TL103W: Optocoupler biasing with TL103W

Denis Kotlar

Part Number: TL103W

I have a problem with understanding why huge capacitor is frequently used in many examples and eval. boards when optocoupler LED is driving with TL103W. Here is an example:

Is it possible that is a sort of soft-start?

Thanks in advance.

over 5 years ago

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi Denis,

I guess that it has to do with the power-on: Directly after the power-on C15 in uncharged and forces a current to flow through the LED of opto. A bit later R27 is fully charging up C15 and this current path for the opto's LED will dry up.

Kai

0 Denis Kotlar over 5 years ago in reply to kai klaas69

Intellectual 790 points

Thanks a lot Kai for instant reply. As a follow up on the same topic, I presume that circuit on input side of op-amp shown below has the same purpose:

0 kai klaas69 over 5 years ago in reply to Denis Kotlar

Guru 140200 points

Hi Denis,

yes, I think so. And D102 provides a fast discharge during power-down.

Kai

0 Denis Kotlar over 5 years ago in reply to kai klaas69

Intellectual 790 points

Thanks!

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TL103W: Optocoupler biasing with TL103W