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Topic: controlling a simple roaster heating element (18 msgs / 460 lines)
1) From: Tom & Maria - Sweet Maria's Coffee
Hi all - thought I would ask here ... i am trying to control a very 
basic 240 v heating element that is basically hot wires attached to 
elements. Can't I use a simple rheostat i.e dimmer switch? Now, 
finding a 240v one is an issue, but unless I am wrong, there is no 
need for a variac type controller. Any help would be appreciated....
--
                   "Great coffee comes from tiny roasters"
            Sweet Maria's Home Coffee Roasting  -  Tom & Maria
                      http://www.sweetmarias.com                Thompson Owen george_at_sweetmarias.com
     Sweet Maria's Coffee - 1115 21st Street, Oakland, CA 94607 - USA
             phone/fax: 888 876 5917 - tom_at_sweetmarias.com
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2) From: Chad Sheridan
The problem will be finding one that can handle the current levels 
needed for a heating element.
Tom & Maria - Sweet Maria's Coffee wrote:
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3) From: J.W.Bullfrog
Yes. However getting one rated for the power will be the problem.
The cheapest I've seen is has been at Harbour Freight (a cheap tool store)
that was intended to control the speed of the spindle on a lathe or drill.
I sure it was under $30, but don't remember if it was good for 240.
On Fri, Apr 4, 2008 at 1:59 PM, Tom & Maria - Sweet Maria's Coffee <
sweetmarias> wrote:
<Snip>
-- 
I've seen things you people wouldn't believe.
Attack ships on fire off the shoulder of Orion.
I watched C-beams glitter in the dark near the Tannhauser gate.
All those moments will be lost in time, like tears in rain.
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4) From: Jeff Bensen
Tom -
As long as you only want to reduce the power delivered to the element 
then, in theory, yes you could just use a rheostat.
As others have pointed out, however, finding one rated for 240 volts 
that is able to handle whatever power level you may require is 
another matter entirely.
You can buy beefy SCR phase-angle controllers for resistive loads 
(like your heating coil), which will allow a control range from about 
95% down to 0% power. Depending upon the current required, one of 
these might do:http://www.thermalinc.com/power/tbp.htmhttp://instrumentation-central.com/SCRPowerControls/ssrp_series.pdf
Perhaps other list members will have suggestions as well. Feel free 
to call me if you want to discuss this (321-728-4358, answers as 
"Bensen Engineering").
- Jeff Bensen
   Palm Bay, FL
At 02:59 PM 4/4/2008, Tom & Maria - Sweet Maria's Coffee wrote:
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5) From: Mike Chester
Tom,
First off, a light dimmer is not a rheostat.  A rheostat is a variable 
resistance device that is placed in series with the load.  As the resistance 
is increased, the rheostat drops more of the voltage and the load gets less. 
One large enough for this application would be large and expensive.  A light 
dimmer uses a triac that without going into detail chops off parts of the 
electrical waveform.  This results in a lower average voltage output to the 
load.  The amount that is chopped off depends on the setting of the dial. 
This works well with a pure resistive load such as an incandesant bulb or a 
heating element, so this type of device would work well in this application. 
HOWEVER it will be necessary to find one with the power handling capacity 
needed for your heating element.  A standard wall unit will be way too 
small.  You will need to find one designed to work with 220 volts and the 
amount of current that the element draws.  There are industrial devices that 
are capable of this.  I would probably build my own controller, but since 
you are looking for an out of the box solution, I would check with a 
commercial electrical supply house and see what they have.  A variac would 
also work, but again it would need to be rated for the voltage and current 
that you need.  I hope that this is helpful.
Mike Chester

6) From: Rich
You can go to eBay and find solid state relays rated for 23A, 40A, and 
60A.  these are switched on and off with a low voltage DC control 
voltage.  Also you could buy one of the $36.00 PID temp controllers and 
automate the entire process.  The SSRs will handle up to 380vAC on the 
load side.
Tom & Maria - Sweet Maria's Coffee wrote:
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7) From: John Despres
Mike's post has given me a thought.
Aren't most electric stove/ovens 240 volts? Perhaps a call to an =
appliance repair shop will give you the answer. Maybe they'll pull a =
controller out of an old oven and hand it over for very little...
Any thoughts on this idea?
John
Mike Chester wrote:
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-- =
John A C Despres
Hug your kids
616.437.9182
Scene It All Productions 
JDs Coffee Provoked Ramblings =
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8) From: Alchemist John
As you have now heard, load is the problem, and if you find one, 
price will be the other concern.  A variac may well be cheaper.  Why 
are you against a variac?
At 11:59 4/4/2008, you wrote:
<Snip>
John Nanci
AlChemist at large
Zen Roasting , Blending & Espresso pulling by Gestalthttp://www.chocolatealchemy.com/Homeroast mailing list
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9) From: Tom & Maria - Sweet Maria's Coffee
Thanks for the input, folks - I was looking at Variacs (which is not 
really necessary in the application) but I will check out some of the 
leads you guys have given. Our power here runs so hot, normally not a 
problem but in this case it is. Instead of just getting a transformer 
I thought it would be nice go get some greater control. I hadn't 
really thought about the PID route with SS relays but that's a great 
idea.
<Snip>
--
                   "Great coffee comes from tiny roasters"
            Sweet Maria's Home Coffee Roasting  -  Tom & Maria
                      http://www.sweetmarias.com                Thompson Owen george_at_sweetmarias.com
     Sweet Maria's Coffee - 1115 21st Street, Oakland, CA 94607 - USA
             phone/fax: 888 876 5917 - tom_at_sweetmarias.com
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10) From: Rich
It would be the oven temperature control that you wanted.  They are not 
cheap if you have to buy one from an appliance parts place and finding a 
oven that you can pull parts out of can be difficult.  Appliances are 
hard yo dispose of, legally, so they go to a few junkers who scavenge 
all of the usable parts if thy can not resell the unit after minor repairs.
John Despres wrote:
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11) From: Morris Nelson
Take a look at a device that controls electric baseboard heating elements.
You should find someone at Home Depot that can help you.
Morris

12) From: Paul Helbert
Bingo. That should work. The only problem I might foresee is that heaters,
ovens and the like don't need very precise control in the short run so long
as they get it right over longer time.
Paul Helbert
On Fri, Apr 4, 2008 at 9:24 PM, Morris Nelson  wrote:
<Snip>
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13) From: Jim Gundlach
These would be thermostatically controlled.  I think he wants the  
ability to produce more and less heat.  Look at the controls behind  
the knob for an  electric range "burner" element.
      pecan jim
On Apr 4, 2008, at 8:24 PM, Morris Nelson wrote:
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14) From: Steven Dover
Since 240 is 2 leads of 120, why not control/reduce only one side/one of
the two 120 volt leads? Controlling only one side should be enough to make
quite a difference.
Steven D. 
At 02:09 PM 4/4/08 -0500, you wrote:
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15) From: Rich
The electric baseboard heat thermostat is a self contained device that 
may be used to switch both a heating and cooling load.  The temperature 
sensing element is integral with the power switching unit.  And it is 
designed for 50 to 85 degrees or so.  Might be difficult to control a 
heater unless it was an area heater.
Paul Helbert wrote:
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16) From: raymanowen
" Since 240 is 2 leads of 120, why not control/reduce only one side/one of
the two 120 volt leads?"
Exactly right, Steven!
As for the nominal electric range top heating elements, they are not
normally thermostatically controlled. The knob just adjusts a % timer. It's
On/ Off, according to a 0-100% duty cycle.
Electric fry pans that have the removable control/ power cord assembly have
a thermostatic control in the pencil shaped nose that senses the heat of the
pan. Also On/ Off, and designed for 115-120v.
As has already been suggested, if you really want to control the temperature
of some process, a PID controller is best. A 2-pole SSR (most cheap-o's are
single pole devices, but you could use two of them- half as cheap) will work
the best.
Drop a line to Omega Engineering or call, and you will be in contact with an
application engineer that can set this up in his sleep.
They used to save us regularly at the CSM Research Institute.
Cheers, Mabuhay -RayO, aka Opa!
Got Grinder? Is that the best you can do for these beans?
On Fri, Apr 4, 2008 at 8:19 PM, Steven Dover  wrote:
<Snip>
-- 
"When the theme hits the bass, I dance the Jig!" - -Virgil Fox at the Mighty
Wichita (ex- NYC Paramount) WurliTzer- 1976
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17) From: Bob Hazen
Tom,
I read a number a good ideas, but I have a few questions.
How much power are you trying to control?  What temperature do you want the 
elements to operate?  Are you heating a room?  Or, as I suspect, making a 
roaster?  Do you want continuous control where you can set an unvarying 
power level?  Or would an hysteretic control (on/off, like a house 
thermostat) work?  The hysteretic control might not be as bad as it sounds. 
If you have enough thermal mass in the elements and set the hysteresis tight 
enough, it could cycle quickly enough to average out the variations.
Just some questions to ponder before picking a solution.
Bob

18) From: Allon Stern
On Apr 4, 2008, at 10:34 PM, Rich wrote:
<Snip>
How about a dimmer rated for 240VAC? Such as one made for a european  
market?
It all depends on how much current you want to control. Those made  
for theatre can handle plenty.
Or you could go with a variac.http://cgi.ebay.com/GE-0-120-0-240VAC-Variable-Transformer-Variac-8-5-
Amp_W0QQitemZ200212788275QQihZ010QQcategoryZ4665QQssPageNameZWDVWQQrdZ1Q 
QcmdZViewItemhttp://cgi.ebay.com/0-240-VAC-Variac-Powerstat-7-5-Amp-tube-amp-
tool_W0QQitemZ110239855554QQihZ001QQcategoryZ10171QQssPageNameZWDVWQQrdZ 
1QQcmdZViewItem
Those are babies.http://cgi.ebay.com/NEW-AUTOTRANSFORMER-20A-0-280V-50-60Hz-
variac-240V_W0QQitemZ260226941633QQihZ016QQcategoryZ11737QQssPageNameZWD 
VWQQrdZ1QQcmdZViewItem
Okay, THAT one is a baby compared tohttp://cgi.ebay.com/GIANT-3P-185KVA-Motorized-Linear-Variac-
Autotransformer_W0QQitemZ230238605057QQihZ013QQcategoryZ73153QQssPageNam 
eZWDVWQQrdZ1QQcmdZViewItem
But if you get THAT one, you're crazy. It's 3 phase :)
-
allon
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