# Triac advantage?



## te12co2w (Jun 3, 2007)

I need to replace a fireplace blower fan speed control. There was a K&B controller in there that melted when the fan motor seized up because there was no more circulating air. It looks like there were 3 or 4 wires on the controller. I couldn't get the number off of the old control, but I can read the word Triac. What advantage is there to this device? I intend to replace the control with a simple 2 wire control. The motor is 1/40hp 120v Fasco double shaft. Thanks.


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## bkmichael65 (Mar 25, 2013)

Speed control


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## oliquir (Jan 13, 2011)

was this something like that
http://www.kbelectronics.com/Fan_Speed_Controls_Triac.html


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## te12co2w (Jun 3, 2007)

oliquir said:


> was this something like that
> http://www.kbelectronics.com/Fan_Speed_Controls_Triac.html


 It did look something like that but,,,,, some one had removed the cooling fins (if it ever had any) and put just the device in the trim of the fireplace insert. I don't know if this was a factory install or not. Very old house, very old fireplace.


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## te12co2w (Jun 3, 2007)

bkmichael65 said:


> Speed control


 I understand that. What would be the advantage of a triac control over a simple 2 wire variable speed control?


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## Big John (May 23, 2010)

As far as I know all fractional horsepower motor controllers use TRIACs to vary voltage to the motor.

There may be different designs, but the operation is fundamentally the same. Just get one that can handle the motor specs.


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## JRaef (Mar 23, 2009)

te12co2w said:


> I understand that. What would be the advantage of a triac control over a simple 2 wire variable speed control?


What do you mean by "2 wire variable speed control?"

Long ago people used to use what were called rheostats as a form of speed control or light dimmer. A rheostat is just a continuously variable resistor, like a potentiometer but without the wiper connection, where all of the power going to the load goes through the resistor first. That means that the circuit itself was constant power, but the resistor burned off some of it as heat. So if you had a 100W load and you wanted only 50W of power, 50W went through the resistor. That was OK for very very small loads when that heat was insignificant but if there was a bigger load, the heat build up around the rheostat would cook it. 

With a triac, voltage is controlled not by dropping it across a resistor, but rather by only allowing a portion of each sine wave to get to the load. So if you wanted your load to be at 50% power, the output is only on 50% of the time, but in a way that the load can only react to it as an average (RMS). The triac puts out heat too, but far far less; about 1-1/2W per running load amp per device. So if you had the same 100W fan at 120V that took 1A if power, the heat rejection (switching losses) of that triac will only be 1-1/2W instead of 50W. Much less stressful on the devices. 

The thing is, a triac needs a control circuit that decides when and how to fire the triac device, so it's more complex than a simple rheostat. There is no free lunch. Still, as a user, you generally don't see that complexity, it's inside of the magic black box. The potentiometer you see on the outside is what it telling the controller how much power you want delivered through the triac, then the circuit board responds to that command accordingly. The thing is however, a triac controller can work in several different ways, depending on the nature of the load; AC, DC, heater, motor, lights, voltage regulation, proportional response, it all makes a difference. First step it to fully and accurately identify the nature of the load.


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## te12co2w (Jun 3, 2007)

JRaef said:


> What do you mean by "2 wire variable speed control?"
> 
> Long ago people used to use what were called rheostats as a form of speed control or light dimmer. A rheostat is just a continuously variable resistor, like a potentiometer but without the wiper connection, where all of the power going to the load goes through the resistor first. That means that the circuit itself was constant power, but the resistor burned off some of it as heat. So if you had a 100W load and you wanted only 50W of power, 50W went through the resistor. That was OK for very very small loads when that heat was insignificant but if there was a bigger load, the heat build up around the rheostat would cook it.
> 
> ...


 For instance Leviton 6616-1XI. Described as single pole fan speed control. A lot of different brands and model #s available. I don't know how this differs from a triac 4 wire control, but I think I will use this or something similar.


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