Thomas McGahee wrote:

> 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

Could You send me the schematics of  this project? I'd like to make a power swit
ch
controlled by PIC, but with zerocrossing detect.
Thx, Peter