At 10:42 AM 2/18/02 -0600, you wrote:

Hi, Dale:-

>My question relates to how to switch the 12VAC to the lights on and off.
>I'm thinking a sensitive-gate (logic) TRIAC on each leg of the AC line to
>the lights, with both gates controlled by the PIC.  From the looks of what
>I have seen in various Web resources about TRIACs, it looks like I could
>use one instead of two, but it just seems to me that for safety's sake
>using two would be like using a DPST switch and completely remove any
>potential from the lights when turned off.  Or should I even worry about
>that?

You don't need two, the 12VAC is isolated and two won't help. The voltage
losses will be significant enough from just one. Why don't you use a
relay? No voltage loss to speak of, and at 2 operations per day, a relay
rated at 50,000 operations (tungsten load) should last the rest of your life.

(Do be sure to get one with a suitable 'tungsten' rating, otherwise
there's a risk of contacts welding "on" or short life)

It will also run cool. A 20A triac switch will dissipate ~20W (that won't
go to your lights, but you'll have to get rid of it in a heat sink, even
on a hot day). 20A @12V is only 240W, not a lot of exterior lighting.
If you do insist on using a triac, get a 40A or higher one (de-rate) and
you'll probably have to put a driver in there for the gate, maybe 50mA,
because logic-level triacs are only low current. If you *want* to be
fancy rather than just having your lights go on and off, make yourself
a hybrid SS/mechanical switch, but at those rep rates it's not required.

>Ebmarrassingly, I have never used a TRIAC before, but it looks from the
>data sheet like I can turn the AC on and off with nothing more than a
>logic HI to the gate(s).  Am I reading that right?  Is there a better way
>to do this?  I guess I could use a SSR, but a couple of TRIACs will be a
>lot cheaper and a LOT smaller.

They work better if you drive them with a negative gate current. Some
are not rated or guaranteed to operate properly in Quadrant IV
(positive gate/negative MT)

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

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.