12v) motor turned perfectly on pin 5-8
When connecting the first (4v) motor to pin 5-8 it didn’t turn.

When connecting the second (12v) motor to pin 1-4 it did not turn

So now I’m left wondering if I blew my opto-coupler (IC4) again or if I damaged the PIC (IC3)…

The strangest thing is that I can “handcrank” the 4v engine using my power supply and the wires but I can’t get it to run on pin 5-8 whereas the 12v motor can …

My apologies for the wall-of-text, feel free to ask me to clarify if there is something you can’t understand.
Thanks in advance,
-Bed.

I also just measured the voltages on pins 5-8 when the 12v motor is connected.
Pin 5 gives 5 volts and pins 6-8 give 3…
Yet the motor turns, I presume it doesn’t need alot of current to turn.

Greetings,
-Bed

You are right - Most probably the IC4 ULN2803 blew up due to too high current through the 4V stepper motor.
BTW: What is the coil resistance of your 4V stepper motor?

4 volt motor with 0.9 amps / step translates into 4.4 ohms?
The label doesn’t state the coil resistance, only the amperage / step…

This too high current is the reason to the damage.
For the K8055 specified 100mA max.
The absolute maximum current specified for the ULN2803 is 500mA.

I see, would using relais be a solution?

-Bed