🚀 go-pugleaf

<ib.mossin@hotmail.com> wrote in message
news:1154722762.371469.113340@i42g2000cwa.googlegroups.com...
> Hi,
>
> I'm trying to figure out how thyristor and triacs work. I understand
> that a thyristor conducts when it's gate recieves a current pulse and
> continues to conduct as long as forward biased. How does this differ
> from a ordinary bijunction transistor operating in cutoff and
> saturation with base working as "the gate"?
>
> A triac is two thyristors connected together. Right? I don't see the
> practical use for such a device. Can you give me a hint?
>
> Best regards,
> John
>
This is an ascii Triac regenerated from a Spice model, as there seems
surprisingly few Triac descriptions out there.
The triac is OFF.
If MT2 is positive and you lift the "Gate" voltage higher than 0.7V, then Q1
starts to switch on.
It's increasing collector current can only come out of the base of Q2. Which
means Q2 starts switching on. Which means Q2's increasing collector current
pumps more base current into Q1. Which means ... . So Q1, Q2 hold each other
hard ON and external gate drive is no longer needed (this is Thyristor
action). The reverse happens when MT2 is negative wrt MT1.
Idea is that the Triac makes a fine  AC switch.

       MT2 o---------o---------o-------o
                     |         |       |
                     |         |      .-.
                     |         |      | |
                     |         |      | |
                     |        e|      '-'
                     |          <| b   |
         (Triac)     |      NPN  |-----o Q3
                     |          /|     |
                     |        c|       |
                    e|         |       |c
                      >| b     |     |/
              Q2  PNP  |-------o-----|  PNP Q4
                      /|       |   b |< e
                    c|         |       |
                     |         |       |
                     |         |       |
      Gate o---------o         |c      |
                     |     b |/        |
                     o-------|  NPN Q1 |
                     |       |>        |
                    .-.        |e      |
                    | |        |       |
                    | |        |       |
                    '-'        |       |
                     |         |       |
       MT1 o---------o---------o-------o

(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Not explicitly drawn, is the Gate path  to the Q3-Q4 devices.

john