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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 │
e\│ + Anode +
PNP ├────┐ Collector P e P
/│ │/ = N---N b = N
Gate──┴────┤NPN Base... P---P c P ...Gate
│\e N N (+0.6V)
Cathode│ 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
│▌Triggerd Unlike transistors, SCR & TRIACS peak
│▌ _.---─────── current handling is very large like
0.5V│▌/~ Triggred diodes, with a 1 Amp rated device is
│▌ able to withstand 40 Amp surge. ──────────┼──────────────I
▐│
▐│
_________▐│-1000V
Breakdown │

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

Breakdown│+30V
Bases not connected ______/▐│
__│___ __│__ ────────┼─────── I
____/ \e____e/ \____ │▌/~~~~~
-30V│


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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