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Subject: Re: Triac questions
Date: Thu, 21 Nov 2002 18:01:41 +1300
Organization: Attica Communications
NNTP-Posting-Date: Thu, 21 Nov 2002 05:02:03 +0000 (UTC)
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John Jardine wrote:
> 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
> > twice.
> > 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
> > Tom
> 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
> their use.
> 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
> isolation stuff.
Thanks John - many thank for that thorough reply.
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