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[SOLVED] ULN2803 using pull ups on the output?

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Bob Edge

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View attachment plc.pdf
Hi,

Could somone please take a look at the attached pdf. The part I am concerned about is U4, & U5. These are 24V inputs to the plc.

I think the input side is fine, but I am not sure if using the pull up resistors on the output side will work. I dont have the facilities to test it, I just have to send files away to have the boards manufactured. So any advice is greatly appreciated, as mistakes are very costly.

Regards
Bob..
 

The inputs at U4 and U5 are pulled low by the resistor and LEDs and also by an internal resistor path inside the 2803. It should be driven by 5V and will draw about 2mA from the input source, the 2803 will withstand higher input voltages but 24V is pushing your luck.

The outputs at U2 and U3 are pulled up by the LEDs and resistors to about 23V in the 'off' state but don't forget these ICs are switches to ground, they are designed to sink current (turning the LEDs on) when active.

Brian.
 
well for U4 and U5 inputs you should add suitable series resistors...
for U2 and U3 you should make sure that you are providing the right logic to the outputs since ULN's are open Collector outputs can only be used to sink current as mentioned by betwixt.....
 
Thank you both for your replies.

The data sheet says the input is good upto 30V, but I have added a series resistor anyway, just to be sure.

I can go ahead, and have the boards made without any worries now.

Kind regards,
Bob...
 

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