| |
G8MNY > TECH 03.01.03 18:07l 38 Lines 1819 Bytes #999 (0) @ WW
BID : 36278_GB7CIP
Read: GUEST OE7FMI
Subj: Transistors, SCRs & TRIACs
Path: DB0MRW<OK0PPL<RZ6HXA<SP7MGD<GB7YKS<GB7PZT<GB7MAX<GB7COV<GB7KHW<GB7CIP
Sent: 030103/1323Z @:GB7CIP.#32.GBR.EU #:36278 [Caterham] $:36278_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
The SCR is 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 +0.6v is now placed on the 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. Only the removal of the supply current
will turn off the SCR switch. With the SCR a -ve gate signal does nothing, not
even turn off the SCR, this is due to the gate connection only being on the
edge of the that layer. There are turn off capable types made.
False triggering is possible if the anode pulses +ve very quickly (dV/dT).
Unlike transistors, SCR & TRIACS peak current handling is very large, with a 1A
rated devices able to withstand 40A surges.
2 transistors SCR TRIAC
+ Anode + Anode'' (+/-) [case]
Collector P e P PN
N---N b N N
Base... P---P c P ...Gate (+0.6v) P-\....Gate (+/-0.6v)
N N PN/
Emitter - Cathode - Cathode (-/+) [Anode']
A Triac is an AC triggered switch, like a Silicon Controlled Rectifier (SCR or
Thyristors) but have 2 additional junctions which are shorted together, it is
a complex 3d device in practice. It can be triggered to the on state with
either +/-0.6V on the Gate with either +/-on the Anode'' although sensitivity
varies over the 4 states. The Gate can only handle low power so narrow pulse
triggering is often used. A diac (not the battery) with a CR provides simple
dimmer AC phasing shift pulse gate triggering.
73 De John, G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
