From: John Woodgate
Subject: Re: Triac Hack?
Date: Mon, 23 Sep 2002 09:53:38 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 23 Sep 2002 10:11:32 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that EskWIRED@panix.SPAMBLOCK.com wrote
(in ) about 'Triac Hack?', on Mon, 23
>>> : EskWIRED@panix.spamblock.com wrote:
>>> : I use a standard 600 watt dimmer switch in an unconventional way - to
>>> : control the speed of a fan motor in a hot air popcorn popper which I use
>>> : to roast coffee. It works well.
>> The circuit you will need is a switch mode power supply. By switching the
>> motor on and off rapidly and adjusting the 'on' and 'off' timing a very
>> efficient motor control system can be made. I know what you are thinking....
>> Bloody complex. It really isn't though.
>But doesn't my light dimmer do exactly that? It uses a triac to switch
>the power off every time the AC waveform passes the zero line, and then
>turns on again later in the waveform.
Well, yes, but it still isn't the same as an SMPS, which switches at a
much higher frequency.
>It currently works well enough. I'm not looking to replace the whole
>thing, but rather, to refine it by increasing the resolution.
Quite: some people here can't resist proposing radical solutions,
usually switch-mode, for everything. If not switch-mode, then a PIC, or
>It is a standard rotary light dimmer switch, rated at 600 watts. I have
>it feeding the primary side of a simple 24 volt transformer, which in turn
>feeds a simple full bridge rectifier.
>All I want to do is to figure out what value pot to replace the standard
>one with, so that I can turn it further (for more resolution) and ideally,
>so I can turn it all the way and still have to motor turn enough to
>prevent the heater coils from getting excessively hot.
You need a lower resistance-value pot and two fixed resistors. The sum
of the three resistances must be near the value of the original pot. You
replace the original pot by resistor-new pot-resistor, keeping the pot
slider connection unchanged. This reduces the speed range you can get.
Without a LOT more information (some of which you are unlikely to be
able to provide), one can't suggest resistance values.
Just as an example, you might replace a 100 k ohm pot by a 47 kohm
resistor, a 25 kohm pot and a 27 kohm resistor, totalling 99 kohms, and
giving *very roughly* the middle quarter of the speed control range.
>Another idea ocurred to me, which is to put a resistor in parallel with
>the pot, so that the effective value of the pot is reduced, and when the
>pot is turned to full value, enough juice still flows.
No, that may have other, very undesirable effects.
>How can I determine the optimal value for the pot? Or the resistor?
ResistorS. You will have to experiment.
>And which method makes more sense?
The one I'm proposing. (;-)
>And why is everyone telling me to replace the whole thing, when it
>currenly works pretty well? All I want to do is to tweak the value, so it
>works somewhat better.
See above. Some people can't, or won't, 'think simple'.
BTW, some people will say you can't use an ordinary dimmer to feed a
transformer, because 'it's an inductive load'. It ISN'T, unless you run
it with no load on the secondary winding, and in that case, who cares
what the dimmer does (it won't explode)? (;-)
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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