G8MNY > TECH 26.11.21 08:00z 85 Lines 3275 Bytes #28 (0) @ WW
BID : 54111_GB7CIP
Subj: Transistors, SCRs & TRIACs
Sent: 211126/0756Z @:GB7CIP.#32.GBR.EURO #:54111 [Caterham Surrey GBR]
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.
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
False triggering is possible if the anode pulses +ve very quickly (dV/dT) due
to stray capacitance to the gate.
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.
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
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.
Bases not connected ______/Ž³
__³___ __³__ ÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄ I
____/ \e____e/ \____ ³Ż/~~~~~
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
Read previous mail | Read next mail