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PICList Thread
'[OT]: TRIACs in parallel'
2001\06\16@122201 by Anand Dhuru

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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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2001\06\16@125309 by Spehro Pefhany

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At 09:44 PM 6/16/01 +0530, you 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?

It isn't, not very. Devices like triacs tend to "hog" current when there is
a temperature
differential. You *could* do it, by mounting them all on a common heat sink
and
arranging some kind of ballast resistors, but why bother?

For a commercial application with a (real) 10A (resistive) load, I'd use an
inexpensive 25A triac, most likely, or even a 40A unit. Even then,
protection (fusing)
can be an issue. You can cheap out and just use the fuse to protect wiring and
assume the triac will go if there is a short. You generally need a *lot*
more margin
than on relay contacts for long-term reliable operation in a production
situation.
Keep them cool, and keep in mind that failure of the eutectic die bond
(leading to
overheating and failure "on") is one of the common failure modes, so thermal
cycling should be minimized.  You'll have 10W, give or take, to get rid of.

Best regards,

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2001\06\18@093947 by Lawrence Lile

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Yeah, margin on relay contacts - I wish I had any.  We often use 10 amp
relays on 13 amp loads!

We can get away with this because the loads are resistive, and the relays
are rated for inductive loads.  I've actually approached manufacturers and
had them get a special UL listing for resistive loads only, so we can
squeeze every last watt out of the relay contacts.  Needless to say, this
isn't on a rocket science, nuclear, medical, downhole or other mission
critical applicaiton, just a toaster.


I'd agree, don't parallel Triacs, it's asking for trouble.  One gets hot,
it'll hog all the current and blow.   I stay away from Triacs as much as
possible, they are nothing but trouble.  Relays, which everybody thought
were history 20 years ago, are doing more volume than ever.  They run cool,
don't take up as much room as a heatsink, and last for millions of
operations ( I life tested them) even when overloaded.

-- Lawrence Lile

{Original Message removed}

2001\06\19@005805 by M. Adam Davis

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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:

{Quote hidden}

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