Thyristors and triacs

Look at a 'discrete' version of a thristor and it'll al be clear.

It's called AC.

Graham

The thyristor keeps conducting after you take the gate signal away; a BJT would turn off then. That is, a BJT is a switch and a thyristor is a latching switch.

The last 2 letters of "triac".

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 | |/ 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

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

john

almost.. thyristor latches and maintains when you remove the current from the gate that switched it on. what opens the thyristor is when the current flowing through its Anode and cathode falls below the holding spec.., at that point is when the thyristor will thus open ..

I'd still like to know why my SCR conducts in both directions with only a positive gate signal. See my previous post "Detecting open gate on SCR switch" of 7/17/06 for more details.

Paul

Or negative even !

Graham

Eeyore skriver:

Would all of you please be so kind no to cross post your thyristor discussion to dk.admin ? The guy who started this thread is only trying to disturb dk.admin with his spam.

Thanks

Klaus