VTTC Staccato Design Help

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Adam Horden" <electronicmadman-at-blueyonder.co.uk> 

Hi List,

I am currently having a few problems with my VTTC staccato controller.

To give you some basic information on what is happening so far. I built
my first controller and to my surprise I kept blowing triacs. They were
dieing short and causing hv to flow back into the control electronics. I
found out of doing alot of reading that triacs have an on current.
Usually Ig in the datasheets. The particular device I have has an Ig of
50mA and is correctly switched on if more than 6 volts is applied to the
gate. ON MY DESIGN I WAS TURNING THE GATE ON WITH 12MA at 12V. I was
basically not turning the device on correctly. Below is my design for an
emitter follower (has the most current gain) output stage.

The circuit is here http://www.acidtech-dot-net/pages/vttc/scat_output.JPG

The scr on the diagram can be a scr or triac. I am currently using
BTA25-600 devices as I got 10 of these units cheap at a local radio
ralley.

MY QUESTION

How do I correctly calculate the resistor values on the circuit? I need
at least 50mA on the output between R1 and R2 to turn on the triac.

Ignore the lamp its just to represent how the cathodes are connected on
my vttc.

 >From my research I know that R3 is used to protect the transistor if the
emitter is shorted to ground and stopping max supply current flowing
through the transistor and burning it out. R4 is used to bias the
transistor. R1\R2 are a divider network to select the current voltage I
require.

Any ideas?

The data for my triac is here
http://www.farnell-dot-com/datasheets/18389.pdf

There is a page explaing the problem in some detail here if you are
interested http://www.acidtech-dot-net/pages/vttc/problem.php

Thanks,
Adam