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From: John Fields
Organization: Austin Instruments,Inc.
X-Mailer: Mozilla 4.01 [en] (Win95; U)
Subject: Re: Higher loads on triac light dimmer ???
NNTP-Posting-Date: Fri, 20 Sep 2002 09:40:44 CDT
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Date: Fri, 20 Sep 2002 14:40:44 GMT
Rob Paisley wrote:
> "CS" wrote in message news:...
> > Hi, I'm moving a coffee machine from the USA 110V to Australia 220V which
> > has a heating element rated at 1300W.
> > Rather than using a bulky & expensive step down transformer, I would prefer
> > to use a standard thyristor light dimmer to control the heating element
> > since it is just a resistive load, but every light dimmer I've seen is
> > limited to about 600W.
> > Is it possible to modify something to get this to work, like add a heatsink
> > to the 600W light dimmer ??
> > Thanks.
> And the best suggestions are:
> 1- Buy an new machine.
> 2- Replace the element with one rated for 220 Volts.
> Great idea but probably not be possible.
> 3- Use an autotransformer.
> Best after #2 and probably most practical.
> 4- Place a 1300 Watt clothes iron in series with the load.
> Idea #4 has some merit but it would be better if 1300 watts of
> resistive lighting was placed in series with the coffee machine. Keep
> in mind that these would need to be 220 Volt lamps running at 1/2
> voltage for this solution. (If you don't mind basking in a 1300 watt
> warm glow while you wait for a cup of mud.)
> Seeing as how you are attached to this machine you could also by a
> second machine and run them in series. Just kidding.
> There are some practical considerations with this problem and the
> members of this news group would need to know more about the
> particular coffee machine's actual electrics in order to give credible
> answers to this problem.
> The greatest of these considerations is: What type of brewing
> controls does this machine have, is it an electronic control or a
> bimetal thermostat.?
> If the machine has anything except a simple bimetal thermostat that
> controls the entire load you can forget about idea #4 and anything
> that involves lamp dimmers, stove element controllers, diodes or
> "something clever electronically". Not that these are practical
> If electronic control is used idea #2 would require that the
> controls be adapted or replaced along with the element. A highly
> unlikely event unless the Australians have the exact same machine
> ratted for 220 Volts In which case just buy one of theirs.
> The diode in the line method proposed will fail no matter how many
> watts is evolved because the even though the current is applied for
> only 1/2 of the cycle at 220 volts it will still be twice what the
> heating element is designed for and will blow the internal fuse or
> destroy the element.
here's suggestion #5, the series capacitor reactive voltage dropper:
With a voltage of 110VRMS across the 9.3 ohm coffemaker we'll have
110V/9.3ohms ~ 12A flowing through it, so if we interpose a capacitor
between the coffeemaker and the 220V mains which can limit the current
through the coffeemaker to 12A, that'll do the job. In order to do that
we'll need the impedance of the cicuit to look like 18.6 ohms, so since
Z = sqrt(R² + Xc²) we can solve for Xc by rearranging:
Xc = sqrt(Z²-R²) = sqrt(346 - 86.5) ~ 16 ohms.
Since Xc = 1/2pi*f*C, we can solve for the capacitance by rearranging:
C = 1/2pi*f*Xc = 1/6.28*60*16 = 165.9E-6F for 60Hz and 199.0E-6F for 50
Hz. (I forget if Oz is 50 or 60Hz...)
GE makes some nice oil-filled caps, Z97FXXXX (where the X's are a
numerical suffix describing the capacitance, voltage rating, etc.) which
would prob'ly work nicely. I don't know for sure, though, since I
haven't checked their ESR. The caps might cost more than the
coffeemaker but, hey, if there's an emotional attachment to the
Professional circuit designer
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