> From:         Bob Blick[SMTP:bblick@SONIC.NET]
> Sent:         Friday, March 12, 2004 5:15 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE:] Switching 24V AC inductive load with PIC, coupling & protection?

>> Yes, of course. I just got carried away. The one consideration with
>> driving the triac directly is protecting the PIC if the triac fails
>> (shorts), dumping 24 volts into the port pin. A series resistor is
>> required between the port and the gate of the triac, but the value
>> needed for reliable triggering may be too small to protect the PIC. An
.> optocoupler would be useful here; in fact, that would be the only reason
>> for using one.

> If the optocoupler fails(drastically) there is no guarantee it will
> continue to isolate.

> All the irrigation controllers I've seen have used triacs with no
> optocouplers.

> I'm of the opinion that the "C" equivalent of an optocoupler is a GOTO -
> in other words, if you are using one, you should really think long and
> hard, because there are very few reasons to use them.

Of course the optoisolator will fail if the triac shorts, but it is easier
to replace a generic IC than a programmable chip with a proprietary
program in it.

John Power

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