Before we can choose the right X-10 device, however, we need to know a little more about the load we are wanting to control and how the circuit is wired.

Which One Should I Use? - Part I

Phil Kingery

"Which One Should I Use?"
by Phillip Kingery of Advanced Control Technologies, Inc.,

As easy as they are to use, the many plug-in and hard-wired X-10 compatible devices have caused more than one of us to ask the question, "Which one should I use?". Before we can choose the right X-10 device, however, we need to know a little more about the load we are wanting to control and how the circuit is wired.

Technically speaking, we can categorize the various loads in your home into two neat groups: linear and non-linear. Electronically speaking, a linear load is purely resistive, having no (or a negligible amount of) inductive or capacitive impedance. Conversely, non-linear loads have a degree of inductive or capacitive impedance.

Linear loads used to be the most prevalent, by far. From the earliest days of Thomas Edison, electricity in our homes was used for only two purposes. The first was to power Mr. Edison's new incandescent light bulbs. The other was to heat coils of wire for warming our houses or cooking our food. Occasionally, we used electricity for an inductive load, like a transformer or a motor.

Today, however, non-linear loads far outweigh the linear ones. We have televisions, radios, the electronic timers in our coffee makers, our electronic digital clocks, our computers, alarm panels and anything else you can think of. Even my wife's sewing machine now has a microprocessor inside it.

Sometimes, we may think that a light bulb is a linear load when it really isn't. Halogen bulbs come in a variety of types and styles. Some are 120v bulbs and since they have tungsten filaments, they are linear loads. Some halogen lights are really 24v and so are powered by a transformer or electronic power supply. In those instances, the entire load is non-linear.

Figure 1Another consideration is the circuit itself. If this is to be a retrofit situation, we may be limited in our choices simply by how the circuit is wired. We often think of a circuit as being wired from the panel, to the switch, then to the load (as in Figure 1). Because of the electrician's prerogative and what was convenient at the time, it is also common to find that the circuit was wired from the panel, to the load and then to the switch.


Figure 2Figure 2 shows, what is commonly called a "switched loop" circuit. As far as the electrons are concerned, they pass through the switch before going to the load in both cases. The only difference is the arrangement and order of the junction boxes.

Our confusion begins when we try to replace a standard mechanical wall switch with an electronic X-10 switch. A regular mechanical wall switch gets it's "power" from your finger. Even though it is connected to a wire that has 120v on it, the old mechanical switch does not "use" that power. When you want to "switch" on the load, the power to do that, comes not from the wire, but from your finger. The purpose of that mechanical switch is to complete the circuit or interrupt the circuit depending on which you desire, but it is not an electrical load in and of itself. The same is also true of most of the standard, hardware-store-variety, dimmer units. You supply the power to turn the knob.

Figure 3Most "do-it-yourself"-ers are fortunate enough to have begun their journey into the world of X-10, remote control and home automation, by plugging a table lamp into a "lamp module". Later we may have replaced a mechanical wall switch with an X-10 brand or Radio Shack "electronic" wall switch (figure 3) on a simple lighting load circuit. These work especially well in those instances where there are only 2 wires in the box.

We might have been disappointed to learn that we could turn it "on" and "off" locally, but had to use a transmitter to do any dimming. (Only later did we discover that we could open it up and restore the local dimming feature.)

Figure 4Only a few of us noticed that the Radio Shack or X-10 wall switch had only "two" wires and wondered why it did not require a connection to neutral. After all, it was an electronic gizmo and as such, did it not need its own supply of electricity? Even ACT's own RD130 (figure 4) is a 2-wire dimmer. It is very close in design to the standard X-10 or Radio Shack units. All of them are made mostly for those retrofit situations where the installer wants to replace a standard mechanical wall switch.
Figure 5Since they are the simplest of the X-10 receivers, they can be used in both circuits types. A switched "loop" circuit (figure 4) has no neutral wire. Oh sure, there is a white wire (I use yellow in my illustrations) but if the electrician followed code, he used a marker, paint or tape to color the wire end "dark" to signify that it was now being used as a switched line wire. When used in a switched line circuit (figure 5) a true neutral is available but in this case, its isn't used.

These 2-wire X-10 dimmers connect to only line and switched line (or hot and switched hot) and operate from the trickle current through the load. In other words, even when the light bulb (resistive linear load) is "off", there is still a little current through it to keep the 2-wire dimmer operating.

Only a few unlucky DIY'ers discovered the problems when they tried using one of these 2-wire dimmer units on a circuit with a radio or TV on it. Not only did the receiver not work, the radio or TV was probably damaged.

While these 2-wire dimmers (intended only for use with incandescent or "linear" loads) seem to be the most prevalent of the hard-wired devices, they suffer from one deficiency that occasionally results in unreliable operation. Not only does their operating voltage come through the light bulb, the X-10 signal must also do the same. If the signal strength is high, then there is usually more than enough signal that gets through. Sometimes, however, the X-10 signal is just barely high enough for the receiver to turn "on". Once "on" the signal is partially blocked by the bulb and there is now too little signal to turn it "off" again.

What has happened is that when "off", most of the (differential) signal voltage is across the receiver, but when on, more of the signal is across the bulb. When ever the installer has a neutral in the wall box, I always suggest using a 3-wire dimmer (figure 6) so that the receiver circuit always has its own connection to line and neutral and no longer has to rely on trickle current to operate (figure 7).

Figure 6 Figure 7

There is another interesting and often confusing aspect of home automation, and that is, "speed" control. I am constantly amazed by just how easily we all associate "dimming" with motor control. As I said earlier, incandescent lights are resistive loads while motors and transformers are inductive loads.

What really gets confusing is when a triac based dimmer unit is used on inductive loads. Most modern dimmers, including all X-10 compatible dimmer units, have a triac output. A triac dimmer does not attenuate the 60Hz sine wave, instead it cuts out varying chunks of the sine wave. Since the output is non-sinusoidal, the motor in a ceiling fan, for instance, begins to exhibit some unpleasant side effects. Sometimes the fan begins to hum or buzz. It may also run hotter than normal since it is no longer receiving a smooth sinusoidal wave as its power source.

Receivers like the RD140, from ACT (figure 7), is not only a 3-wire dimmer, it is also rated for non-linear loads. Many users find satisfactory operation using this dimmer in those applications where speed control of a ceiling fan, or a transformer is desired. Giving the dimmer its own connection to line and neutral makes it possible to have a power source that is unaffected by the on/off condition of the load. This will also make sure that the X-10 signals do not have to squeeze through the load (the motor, in this case) in order to get to the dimmer unit.

I know of many people who have attempted to use a two-wire dimmer with a motor (or transformer for low-voltage lighting) and discovered that if it works at all, it is very intermittent. Either it will not stay on, will not stay off, or they can not turn it on and/or off remotely.

However, just because the RD140 is rated for non-linear loads does not automatically mean that your non-linear load wants to be controlled by a triac based dimmer. We all know that we will damage a television or radio if we try to control them with a dimmer unit. Trying to control a television's picture brightness or a radio's volume with a dimmer is like trying to control your car's speed by how fast you fill the gas tank. The same is true for some "non-linear" loads like fluorescent lights, low-voltage lighting systems and motors.

Figure 8Some non-linear loads just should not be controlled with a dimmer at all. Even when the dimmer is full on, it is still not a true sine wave and therefore causes the load to hum, buzz, run hot, or worst case, burn out. In these cases, a switch that uses a relay instead of a triac is required (figure 8). A relay, of course, is either on or off: it isn't a variable output.
Figure 9These kinds of receivers are known as "hard-contact" receivers and are also available from ACT as our part numbers RS120. Since these have a relay as their output device, they can carry much larger amounts of current than a comparably sized dimmer unit. Our industrial grade devices are rated at 20 amps. They will require their own connection to line and neutral, (figure 9) and so they will not work in wall boxes where there are only two wires.

While the question of "which one should I use?", has been only partially answered in this article, I hope that the next time you want to replace an old mechanical light switch, you will be able to make a more informed choice.

Phillip Kingery is the X-10 support representative for Advanced Control Technologies, Inc., Indianapolis, Indiana and is a well known instructor of X-10 technical classes routinely held around the country. His email address is:

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