ULN2803 doesn't turn off

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Menticol

Active Member
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.
 
Last edited:
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.
 
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Pin 10 has to connect to +12V if you have an inductive load. Otherwise, you can leave it floating
Click to expand...
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.
 
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.
 
Last edited:
alec_t said:
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.
Click to expand...
What do you think the consequences would be if pin 10 were left open on a unit where all loads were resistive??
 
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Roff said:
What do you think the consequences would be if pin 10 were left open on a unit where all loads were resistive??
Click to expand...

Sorry by invading the conversation, I tried it, and the resistive load worked normally.
 
Last edited:
What do you think the consequences would be if pin 10 were left open on a unit where all loads were resistive??
Click to expand...
Oops, my bad. No consequence. Brain not engaged.
 
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