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Does this Digital output design correct?

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oPossum:

--- Quote from: MarkF on January 02, 2020, 12:17:12 am ---However, the ULN2803 inputs will be floating when the opto is OFF.

--- End quote ---

That's not a problem. External pulldown is pointless on these extremely slow devices.

MarkF:
Back up the horses...

I just realized you intend to drive the opto's with +24V.
The ULN2803 inputs can NOT exceed +5V.  So, you need to limit the current to about 1mA.

And assuming you do not want the pull-down resistors:

Danesh_SA:

--- Quote from: MarkF on January 02, 2020, 04:17:41 am ---Back up the horses...

I just realized you intend to drive the opto's with +24V.
The ULN2803 inputs can NOT exceed +5V.  So, you need to limit the current to about 1mA.

And assuming you do not want the pull-down resistors:
(Attachment Link)

--- End quote ---

Dear MarkF ,the optos and ULN2803 is the same as my design .
1-As you explain about pull up and down resistors in perviuse post,here is just a current limit resistor ,There is no Pull down resistor in this design(which is prevents ULN2803 floating when optos are turn off),why?

2-there is no way to protect ULN2803 reverse polarity protection ,with your schematic,Right? :(

MarkF:

--- Quote from: Danesh_SA on January 02, 2020, 07:52:58 am ---Dear MarkF ,the optos and ULN2803 is the same as my design .
1-As you explain about pull up and down resistors in perviuse post,here is just a current limit resistor ,There is no Pull down resistor in this design(which is prevents ULN2803 floating when optos are turn off),why?

--- End quote ---
Since your last circuit did not have them, I left them out.
As @oPossum stated, they are really not needed because of the ULN2803's input configuration.
Look at the ULN2803 schematic.  When the input is open, the 7.2KΩ and 3KΩ resistors pull it to GND.
(This is unique for the ULN2803.  For a logic gate or a MCU input, you would want them.)




--- Quote ---2-there is no way to protect ULN2803 reverse polarity protection ,with your schematic,Right? :(

--- End quote ---
Look at the schematic for the ULN2803.  For reverse polarity, the internal diodes (blue above) on each output will conduct and the solenoid (or whatever) will always be ON.

The internal diodes protect against reverse polarity on the inputs and outputs.

The bigger question-  How to protect against total power dissipation?
Remember, the total current sunk with all outputs conducting is 500mA.
How do you control what devices are connected to the outputs?

Danesh_SA:
Now i got it ,there is internal pull down resistor inside ULN2803 and we just need current limit resistor because ,optos collector is 24V. :-+
I use 250mA fast fuse ,series with all output pins of ULN2803 and output connector,and there is one 1Amp fuse(F1) for total current limit for this chip.

I have another big concern!
when i check related topics,a few days ago,i noticed someone had issue with ULN2803 internal free wheel diodes on each outputs,he said  THESE ARE NOT ROBUST FOR INDUCTIVE LOADS SUCH AS BIG 24V RELAYS,AND IT CAUSES ULN2803 BURN!
I think add discrete free wheel diode is much relaible for this chip!
what do you think?(and why you say "DO NOT USE BOTH the internal diodes (i.e. pin 10) and the discrete diodes.")

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