Subject: Re: Dimmer through mosfet?
From:
[email protected]
Date: 10/4/2004 8:44 AM Central Daylight Time
Message-id: <
[email protected]>
Any tip on a semiconductor that can turn on the load in each phase with a
slower di/dt without becomeing an electrical heater
..?
kind
Optocoupler is so natural for me safe-wise that I didn't put it in the
generic
schematic.
When working with dangerous voltages I always check things twice, and make
sure all parts are properly fastened at safe distance from eachother. And
analyze what happens should a component fail, or if thunder, spikes etc..
would occour.
I know very well about triacs, my idea was to find an alternative that will
avoid emitting RFI.
Maybe generating pulsed sinewave with a mosfet would be better?, as the di/dt
might then be lower (U=220*abs(sin(w))).
Nice chip.. now the thing is to get it
Btw, is it feasable to modify an of the shelf dimmer by replaceing the dial
with a transistor-optocoupler in some way?
The key to reducing RFI from dimmer switching is to just take the edge off the
very fast transistion from no current to full current, particularly when
dealing with cold light bulbs, which grab a lot more current than ones that are
already on. A small choke will do this well. There are many chokes, both
toroidal and non-toroidal, which are made for this application. Another thing
is, you only need one for all the triacs -- just put it in series with the line
before your loads like this (view in fixed font or M$ Notepad):
L1 ___
o----UUU------o-------o-------o-------o------------.
| | | | |
| | | | |
,---. ,---. ,---. ,---. ,---.
| X | | X | | X | | X | . . . | X |
'---' '---' '---' '---' '---'
| | | | |
_|_ _|_ _|_ _|_ _|_
V_A V_A V_A V_A V_A
/ | / | / | / | / |
| | | | |
| | | | |
L2 | | | | |
o-------------o-------o-------o-------o------------'
created by Andy´s ASCII-Circuit v1.24.140803 Beta
www.tech-chat.de
All the opto stuff has been assumed here.
You can get a 10uH 11A choke for $3.99 and a 25uH 5.5A choke for the same price
from Jameco:
Jameco #: 208266 25uH, 5.5A
Jameco #: 208258 10uH, 11A
$3.99 USD ea.
http://www.jameco.com/webapp/wcs/stores/servlet/CategoryDisplay?storeId=10
001&catalogId=10001&langId=-1&categoryId=11364
If you have enough current to require the 10uH chokes, you might want to put
two in series if 10uH is still giving you too much RFI. The easiest way to
tell is to get an old AM transistor radio, and dial over the frequency range
while your dimmer circuit is working. If you can't hear the buzz when your
dimmer is working, you're probably OK. Try various antenna positions and
orientations, as well as different power settings on your dimmer.
Looking at MOSFETs for this is probably the wrong direction. A fast switching
MOSFET which would give you a simulated sine wave would have many more
opportunities per line cycle to generate current pulses which would cause RFI.
The above is the simplest, easiest way to do this. Also, the thermal constant
of light bulbs and heaters is such that you won't gain anything by using high
frequency PWM for voltage control.
Is it possible you're concerned about finding the zero crossing? If you're
looking at using computer control for TRIAC phase control, all you need is a
way to mark the line voltage zero crossings, and then knowing if you've got 50
Hz (10 ms) or 60 Hz (8.3 ms), you can time out to where you want to turn on the
TRIAC. Most people use the low voltage AC coming off the power transformer for
the computer power supply to generate the zero crossing interrupts. If you're
using a PC, you will need a separate transformer, as well as a way to get the
interrupt into the PC (I'd try using assembler/C and one of the status pins on
the printer port to generate interrupt, if you don't have a separate card for
this).
The MOC3051 is available at Jameco as their #: 277860 for $0.74 ea.
http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&s
toreId=10001&catalogId=10001&productId=136747
Good luck with your project. Try the TRIAC/opto setup. It's what everyone
else does, it's more efficient, costs less, and it's really what it's made for.
Chris