ULN2803 doesn't turn off

[Menticol]

Hello again guys!

I'm having some problems with my parallel port + ULN2803 setup. When I connected my stepper motor all the coils were continuously activated, no matter the status of the parallel port pins.

The problem repeats using another ULN2803, a resistive load (12 light bulb) and a +5v power supply for the controlling pin (replacing the parallel port signal). Result is the same, bulb keeps continuously ON. Connecting or disconnecting the control pin just increases the bulb brightness a little bit.

Attached is the schematic of what I'm trying to do. Any help will be extremely appreciated.

 

[Roff]

Hello again guys!

I'm having some problems with my parallel port + ULN2803 setup. When I connected my stepper motor all the coils were continuously activated, no matter the status of the parallel port pins.

The problem repeats using another ULN2803, a resistive load (12 light bulb) and a +5v power supply for the controlling pin (replacing the parallel port signal). Result is the same, bulb keeps continuously ON. Connecting or disconnecting the control pin just increases the bulb brightness a little bit.

Attached is the schematic of what I'm trying to do. Any help will be extremely appreciated.

Pin 10 has to connect to +12V if you have an inductive load. Otherwise, you can leave it floating. As you have it now, your current is flowing from +12V, through the load, through the diode, and to ground.

 

[alec_t]

Pin 10 has to connect to +12V if you have an inductive load. Otherwise, you can leave it floating

That's not my understanding of the IC requirements. Pin 10 must connect to +12V regardless of load type, because the IC outputs are open-collector.

 

[Menticol]

Thank you so much Ron, It worked like a charm! Without your help the project presentation would be failed.

I'm sorry for taking so long to answer.

 

[Roff]

That's not my understanding of the IC requirements. Pin 10 must connect to +12V regardless of load type, because the IC outputs are open-collector.

What do you think the consequences would be if pin 10 were left open on a unit where all loads were resistive??

 

[Menticol]

What do you think the consequences would be if pin 10 were left open on a unit where all loads were resistive??

Sorry by invading the conversation, I tried it, and the resistive load worked normally.

 

[alec_t]

What do you think the consequences would be if pin 10 were left open on a unit where all loads were resistive??

Oops, my bad. No consequence. Brain not engaged.

 

[Roff]

Oops, my bad. No consequence. Brain not engaged.

I can relate.