Message 28533 in TECH.WW
From: G8MNY
Date: Sun, 23 Aug 26 08:03:00 Z
Newsgroups: TECH.WW
Subject: Transistors, SCRs & TRIACs
Message-ID: <3057_GB7CIP>
Path: N2MH4|N3HYM|W9GM|N6PNK|VE3CGR|VK2RZ|GB7CIP
R:260823/0806Z 35901@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260823/0805Z 15939@N3HYM.#FRDK.MD.USA.NOAM BPQ6.0.25
R:260823/0805Z 7887@W9GM.#SWWI.WI.USA.NOAM BPQ6.0.24
R:260823/0804Z 11021@N6PNK.#NCA.CA.USA.NOAM LinBPQ6.0.25
R:260823/0804Z 91931@VE3CGR.#SCON.ON.CAN.NOAM LinBPQ6.0.25
R:260823/0804Z 21137@VK2RZ.#SYD.NSW.AUS.OC LinBPQ6.0.25
R:260823/0803Z @:GB7CIP.#32.GBR.EURO #:3057 [Caterham Surrey GBR] $:3057_GB7CIP
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 and one
NPN with their collectors and bases connected to the other transistor's base
and collector (swapped around).
If an on condition of +0.6V is now placed on the NPN's base, and 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.
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
_________-1000V
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
PN on state with either +/-0.6V
N on the Gate with either +/-
NP/ triggering sensitivity
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 and 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 ______/
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
← Return to Bulletin List