Path: utzoo!attcan!telly!lethe!yunexus!xrtll!rsnider
From: rsnider@xrtll.uucp (Richard Snider)
Newsgroups: sci.electronics
Subject: Re: Light Dimmer
Message-ID: <1990Nov26.181619.9855@xrtll.uucp>
Date: 26 Nov 90 18:16:19 GMT
References: <2570007@hppad.HP.COM>
Reply-To: rsnider@xrtll.UUCP (Richard Snider)
Organization: XRTLL - Almost Public Unix, Toronto, Ontario, Canada
Lines: 37

In article <2570007@hppad.HP.COM> lindner@hppad.HP.COM (Steffen Lindner) writes:
>control about 24 individual channels.  Each channel consists of one or more
>lights (incandescent).  The maximum load on each channel is about 3A.  The
>dimmer must allow a minimum of 16 brightness settings (more would be better).
>The system uses a microcontroller for I/O and triggering the TRIACs. 
>
>Problem:
>A regular dimmer setup (controlling the phase angle of a TRIAC) produces 
>substantial current spikes, since the load current is turned on under
>non-cross-over conditions.  This causes a lot of interference.         

While on the topic of problems, depending on the type of incandescent
lights you are using, you probabbly will need some form of "preheating"
current through the fillament in the bulbs if you want them to come on
from some minimum setting without "popping" on.  This is becuase the
current required to get the filament glowing from a "cold" condition is
considerably higher than that which is needed to keep it glowing.

Your idea of skipping cycles to dim the bulb will work, you can also
trigger on half cycles as well, to cut down on flicker.  If you are
expecting these bulbs to be used in any kind of lighting situation where
people are not looking directly at the bulbs, then the flicker won't
be noticed at all.  Also, if dimming to 1/16th, chances are there won't
be much light to look at anyway, since that is less than 10V and most
120V incandescant bulbs (I assume you are dimming 120V circuits) will
not light up appreciably (sometimes not at all) with that kind of
voltage through them.

>Is there any way to trigger the TRIAC "gently" (ie. soft start)?

Yes, use an inductor in series with your triac and load, this will
remove a considerable amount of the spike associated with turning
the Triac on during part of the cycle. The calculation of the inductance
is left as an excercise for the student  :-)

					.....Rich
                                        rsnider@xrtll