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RE: Variac vs. Fan speed control
Subject: RE: Variac vs. Fan speed control
Date: Thu, 8 May 97 14:10:15 UT
From: "William Noble" <William_B_Noble-at-msn-dot-com>
To: "Tesla List" <tesla-at-pupman-dot-com>
the way an SCR or triac works is that it starts to conduct when you put
a
pulse of current into the gate, and stops when the current reverses
direction.
The purpose of a diac or avalance diode is to allow you to have the
device
fire at a predictable point. You build a lag (an RC circuit) which
charges
gradually over the 1/120 sec of a half cycle. The pot taps into this
circuit
to in effect multiply that voltage by a gain between 0 and 1. When the
voltage comming out of the pot exceeds the avalanche voltage, the diode
conducts turning on the traic. Without the avalance device triggering
is
unstable. (the gate is very sensitive) and without the RC lag, you
can't
trigger in the second quarter cycle (e.g. 90deg to 180 deg).One ref on
this
subject that I had handy was page 520 of an old RCA transistor manual
(aargh,
dated 8-67). If there is serious interest on the list, I can scan a
couple of
pages and OCR them and post them, but you should be able to use
altavista to
find web articles by using and advanced search for the followng string:
(SCR OR triac) AND (theory OR lamp)
and tell it to sort RCA electronics and construction to the top
hope this helps
[Bill] snip
Does someone know what purpose the diac or avalanche device on the
gate of a triac serves? Does it have something to do with slamming
the triac on instead of gradually?
DL Huffman
[Bill] snip