The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "John Jardine"
Subject: Re: Triac questions
Date: Thu, 21 Nov 2002 01:11:00 -0000
NNTP-Posting-Date: 21 Nov 2002 00:59:30 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Tom wrote in message
> I posted this yesterday but I think there is something wrong with my
> newsgroup server so I am going via Google this time. Appologies if it
> What exactly is meant by snubbing? I realise this is a CR network but
> what does it do and is it necessary for all Triacs?
> Does a Diac just act as a big voltage dropper when used to trigger a Triac
> (like a diode drops 0.7v) or does it have a more special porpose?
> Will a Triac rated at 400v 4A be ok to switch a light in a 240v rms
> mains system?
> Does anybody have a good method of phase control that can be driven by PC
> (other than the really complicated Microchip article I saw on PIC ref 4
> for the PIC12C508)
> regards and thank you
A 400V 4A Triac is excellent for a few hundred watts of lighting.
A Diac just looks like a voltage sensitive switch. It looks like an open
circuit until the voltage across it gets to say 70V then it switches ON very
rapidly and then goes open circuit again when the current through it falls
below a few ma's 'threshold'. (can be looked as say a tiny spark gap).
Some Triacs are sold as not needing snubbers. Most Triacs can benefit from
The main problem with Triacs, is that they are, or can be, sensitive to the
rate of change of the incoming mains voltage on their main terminals (MT1
If this rate of change of incoming voltage is sufficiently fast, then the
Triac can 'switch on' (self trigger) by itself without any prompting from
the normal gate terminal.
Under normal conditions this aspect is not a problem, as the Triac is being
fed with nice, smooth, gently undulating, sine waves. But ...
Say you have a 240Vac live, feeding an on/off isolation switch, then a
Triac, then (say) a 100Watt light bulb and back to neutral.
If you switch the on/off switch to 'ON' to connect mains power to this
dimmer circuit you can easily have a situation, where at the moment in time
you threw the switch, the incoming mains voltage was near one of its peaks
of + or - 340V.
From the Triac's point of view it's sitting there one moment without a care
in the world seeing no voltage at all, then all of a sudden a massive high
voltage step edge turns up (say 340V worst case).
From the time you threw that switch to the time this tidal wave of a voltage
turns up at the Triac connecion can easily be less than (say) 1uS, so the
Triac will suddenly be looking at a rate of change of incoming voltage of
340V per uS. This 340V/uS may well be a lot higher than the Triac data sheet
value of (say) a max of 50V/uS.
The Triac will thus switch ON by itself for the remainder of that particular
incoming cycle and then switch off again. Subsequent cycles will of course
now all be nice smooth sine waves, so this problem will not happen again.
That single unwanted and uncontrolled, part cycle that was fed through to
the load can though cause fuses to blow etc etc.
Put a 'snubber' capacitor (say 0.22uF) across the Triac's terminals and it
will charge up via this incoming voltage edge and through the series
resistance of the load on the other side of the Triac.
The charging of this snubber cap takes energy or electrical power from the
incoming transient edge and causes the rate of it's voltage rise to slow
down sufficiently to not exceed the Triac's dv/dt limit.
In the case of the switch circuit above. One would think that as the voltage
edge is backed up by the local power station then the snubber cap will fully
charge in an instant without the incoming transient even noticing. This
would indeed be the case except the the incoming wiring (fortunately!) has a
bit of series inductance which acts nicely to limit the max energy
throughput and the snubber cap can still do it's job. (it has only to reduce
the fast rate of change and not kill it completely)
The series resistance you find built into purpose made snubbers (say
100ohm) is purely to limit the damageingly high transient charging current
that may flow through the capacitor if the load on the other side of the
Triac is very low.
The action of the snubber is thus to be continuously effective against any
nasty spikes, drop-outs, noise and transients that can (and do) appear on
the incoming AC supply.
Circuits for PC (phasing) dimmers tend to get messy as circuitry is allways
needed to provide a synchronised incoming mains time reference (eg start a
10ms timer after the incoming sine crosses a 0V threshold). Plus safety
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup