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G8MNY  > TECH     26.11.21 08:00z 85 Lines 3275 Bytes #28 (0) @ WW
BID : 54111_GB7CIP
Subj: Transistors, SCRs & TRIACs
Path: SR8BBS<SR1BSZ<IW0QNL<ON0AR<GB7CIP
Sent: 211126/0756Z @:GB7CIP.#32.GBR.EURO #:54111 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                           (Updated Feb 10)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

SCRs or THYRISTORS (trade name)
Silicon Controlled Rectifiers are equivalent to 2 transistors one PNP & one NPN
with their collectors & bases connected to the other transistor's base &
collector (swapped around).

If an on condition of +0.6V is now placed on the NPN's base, & there is a +ve
supply/load on the PNP's emitter, current flows into the PNP's base turning
it on, which in turn hold the NPN fully on.

         2 TRANSISTORS                  S.C.R.
Anode ³
      e\³                  +            Anode +
    PNP ĆÄÄÄÄæ     Collector   P e        P
       /³  ³/   =          N---N b   =    N
GateÄÄĮÄÄÄÄ“NPN    Base... P---P c        P ...Gate
           ³\e             N              N    (+0.6V)
      Cathode³     Emitter -           Cathode -

Only the removal of the supply current, will turn off the SCR switch. With the
SCR a -ve gate signal does nothing, not even turning off a turned on SCR, this
is due to the gate connection only being on the edge of that layer & not
affecting the currents flowing further in. There are turn off capable types
made though.

False triggering is possible if the anode pulses +ve very quickly (dV/dT) due
to stray capacitance to the gate.

         V  Not
          ³ŻTriggerd                Unlike transistors, SCR & TRIACS peak
          ³Ż  _.---ÄÄÄÄÄÄÄ          current handling is very large like
      0.5V³Ż/~    Triggred          diodes, with a 1 Amp rated device is
          ³Ż                        able to withstand 40 Amp surge.
ÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄI
         Ž³
         Ž³
_________Ž³-1000V
Breakdown ³

TRIACS
These are AC triggered switches, like SCR but have 2 additional junctions which
are shorted together, it is a complex 3d device in practice.

       T R I A C

     Anode'' (ń) [case]             It can be triggered to the
      PN                            on state  with either +/-0.6V
      N                             on the Gate with either +/-
      P-\....Gate (ń0.6V)           on the Anode'' although
      NP/                           triggering sensitivity
   Cathode  (ń) [Anode']            varies over the 4 states.

The Gate can only handle low power (e.g. 1/100,000 the anode switching power)
so narrow pulse triggering is often used.

A diac (not the battery) is like a triac with internal 30V zeners anode to
gate. So over 30V it goes short until the power is removed. With a suitable CR
it provides simple AC phase shifted gate pulse trigger, used in mains light
dimmers.

DIACS
These are triggering devices used with TRIACS gates & a capacitor to provide a
short sharp high current trigger pulse. Basicly they are 2 like transistors in
series with the same 30V breakdown voltage.

                            Breakdown³+30V
 Bases not connected         ______/Ž³
   __³___   __³__            ÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄ I
____/ \e____e/ \____                 ³Ż/~~~~~
                                 -30V³


See my buls on "TRIAC Light Dimmer", "Auto SCR charger for Car Batts". "Simple
SCR Regulated Charger" & "SCR Drill Speed Controller".
 

Why don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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