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Forum Index : Microcontroller and PC projects : Opto-coupler for DIs

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PhenixRising
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Joined: 07/11/2023
Location: United Kingdom
Posts: 1324

Posted: 07:18am 24 Jun 2025

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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

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Joined: 11/02/2018
Location: Australia
Posts: 2566

Posted: 08:21am 24 Jun 2025

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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
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Joined: 07/11/2023
Location: United Kingdom
Posts: 1324

Posted: 08:50am 24 Jun 2025

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Cheers Phil. 2.5mA feels good

Volhout
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Joined: 05/03/2018
Location: Netherlands
Posts: 5016

Posted: 10:23am 24 Jun 2025

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Phenix,

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

Volhout

PicomiteVGA PETSCII ROBOTS

PhenixRising
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Joined: 07/11/2023
Location: United Kingdom
Posts: 1324

Posted: 10:54am 24 Jun 2025

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Cheers, Harm.

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

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