I'm trying to size the appropriate power supply for a bunch of 12 VAC, 15
Watt bulbs, which are connected to buttons.  There is a momentary single
pole double throw push button (it has a normally closed lead, a normally
open lead and a common lead.)
The bulbs go into their own housing and has two leads for power.

A rectifier bridge to hooked up to a 12 VAC power supply to convert it to
DC power.  This DC voltage from the rectifier is used power the 12 VAC
lights (will this pose a problem?)  I guess the 12 VAC is actually the RMS
value so the peak value is 12/.707 = 16.9 V.  (Does this mean the 12 VAC
bulb can handle a 16.9 VDC?)

We are doing this because we need to interface the button to my PIC.  The
+12 VDC of the rectifier is connected to the bulb and the other end of the
bulb is connected to the NC lead of the button.  The negative lead of the
12 VDC from the rectifier is connected to COM of the button.  The NO lead
of the button is connected to a pin on my PIC.  My PIC's GND is also
connected to the COM of the button.  Every time someone presses this
button, the light goes out and my PIC will see a "0." (there's a pullup on
the PIC pin.)

I've never had to think about lights in as much detail as on this project
so here are a few questions regarding bulbs and electronics.

Since it is a 12VAC/15W bulb, the resistance is P=V*V/R so R is 12*12/15 =
9.6 ohms.
I = V/R = 12 / 9.6 = 1.25 Amps...I guess bulbs need to draw that many amps
to get hot enough to emit light.  :)

We have about 30 bulbs that we are powering with the power supply.

Here are the questions:
(1) The bulbs are 12 VAC so does this mean that it can handle 16.9 VDC?

(2) Can I drive these bulbs with a DC power supply?

(3) If these bulbs can be driven with DC should I just use a 15W*30 = 450 W
DC power supply for the bulbs?


Thanks.

James

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