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Old 10-22-2011, 08:36 PM   #1
ahave
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Default Any reason why an electric dimmer would NOT work?

So instead of building a PWM unit, why couldn't i use something like this?
http://www.bing.com/shopping/electro...er&FORM=CMSMSP

it is a 1500W electric dimmer.
I would imagine these things do the same as a PWM, just built as one. Or does it reduce the voltage? Would that matter for an electric heater?

I plan on using it with 110v 1500W electric heater. The only downside I can see is if I ever went to 220v service, the PWM could be carried over whereas this could not. Is there anything I am missing from the electric side of things?

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Old 10-22-2011, 08:49 PM   #2
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Those work roughly the same way as PWM, only they operate at the line frequency (60Hz in the US). PWM is a more general term, that even applies to DC. Dimmers use thyristors to chop the AC waveform and rely on the zero-crossing to reset the thyristor, but it's the same principle and for a resistive element it's equivalent to PWM. As long as its rated for the correct wattage you should be fine.

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Old 10-24-2011, 06:16 PM   #3
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A dimmer switch (rheostat etc) is NOT the same as a PWM control nor will they perform the same on a high wattage circuit.

An electric heating element is simply a low ohm resistor that is heavy duty enough to not burn up from the high current. When this resistor has line voltage across it you get current, and this current is high enough that the material gets hot. Imagine overloading an extension cord by 10 times its rating; it will be a heater for a very short time period.

The goal of PWM or a dimmer switch is to limit the voltage applied across the resistive element so you end up with less heat from the resulting current. If you were to put a rheostat in a series circuit with a heating element the total circuit's resistance will not decrease. The resistance will only increase as the rheostat's resistance is increased, the knob is turned. To lower the heat (result of current) in the heating element you must have a higher resistance than the element in the circuit. A large enough resistance rheostat, the 1000W+ kind, will essentially be absorbing more voltage than the heating element, therefore lowering the heat created in the element.

However, by absorbing the majority of the circuit’s voltage the rheostat will also be using the majority of the circuits power. What you end up with is a heating element, the rheostat, inside your control box instead of in your tank. Imagine that a heating element puts off a certain amount of heat, and two of them in series would put off half as much each.

On the other hand PWM accomplishes the same thing by fast switching the circuit. The voltage across the element isn’t reduced at the high frequency that the PWM controller switches but averages out to less voltage at line frequency to the element. I always describe it as a rocket ship that has full on or full off thrust. If you were to “feather” the throttle very fast you could make the rocket hover, neither drop nor rise.

In a lighting circuit, lower power than heating water, a rheostat would work great because it would only have to dissipate the wattage of a bulb, 100W or so. However for a high load circuit you would have very short component life and some unsafe conditions.

I hope this helps you some. Drop me a PM if you have more questions.

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Old 10-24-2011, 07:13 PM   #4
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Quote:
Originally Posted by clay9_24 View Post
A dimmer switch (rheostat etc) is NOT the same as a PWM control nor will they perform the same on a high wattage circuit.

An electric heating element is simply a low ohm resistor that is heavy duty enough to not burn up from the high current. When this resistor has line voltage across it you get current, and this current is high enough that the material gets hot. Imagine overloading an extension cord by 10 times its rating; it will be a heater for a very short time period.

The goal of PWM or a dimmer switch is to limit the voltage applied across the resistive element so you end up with less heat from the resulting current. If you were to put a rheostat in a series circuit with a heating element the total circuit's resistance will not decrease. The resistance will only increase as the rheostat's resistance is increased, the knob is turned. To lower the heat (result of current) in the heating element you must have a higher resistance than the element in the circuit. A large enough resistance rheostat, the 1000W+ kind, will essentially be absorbing more voltage than the heating element, therefore lowering the heat created in the element.

However, by absorbing the majority of the circuit’s voltage the rheostat will also be using the majority of the circuits power. What you end up with is a heating element, the rheostat, inside your control box instead of in your tank. Imagine that a heating element puts off a certain amount of heat, and two of them in series would put off half as much each.

On the other hand PWM accomplishes the same thing by fast switching the circuit. The voltage across the element isn’t reduced at the high frequency that the PWM controller switches but averages out to less voltage at line frequency to the element. I always describe it as a rocket ship that has full on or full off thrust. If you were to “feather” the throttle very fast you could make the rocket hover, neither drop nor rise.

In a lighting circuit, lower power than heating water, a rheostat would work great because it would only have to dissipate the wattage of a bulb, 100W or so. However for a high load circuit you would have very short component life and some unsafe conditions.

I hope this helps you some. Drop me a PM if you have more questions.
This is all correct info, but the dimmer ahave linked to is most likely not a rheostat. Modern dimmers are typically thyristor based, not rheostat based, so they don't work by "burning up" the rest of the power...they work by chopping the waveform. It's basically the same result as a PWM operating at a set frequency, (although not exactly the same, as PWMs don't chop the waveform), as bettersense said....so since it's rated for 1500W, and he's using it for 1500W, it should work fine.
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Old 10-26-2011, 04:47 PM   #5
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sooooo
When I go the the hardware store to get a PWM dimmer for my new stir plate project, what do I look for? Searched the Loews website and found nothing listed as PWM. Do i look for a dimmer that will dim fluorescent? I assume that "incandescent only" dimmers would most likely be simple resistors.

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Old 10-27-2011, 08:08 PM   #6
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Originally Posted by william_shakes_beer View Post
sooooo
When I go the the hardware store to get a PWM dimmer for my new stir plate project, what do I look for? Searched the Loews website and found nothing listed as PWM. Do i look for a dimmer that will dim fluorescent? I assume that "incandescent only" dimmers would most likely be simple resistors.
Look for a dimmer that can handle the wattage.

You would be hard pressed to find a 110v dimmer that worked using a resistor...if only because it's a far more expensive and less effecient way of dimming a lightbulb. They'll be of the type you want.

and electrically, and heater is the same as a really big light bulb.
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