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Dimmers/Speed controls of various types

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#1 Orpheus


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Posted 05 November 2009 - 11:43 PM

This is actually a pretty diverse topic that I was planning to address over a year ago, before my parents fell ill.

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
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.

Variable Transformers
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.

#2 Orpheus


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Posted 28 May 2010 - 12:49 AM


What simpler speed control is there than "on/off"? Let's start here, to get some basics out of the way. There are all sorts of switches out there, and sometimes the right switch can replace an entire circuit

Let me state the obvious here: a switch (or circuit) is open, if there isn't a continuous conductive path (i.e. no power can flow, aka "off") and closed if the contact provides a conductive path (allowing power to flow, aka "on") "Open" and "closed" are better terms to describe a switch position than on and off, because sometimes you turn a circuit on by flipping a switch to the "off" (open) position. Open and Closed are also used when we're debugging a circuit: a circuit may be open if a wire breaks or a component blows/burns out, and fixing it "closes" the circuit again. Sometimes a component "fails closed", instead of "failing open" -- it may melt into a little conductive metal blob instead of burning out and breaking the circuit. It's still broken, but the symptoms are different (and it may be dangerous, with the power on).

The famous "short circuit" is another kind of closed circuit failure, with a very specific meaning: it's when the circuit is closed by a conductor that leads somewhere unintended. If the wiring insulation in a lamp fails, the voltage can "short" (take a short or easy path) from wire to wire without passing through the bulb (which would force the electricity to do work, which it abhors) Naturally electricity stampedes through shortcuts uncontrolled--hence the sparks often associated with short circuits (until the fuse blows). However, electricity can also short to, say, a lamp body or fixture, which can be much more dangerous. Without a clear path to ground, the stampede doesn't occur -yet- but if someone touches it who also happens to be a shortcut to ground--cowabunga. A fuse only trips if a pretty big stampede happens, and a pretty small electrical stampede can knock you flat or kill you. There have been instances of four or more people felled by the same lamp in rapid succession (not necessarily killed) because each new person saw the bodies and reached to turn on the light

Sorry. I've always found that an amusing image. I'm a horrible person. Just consider yourself warned.

Poles and Throws
The most basic switch is a piece of metal that is physically pushed to make contact with another, creating a path for current can flow. This is a Single Pole Single Throw (SPST switch). It has one input (pole) and one output (throw) connected by a movable contactor. (Technically, the "pole" is the number of contactors, not inputs) Most common light switches are SPSTs. Either the pole is connected to the throw (on) or not (off). Doorbells are usually like this, too, only they are momentary contact switches, a spring inside them turns them off as soon as you let go. You can imagine how annoying the alternative could be!

Sometimes you want to switch between two outputs [A/B]. Maybe you want an outlet switch that will let you use your dishwasher *or* microwave, but not both together lest you blow a fuse. You need an SPDT switch (single pole double throw): a single contactor pole, which can be "thrown" to two electrical positions (throws), A or B. A SPDT can be used as an SPST (just leave B unconnected) but SPSTs are cheaper

You may see DPST (double pole, single throw) switches but people usually use DPDTs (Double pole Double Throw) instead, so those are easier to find. DPSTs can be made in three configurations. The first hooks two SPST switches to turn on/off together (if you want two things to turn on at once; or disconnect both wires at once to be *extra sure* no voltage strays into a device when you're working on it) the second sets them oppositely (when one turns on, the other turns off -- useful if the two devices need different voltages, so you can't use an SPDT).

The third configuration of DPST is often drawn in schematics as a pushbutton (suggesting momentary contact) even when it isn't a momentary. It's analogous to the misleadingly named "three-way" wiring ("Two-way" would be a better name) used to switch, say, a stairway light from both the top and bottom of the stairway. A three-way doesn't have an on or off position. It turns on or off depending on whether its two SPDTs (connected via two wires called "travelers", here marked 1 and 2) agree on which wire to use. If they do, that traveler closes the circuit so power can flow. If one switch is set to "1" and the other to "2", there's no continuous path for the power to follow. This works well for lighting because if the light is on, we obviously want it off, and if the light is off, we obviously want it on -- else we wouldn't bother with the switch at all. If we're wrong, well, a quick flip back doesn't hurt anything (obviously, this kind of three-way switch could be disastrous with heavy machinery!)

The "third type of DPST" (which I consider a DPDT) is like a three-way, except the two SPDT switches are right next to each other, and physically tied together, so the both flip at the same time in the same direction.

As I said, a regular DPDT can work as any type of DPST, and as a DPDT as well, so it's more popular and often cheaper


More complicated switches are common in commercial devices you may fix, but you'll rarely design with them. A SP4T might be used to choose whether to power up a car's AM, FM, CD or phone (you wouldn't want more than one at once). An SP8T might be used in a music studio to route a monophonic audio channel to one of 8 different audio devices. a DP8T might do the job for two stereo channels. Conceptually, non-digital washing machine controllers (aka 'timers') are little more than a multipole, multithrow switch with maybe 8+ poles (input water valve, spin motor, agitator, drain valve, lid lock, softener dispenser, temperature sensor, heater, and whatnot) and 20-40 or more positions, with a motor that advanced the switch through each of those positions in order. Imagine a position for each minute of operation, with the the designer hooking up all the devices he needed that minute to the corresponding "throw position". The wiring around the switch could be a rat's nest, but they were reliable and easy to diagnose.

Switch specifications
A contact point has a slightly higher resistance than a solid metal connection, so that tiny point may heat up or briefly generate a spark. This is pretty harmless. You don't see it inside the case, but it does put a limit on how much current a given contact should handle. Even a little abrasion or sparking adds up over 10K, 100K or even 1M operations. Switches usually have a spring-loaded actuator mechanism that quickly clicks the contact open/close when the switch passes a certain point to minimize spark time. You've felt that click. Though some switches are supposed to glide, avoid one that is so cheap/worn that it doesn't actuate crisply. It may switch on or of by itself, or loose contacts may even "strike an arc" (a continuous spark) like an arc welder, and start a fire

Switches have many specs that are easy to ignore. Voltage is one biggie: the switch may have a small switching gap, that a high voltage spark may jump; the contact material may vaporize or pockmark at high voltages (independently of the current); it may be "vented" and prone to collect dust or oil; the case or wiring insulation may not be robust enough  or may not stand up well in the long term under HV ozone, heat, sunlight or artificial UV etc. The dangerous part is: the markings can wear off, and a switch that is underrated for the job may work fine for minutes or years--then fail catastrophically! Size is no indication. By proper choice of material, a microswitch may easily handle 250V, while some much bulkier "project" or "panel" switches can't go above 50-100V. You'll run across this more if you use surplus or salvaged switches (as I often do: new switches can be expensive)

BTW, you may think a 250v switch would be adequate for UK 230VAC and positively virtuous for US 120VAC, but  for safety it should handle *at least* 400V (US) or 750V (UK) to be hooked up to line (mains) voltage. Sadly, switch makers often mark household switches with the *intended* voltage, not the maximum voltage. A "250VAC" household switch may be fine in US/UK, but a (fancier!) 250V = 250VDC MAX instrument switch wouldn't be (look up RMS in TC)

If a switch says "for lighting use only" or "for resistive loads only", believe it. The main thing to avoid is inductive loads -- motors, transformers, etc.-- which can generate a "kickback" voltage surge that can spark and destroy lightly built switches. Power supplies, UPSs and (esp. older) electronics can have transformers in them, and many fluorescent fixtures have inductive ballasts (this is changing). Electric heaters, which would seem like classic resistive loads, may be inductive if they use coiled elements, or have a (motorized) fan inside

As a general rule, think twice about any switch you use to provide power to inductive loads, power supplies or fluorescent lights. These loads are tricky for many controlling devices. They aren't huge problems, but you do have to be aware of the issues, and either use the right part or make sure there's suitable protection.

A switch must reliably "break" as well as "make" a connection. Sometimes (esp. with multiposition switches) it can be critical that one throw or another is *always connected* (e.g. when switching a transmitter or generator from its working output to a dummy load, the energy must *always* go somewhere predictable and safe). This is "make before break": make the new connection before breaking the old one. More often, you need "break before make" to assure that two unrelated circuits don't connect to each other unexpectedly via the switch outputs (this is the commonest behavior, but if it's important to your particular job, make sure!)

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