Thanks for the circuit diagram!

I still have one question though, how do I connect the AC supply and the =
5V
DC supply together?

Say I have a 24V transformer, which will drive the load, and I rectify /
regulate this to drive the PIC. Where is 'ground' in this case for the
rectified/regulated supply and where is ground for the two AC wires =
coming
out of the transformer? And do I just connect these two grounds =
together?

Sorry if you've answered this already. (And apologies about my poor =
ASCII
art!). Are there any connections missing from the below schematic? And =
would
it work like that? I know the supply is missing some filter/smoothing =
caps.

                                 7805
                    ,----.     ,----------.   =20
-------)------+-----|~  +|-----|IN  G  OUT|------ +5V to PIC
 24V AC       |     |RECT|     `----+-----'
-------)-+----|-----|~  -|----------+------------ GND to PIC
         |    |     `----'     `
         |    |               ____
         |    |  ------------|____|-------------- Trigger from PIC
         |   _|_/              Rg
         |   V_A
         |    |
         C|   |
Solonoid C|   |
         C|   |
         |    |
         +----+

_____ =20

Andr=E9 Miller =09


-----Original Message-----
From: pic microcontroller discussion list =
[mailto:PICLIST@MITVMA.MIT.EDU] On
Behalf Of Spehro Pefhany
Sent: 15 March 2004 04:02 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] Switching 24V AC inductive load with PIC, coupling &
protection?

At 10:20 AM 3/15/2004 +0200, you wrote:
>Say for example I want to attempt driving the triac directly without an
>opto-coupler, how would I connect the triac and its AC load to the =
driving
>pin of the PIC?




                      +5, 24VAC #1
                   Rg
      A            ___    |
       ---------+-|___|-  |
     PIC        |       \_|_
        ___   |/         V_A
      -|___|--|           |
     B  Rb    |>          |
                |         C|
                |         C|  Solenoid
               =3D=3D=3D        C|
               GND        |

                        24VAC #2


1)      Make sure no load current gets drawn through the
         5V line (tie it together with the 24V line at only one
         point, and make sure the 24V line goes directly to
         the triacs)

2)      No snubber parts shown, with normal 400V
         alternistors/snubberless triacs they should not be
         required @24VDC for commutation but you may wish to add
         them for EMI reasons.

3)      Direct drive with PIC A  OR with transistor B. B is more
         rugged in case the triac gets killed, and you can lower
         the resistor values to drive more insensitive triacs.
         Method A will also cause the protection diodes on the PIC
         to conduct maybe 2mA peak. Of course, using A LOW on the
         PIC =3D ON, and it's the opposite with B.

         With a typical alternistor (eg. Q4008) you'll NEED to use
         circuit B, as the trigger current is 35mA or so. In that
         case Rg is maybe 100R and Rb can be 1-2K. If you have
         sensitive gate triacs, you can use direct drive (A) but
         you'll probably need the snubber. For example, you
         might use Rg =3D 330R (and no transistor, Rb, of course).
         for a ~10mA gate current.

Note: take care with this if you happen to have an emulator or other
expensive equipment hooked up to your prototype. It's best to put
5V-only opto-isolation in there during development in such cases.
(and be very anal with the prototype wiring and securing it to the
bench etc.). Serial ports can usually take 24VAC, but anything like
printer ports (and maybe further into your computer) or ICE-2000 =
emulators
is vulnerable to complete destruction in case of a momentary short or
other lapse of concentration.

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

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