I have had to build some systems that drew about 20 amps continuous
and 40 amps peak (turn-on). Not wanting to have to put up with
either arcing relay contacts or excessive heat at the Triac,
I chose to design a circuit that has the best features of both.

I have a control circuit that upon receipt of a closure command
waits for zero crossing of the AC mains, then activates BOTH
a heavy duty Triac (with NO heat sink!) and a relay capable
of handling the 20 amps continuous current. Naturally the Triac
will respond first. Some tens of milliseconds later you can be sure
the relay contacts have closed (with NO arcing, due to the Triac
already being on!)

Triac control was via optoisolators. Relay control was direct,
but the relays had a separate power supply and ground buss. The
PIC and relay ground busses then shared a common connection
so the relay driver transistors could operate properly. This
saved me from having to use a bazillion additional optoisolators.

When the "open" command arrives I keep the Triac on, but turn off
the relay. I wait several tens of milliseconds to ensure that
the contacts have opened (with NO arcing!), and then I turn
off the Triac at a time such that inductive effects are minimized.
Keeps the lines "clean".

The Triac does not need any heat sink at all, because it actually
conducts for only a few tens of milliseconds during turn on and
turn off. The rest of the "on" time it is the relay contacts
that carry all the current.

Simple, effective, and right up a PIC's alley.
Of course, I have also designed PIC-less versions of this
circuit using monostable timers and an XOR gate, but the
PIC version is nice because it can control MANY such Triac/Relay
sets at once. Using shift registers it is amazing how many
such sets can be controlled with a resolution of a millisecond
or so. An RS232 type serial port set allowed remote control
of all the Triac/Relay sets and even allowed a deferred and
synchronized mode where the data for all sets was loaded and
then implemented upon receipt of a syncronizing command.

This made it easy to control a wide variety of machinery.

Fr. Tom McGahee

----------
> From: Harold M Hallikainen <haroldhallikainen@JUNO.COM>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: 16A heater control? Triac? SCR?
> Date: Friday, May 14, 1999 10:25 AM
>
>         I think a triac is about the most efficient way of handling the proble
m
> (unless you want to go to an electromechanical relay).  The voltage drop
> across the triac would be about 1.5 volts, so multiply that by the
> current to get dissipation in watts.  Multiply the dissipation by your
> heat sink's thermal resistance to ambient (degrees C/watt) to get
> temperature rise above ambient.
>         For typical drive circuits for the triac, see the datasheet on the
> MOC3020 or similar opto couplers.  Also, for simplicity, you can just buy
> an opto coupled solid state relay.  The manufacturer of the SSR has
> solved all the drive and snubber problems for you.  You can drive the SSR
> directly from a PIC.
>
> Harold
>
>