At 10:31 AM 6/18/02 +0100, you wrote:

>What does this achieve?

Better isolation.

>The high current side of the relay is already
>(hopefully) isolated from the coil.

This is the analog world. Isolation may be pretty good at DC but
rather less so in the MHz region.

>Adding a diode to prevent back EMF
>spikes from destroying the driver should be enough protection.

This is not a safe assumption, in general.

>  I have
>dismantled many OEM automotive electronic modules and I've never seen a
>manufacturer resort to opto-isolators to drive a simple relay, even
>extremely inductive components such as the ignition coil and injectors don't
>appear to be opto-isolated within the ECU.

It's really not uncommon in conservatively designed industrial electronics.
The car manufacturers have the resources to work out the bugs in using
low-cost circuits. If you want something to work for sure without having
trouble calls, in many installations, without spending engineering time
dealing with EMC issues, the advice given may prove useful.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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