Those optos are SLOW! Investigate with a scope if you need confirmation. The
datasheet performance rating can probably only be reached with heavy bias.

If cost is critical use of the C-B junction of the opto as a reverse biased
photodiode works much better than the conventional circuit, see the Maxim
isolated interface for one way to implement it. Otherwise go with a faster
device.

The 74OL6000 series devices are ideal but expensive!

Oliver.

----- Original Message -----
From: "Peter Crowcroft" <peter@KITSRUS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 12 January 2001 04:58
Subject: [OT]: opto-isolater IC efficiency


> We are trying to develop a kit where we need to implement an electrically
> isolated serial port between a kit and
> a standard PC serial port.
>
> We have a circuit using opto-couplers (4N25's) but it is not reliable. It
> seems too
> dependant on the IC parameters so that changing the opto-coupler for
> another of the same type affects whether the circuit works or not. Is has
> to do with the turn-on and turn-off delay times of the opto-coupler.
>
> Does anyone have any suggestions or even a (reliable) circuit to do this?
> There is no power supply available on the PC side except what comes out
the
> serial connector.
>
> Any advice gratefully accepted.
>
>
> regards,
>
> Peter Crowcroft
>             DIY Electronics (HK) Ltd
>       PO Box 88458, Sham Shui Po, Hong Kong
> Voice: 852-2720 0255   Fax: 852-2725 0610
> Web:  http://kitsrus.com     Email: peter@kitsrus.com
>     Number One in Electronic Kits Worldwide
> ------------------------------------------------------------------
>
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