Newsgroups: sci.engr.lighting,sci.electronics.misc
Subject: Re: Digital dimmer: Triac vs MOSFET in a circuit?
References: <[email protected]>
[email protected] (Jean) writes:

> Digital dimmer: Triac vs MOSFET in a circuit?
> 
> I want to dim a 120Volt-60Watt light with a digital controller.
> I already have a circuit that use a Triac but I would prefer to use a
> power MOSFET.
> 
> *******************************************************
> This is not the purpose of this post.
> 
> For the ones who want to know why I would prefer MOSFET, some reasons
> are summerised at
> http://www.epanorama.net/documents/lights/lightdimmer.htm

The correct URL for this document is 
http://www.epanorama.net/documents/lights/lightdimmer.html

> like these:
> Reverse phase controlling has some advantages over traditional dimmers
> in many dimmer applications. The manufactuers of inverse phase dimmers
> adverstise their products to be more efficent and less noisy. Using
> proper controlling electronics it is possible to build a reverse phase
> dimmer without any magnetics or vibrations caused by them.
> Because turning on point is always exact at the zero phase there are
> no huge current spikes and EMI caused by turn on. Using power MOSFETs
> it is possible to make the turn-off rate relatively slot to achieve
> quite operations in terms of EMI and acoustical or incandescent lamp
> filament noise.
> 
> For the ones who want to know the basic functionning of a light dimmer
> with Triac:
> http://www.interq.or.jp/japan/se-inoue/e_ckt24_2.htm
> *******************************************************
> 
> The Triac is turned on with the impulse current on the gate and shuts
> off when the AC is zero.
> The MOSFET turns on when the AC is zero and shuts off with the impulse
> current on the gate.
> 
> Besides this basic functionning, my question is:
> Is there any other considerations I should take into account by
> replacing a Triac by a MOSFET in a circuit?

The MOSFET should be suitable for the given application 
(vortage and current ratings). Triacs can pass 
AC power to both directions. 
MOSFETs work only on one direction, so you need to either 
use two MOSFETs (one for each AC current direction) or use 
one MOSFET plus a rectifier.
On the control circuitry you need to take care of the following
things:
1. Triacs are triggered with current usually at low voltage, 
   while MOSFET is a voltage driven (make sure control circuit gives 
   high enough voltage)
2. MOSFET needs drive signal all the time it needs to conduct, 
   for a triac only pulse is enough. Some TRIAC circuits 
   send contant drive from the start of conduction to the end 
   of phase to triac (those suit for FET as well) while some 
   other designs just send s brief pulse at one point of 
   phase where triac is needed to start conducting (does not 
   suit well for FET application).

Those are the first points that come to my mind on this.
 
> Do you have a dimmer circuit with MOSFET to suggest?

CMOS gate implements reverse phase control - this circuit implements a
"reverse" phase control, using only a single CMOS 4001 quad NOR gate,
conduction begins at the zero crossing of the ac sine wave and the
turn-off timing is adjusted based on dimmer setting
http://www.e-insite.net/ednmag/archives/1998/052198/11DI.htm#CMOS%20gate%20implements

> Where in the net I could have more information about light dimmer
> circuit with MOSFET, IGBT...?

Take a look at the links at 
http://www.epanorama.net/links/lights.html#dimmer

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/