Opto-coupler for DIs

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PhenixRising
Guru

Joined: 07/11/2023
Location: United Kingdom
Posts: 2050
Posted: 07:18am 24 Jun 2025      

I am using the PC817 for digital inputs. Would an If ~5mA suffice? The datasheet warns about <1mA and I want to be safe.

PC817.pdf
phil99

Guru

Joined: 11/02/2018
Location: Australia
Posts: 3348
Posted: 08:21am 24 Jun 2025      

  Quote   Current transfer ratio (CTR : MIN. 50% at IF=5 mA, VCE=5V)
So it appears the output load can be up to 2.5mA for a drive of 5mA.
If the supply on the output side is 3.3v and the load resistor is 3.3kΩ just 1mA can flow so it should be ok.

Edit.
Fig.6 Current Transfer Ratio vs. Forward Current shows best CTR @ If = 2.5mA.
Edited 2025-06-24 18:29 by phil99
PhenixRising
Guru

Joined: 07/11/2023
Location: United Kingdom
Posts: 2050
Posted: 08:50am 24 Jun 2025      

Cheers Phil. 2.5mA feels good  
Volhout
Guru

Joined: 05/03/2018
Location: Netherlands
Posts: 6029
Posted: 10:23am 24 Jun 2025      

Phenix,

Speed on these PC817 is not overwhelming.
For more speed use digital isolators, or digital optocouplers.

Volhout
PhenixRising
Guru

Joined: 07/11/2023
Location: United Kingdom
Posts: 2050
Posted: 10:54am 24 Jun 2025      

Cheers, Harm.

These are only 24V signals from proximity sensors. I software debounce anyway so speed isn't an issue.