SCR dimmer control for portable electric heater??

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snipped-for-privacy@privacy.net wrote in wrote:

the triac may shut off current flow at a part of the sine wave less than peak voltage,so it -can- reduce the voltage to the load. It would have to be at less than 50% setting,though.
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On 3/21/2010 4:02 PM, Jim Yanik wrote:

It -does- reduce the voltage. Connect any volt meter to the load and observe the RMS voltage (what meters display) decrease as you decrease the dimmer setting. You are thinking of p-p voltage which is always constant and isn't what a typical meter displays.
I don't understand the significance of 50%. The moment the triac changes the waveform is the moment the RMS voltage is no longer line voltage.
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wrote:

When a triac is used to control the ammount of heat in a resistance load, a meter can show almost anything up to the peak of the voltage. The common meter with a coil is only calibrated to show the RMS of a sine wave. If any other waveform is measured, the calibration is no longer valid. Some digital meters are suspose to be calibrated for a true RMS, but I would think they have some limitations.
The 50 % part comes in because any conduction angle less than 50% will not have the full line voltage as the peak value. That is the point the meter can not reach the full input voltage to the triac on the output side. I really should say scope instead of meter, but a peak or peak to peak meter with a peak hold function could be used.
All this is for simple circuits. There are some triacs that only cut off and on at the zero crossing and there is a timming circuit that will let it conduct for so many full cycles out of a period of time depending on the ammount of heat wanted. If a voltmeter is put across the load it will try to reach the full line voltage and will almost do it lots of times. It will only stay there for a fraction of a second. Almost looks like a windshield wiper on a car. The same with an amp meter. It will almost reach the full current. The meters can be dampened to show a more 'averaged' value.
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On 3/21/2010 8:51 PM, Ralph Mowery wrote:

I understand p-p vs RMS. And you are correct depending on how well the meter integrates the waveform it may not be accurate. But it still shows an approximation of the RMS voltage value which does continuously change with the dimmer setting.
RMS is used because it allows us to treat the reading as DC for basic calculations.
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wrote:

Not the way they work. The triac turns off when the voltage reverses. The turn -ON is all that is controlled, so the actual voltage would always be the same, but average voltage (and therefore average power) would be reduced.
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On Tue, 23 Mar 2010 02:17:03 +0000 (UTC), snipped-for-privacy@manx.misty.com (Don Klipstein) wrote:

RMS on a sine wave, yes - but a chopped sine wave responds differently.
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On Sun, 21 Mar 2010 10:37:02 -0500, Monty B

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no,because triac dimmers -shut off- current flow during part of the AC cycle,so there's no power dissipation while the triac is OFF. It dissipates only when it's on and conducting,and it's ON resistance is very low,so that's not a lot of dissipation.
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On Sun, 21 Mar 2010 07:57:34 -0400, "John Grabowski"

An SCR (or more correctly a Triac) type dimmer will work very well for controlling the output of an electric heater. The control will NOT be linear - but it will be effective and quite efficient as the triac control turns the power on and off at different points of the sine wave. There is very little dissipation.
There IS, however, significant electrical "noise" and a standard residential dimmer will not do the job because they are generally rated for 600 or 700 watts. You need a 1500, or preffereably 1800 watt dimmer to do the job. They will get WARM - but only dissipate a few watts if running within their design parameters.
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snipped-for-privacy@snyder.on.ca wrote:

Yes.... that's the problem.....finding one for 1500 watt load.....and one that is "off the shelf" and cheap enough
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On 3/20/2010 8:04 PM, snipped-for-privacy@privacy.net wrote:

More efficient in what regard? It takes xxx BTUs to warm the space. Assuming the space heater is providing the heat it costs the same to provide those BTUs whether the heater is operated at a higher output for short periods or a lower output for longer periods.
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On Mar 20, 7:04pm, snipped-for-privacy@privacy.net wrote:

You can buy a 20 A Powerstat or Variac autotransformer on eBay for 40-60 bucks or so. Use it to change the voltage to the heating elements and leave the thermostat function alone. The units are good sized, but from a safety and simplicity standpoint you won't hurt yourself with such a system.
Joe
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wont a variac just dissipate a TON of heat and act like a secondary mini heater?
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snipped-for-privacy@privacy.net wrote:

You may be thinking of a rheostat - which is a resistor.
A variac is a type of transformer and has a high efficiency like a transformer.
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On Mon, 22 Mar 2010 13:46:30 -0500, snipped-for-privacy@privacy.net wrote:

A variac is an autotransformer, not a rheostat, and as such is pretty darn efficient
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A variac is a variable transformer. Different than a resistor.
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On Mar 22, 1:46pm, snipped-for-privacy@privacy.net wrote:

Actually, no. Variacs are autotransformers, just like the tapped coil distribution transformer hanging on the pole outside your house. Their power losses are minimal, and they operate serenely and fuss free just supplying the voltage you dial in. Most can even supply 0 - 140 V if you need it. I keep one in my shop for calming down my monster soldering iron when working on more delicate projects. I've never found it to be even slightly warm after use. Variacs and Powerstats are used universally in chemical laboratories for controlling reactor flask heaters and such. In those environments, stray heat would be a safety problem. The Variac therefore is the best solution, IMO to the OP's problem from simplicity and cost standpoints.
Joe
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Thanks so much Joe!!
I learned something I didn't know today!
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In my opinion the OP doesn't have a problem. I was just willing to suggest a way to do what he wanted to do. I don't agree he needs to do it.
Jimmie
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