=20
I won't use that triac in this application, I just happened to have some =
to
test out how to switch them on and off (I got them for a lamp dimmer
project). For driving the 24V solonoids I'll use a much smaller triac =
which
hopefully wont draw that much current!

The reason I want to try and avoid opto-couplers is to keep component =
count
and cost down, I want to drive up to 16 different solonoids, and having =
16
triacs is already taking up a lot of space.

They won't all be switched on at the same time. At most two will be =
switched
on at the same time, so I'm hoping it won't draw more current than the =
PIC
can deliver. Each solonoid is 24V AC, 2 Watt.

  _____ =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 11:53 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] Switching 24V AC inductive load with PIC, coupling &
protection?


>I then used a pot for that resitor on the gate of the triac, and=20
>noticed that I had to lower the resistance to about 330 Ohm before the=20
>triac would switch on (this is with my test circuit using a 12V AC=20
>transformer, I didn't have any other ones around). I would guess the=20
>function of this resistor is current limiting, and the current needed=20
>to switch on the triac would depend on which triac I'm using. In this=20
>case it was a BT139, which is a bit of an overkill for this =
application,
but it=92s the only one I had laying around.

That triac: http://www2.eese.qut.edu.au/store/pdf/BT139.pdf requires =
25mA to
50mA depending on grade to drive reliably. More than the PIC can handle
directly unless you have the most sensitive type and then it's on the =
edge
(you could parallel two outputs or whatever). This is what's guaranteed =
at
25=B0C (typical units will be better), but at low temperatures the =
trigger
current will go up. Note from the data sheet why I wanted you to use the
triac "upside down"- you avoid the ugly Quadrant IV which can take as =
much
as 100mA to trigger. I'd use the transistor to drive the triac gate in =
this
circuit.

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