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G8MNY > TECH 16.04.04 18:21l 62 Lines 2241 Bytes #999 (0) @ WW
BID : 53062_GB7CIP
Read: GUEST OE7FMI
Subj: Transistors, SCRs & TRIACs
Path: DB0FHN<DB0RGB<DB0MRW<OK0PPL<DB0RES<ON0AR<GB7CIP
Sent: 040416/0854Z @:GB7CIP.#32.GBR.EU #:53062 [Caterham] $:53062_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Graphics Oct 03)
SCRs or THYRISTORS
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 ÃÄÄÄ¿ Collector P e P
/³ ³/ = N---N b = N
GateÄÄÁÄÄÄ´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 the gate.
Unlike transistors, SCR & TRIACS peak current handling is very large, with a
1 Amp rated device is able to withstand 40 Amp surges.
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'' (ñ) [case]
PN
N
P-\....Gate (ñ0.6v
NP/
Cathode (ñ) [Anode']
It can be triggered to the on state with either +/-0.6V on the Gate with either
+/- on the Anode'' although triggering sensitivity varies over the 4 states.
The Gate can only handle low power (eg 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.
/QSL
73 De John, G8MNY @ GB7CIP
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