Often the difference between a regular and a deluxe device costs 50% more is just a speed control, and you often don't need a separate one for each device. A single adjustable power strip that you build yourself, or a dimmer or dimmable remote control module (all under $10) can handle many needs and a $15 motor control unit can handle many more.
I was going to approach this subject differently, starting with theory but I think I'll start with some quick and easy solutions that almost anyone can use themselves -- and some warnings about what is/isn't suitable. You only need a single controller to adjust the temperature of a Ronco rotisserie/Foreman grill, the speed of a a table saw/router/sewing machine, or the brightness of a work/reading lamp. Why pay lots extra for separate (often quite crude) built-in variable controls?
Dimmer switches can be bought in any hardware store, often for under $10. They have a black wire, a white wire and a green wire, just like you household wiring. You can wire it in place of a household light switch to control a light fixture or outlet, install it in a box of its own (with a power cord and outlet), or install it in an appliance/power strip. The light dimmers you get for about $5-10 at a hardware store typically handle 600W (= 5A @120V)
As you know, AC current is always switching directions (100-120 times a second, depending on your country; sometimes more for military electronics) Without going into the circuitry for now, a dimmer switch basically works by turning on the output when the input reaches a certain voltage (which you adjust) and staying on until the voltage passes zero (which it does every time it switches direction). Clearly this isn't suitable for controlling DC (battery-powered devices or inside most circuitry) or even low-voltage AC (which doesn't reach the full range of voltages a home light dimmer is designed to control)
All this switching on/off creates electrical noise, and causes the voltage to be completely "off" for an an appreciable part of each cycle (vs. briefly passing through zero as smooth AC does). Some devices like light bulbs or heating elements don't mind this, but e.g. fluorescents get really cranky about it, as do many power supplies, which may burn out in time.
X-10 is a rather old "home automation" technology (I used it in the 1970s) that has been more widely popularized by X10.com (a website famed for its almost psychotic-looking webpages, pop-ups and (long ago) floods of spam. A lot of geeks don't care much for it, because it can be sometimes be fooled by electromagnetic or line noise, and its commands that arrive in one household circuit may not reach another in some home wiring configurations, but I'e had great luck with it, and it is really quite inexpensive (especially on eBay) You can remote control up t0 4-5 devices for ~$50, and the system can technically control 16 groups of 16 devices -- a total of 255 devices in one house.
There are many different kinds of X-10 controllers. Some just turn on/off, other can also dim (but please don't use dimmer lamp modules to turn on/off devices that can't be "dimmed" -- if they accidentally get turned down a notch or two it puts a real strain on the power supply, and your device may burn out). Some must be installed like a wall switch or oulet, while others plug into wall outlets or screw into lightbulb sockets. Some send and receive signals via household wiring while others use radio waves or infra-red. Some act like manual remotes, while others send signals automatically (timers, computer control modules, motion or IR sensors, magnetic door/window switches, and hobby controllers that respond to an external signal you supply). They even have a wide variety of cameras and receivers.
You should read the distinctions carefully (they aren't always clear) but I consider X-10 one of the easiest and cheapest ways to solve many household problems. A cheap wireless flat switch can be taped anywhere you *wish* you had a light switch without calling an electrician of digging in your walls. Motion detectors and cameras can have many uses besides security (e.g. triggering lights or checking who's at the door). You are mostly limited by your imagination.
Sewing machine foot pedals
These are everywhere. Oddly, it can be cheaper to buy an old sewing machine (they are often practically given away) than to buy a foot pedal alone (but the foot pedal isn't that expensive $25-30). Though there are three basic types: resistive, switcher/dimmer, and air bulb. All but the "air bulb" type can work well in controlling lights and motors. I love them in the workshop for routers, saws or Dremels -- pretty much anything where you'd want to control something continuously while you work, especially if you want to use both hands on your work.
The air bulb type isn't useful as a controller, because it just send pressure to sensor circuit inside the sewing machine
Motor Speed Controllers
These are switcher dimmers that have been beefed up to handle very inductive loads, like motors and transformers (but they don't really work at "controlling" transformers) I mention them only because they are commonly sold for $70 or more to woodworkers and other specialty interests, but I've fixed many over the years, and have been shocked at the markup. They typically contain the 2-3 variations on a certain basic circuit that a hobbyist could build for around $5, even in single unit quantities. They must cost the factory even less. Indeed, lately they've often had the *exact* same innards as a Harbor Freight motor controller (frequently on sale for <$15 or even <$10 with a coupon) in a *slightly* nicer case.
Potentiometers are variable resistors. many of you may remember when TVs and radios had knobs, sliders or thumbwheels for adjusting the voltage or ether parameters. These were potentiometers. Some sewing machine pedals (especially older looking ones) are just big potentiometers.
Since potentiometers are resistors they limit the current (and therefore the power available to a device, and don't have some of the "line noise" problems associated with "switcher" dimmers like the ones described above, but keep in mind that some devices (e.g. pretty much anything with an internal, brick or wall-wart power supply) don't like having their input voltage lowered. They may work for a while, but their speed/intensity won't change, and the device may eventually die..
You may hear (even from me) that a potentiometer "drops the voltage", and in some circuits, this is quite accurate: there is always a voltage drop across a resistor, and sometimes that is the effect that the potentiometer exploits. However I think it is better (and more generally true) to say it limits current (and that therefore the voltage drop across the *device* is less). I make this distinction because of the final type I will describe in this post.
The big problem with potentiometers as direct controllers is resistive heating. A resistor will turn P=EČ/R watts of electricity to heat. That E is the voltage drop across the resistor, which depends on the resistance in the rest of the circuit, so you have to do some math to find the right resistance to control your device over the desired range (depending on device resistance), and calculate the necessary power rating (all that waste heat can burn out a resistor). It's not "Plug and Play" as switching dimmers are, and it's wasteful: to adjust the current over a wide range, a potentiometer must cause a voltage drop comparable to the device it is controlling, consuming a comparable amount of power.
Transformers are used to convert one AC voltage to another. They are the *reason* we use AC in our homes and most factories and powerlines, instead of DC. They operate through induction. That is to say: they run the input AC through a coil to generate an oscillating magnetic field, and then intercept that magnetic field with a second coil to get the output. The ratio between the turns in the input and output coils determines the ratio between the input and output AC voltage.
There are main three kinds of variable transformer: auto transformers, true transformers, and isolated transformers.
Autotransformers (often called Variacs, after one popular brand) are perhaps the commonest, cheapest, and most efficient on the surplus of electronics market, but are less desirable for some applications. They really have only one coil, but the only run the input voltage through part of it. If you connect the full length of coil to the output, you get a voltage that is even higher than the input (typically 120->140 or 220->260), if you connect the output to one end and a sliding contact (moved by rotating a big insulated knob or lever) the voltage depends on the position of the contact on the coil
True transformers have two coils, and can be designed to be quite efficient for fixed value transformers, but are less efficient for variables. They are somewhat safer, because the input power never flows directly into the output coil. Isolated transformer are true transformers that meet special standards for safety and isolation of the two coils -- in this sense, they are like isolation transformers, which are just 1:1 transformers that output the same voltage as the input but limit the current and prevent power line current from directly crossing to the output.
All transformers limit the output current: if nothing else, their miles of wire have resistance, and their coils have inductance, producing a resistance-like effect called "reactance". If you try to draw more current than the transformer is rated for, you'll simply fail, and the voltage will drop -- the magnetic field can only give you what it has -- and you may damage it in time. However, unlike potentometers, transformers operate by truly changing the voltage, not by limiting current.
Variable transformers are not generally good units for household use: they are heavy (hence expensive to ship), expensive (~$50 for a 10A surplus unit) vs. switching dimmers for household currents, and may not be in as good shape inside as outside, if they weren't properly stored. Speaking of "inside" many are sold without cases, which is fine if you're an electrician or technical hobbyist who knows what they are doing, but "if you have to ask, then it's not for you".
There are many other approaches, but for those we really need to look at some theory first.
Edited by Orpheus, 07 November 2009 - 02:45 PM.