It is generally bad form to parallel semiconductor components for larger
loads than a single component can handle without long and hard
consideration to what exactly is going to happen.  This is where the
rubber (theory) hits the road (real life).

In theory each triac will handle exactly one-third of the current.  This
is due to the theory that each triac has exactly the same on resistance
as the other traics.  In practice no two components are exactly alike,
and so one component will handle more current than the other two.  This
additional current will cause additional heat, lowering the on
resistance of the affected triac, and forcing it to handle even more
current.  While this works fine on the bench, it will eventually lead to
that component's eventual heat failure, which will then force (suddenly)
a larger load on the other two (assuming the first failed open), and
they too will fail.  The last one to go will probably let you know quite
audibly.

To avoid this, mount them all on the same heat sink, and put an
additional resistor in series with each triac with its resistance being
a magnitude greater than the individual on resistance of the triacs.

Given that the extra expended effort and cost of additional components
(not to mention the associated headaches and problems) is far greater
than specifying the proper part in the first place, there really is no
reason to try to make three components do the work of one.

It appears, though, that you are restricted to making his design a
certian way, and you've doubtlessly considered using a properly rated
part.  What restriction is keeping you from using a 15 or 20A TRIAC?
Why can't you use a relay?

-Adam

Anand Dhuru wrote:

>I have an application wherein the PIC is to drive an AC load at 240 volts, about 10A. For certain reasons, I cannot use a relay and have to use a TRIAC instead. Instead of using a TRIAC with a high current capacity of the required value, can I 'parallel' two or three of them safely, and expect them to share the load current? I have tried this technique briefly, and nothing blew, but how safe / reliable is it?
>
>I would appreciate comments on this.
>
>Regards,
>
>Anand
>
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