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EI.ECTROMCS FOR EVERYONE On the face of it, switches are easy to understand devices which are either on or off. But there are many different sizes and types. Some are designed for high voltages and high currents but most carry only flea power currents. PT.5: WHAT YOU NEED TO KNOW ABOUT SWITCHES By L-XO-SIMPSON Most electronic devices have an on/off switch and even those that don't, such as some calculators, watches and clocks, still have switches to control their various functions. Switches can range from single contact single position devices to multiwafer multi-position devices that cost big dollars. If you wanted to count up all the different types, sizes, styles, ratings and so on, you would run into thousands. Single pole, single throw The simplest switch of all is the single pole single throw type. It has a single moving contact (the pole) which is either on, so that current flows, or off, so .that current can't flow. This simple on/off action is referred to as 'single throw'. Fig.l(a) shows how a single pole, single throw switch is usually depicted while Fig.l(b) shows a variation whereby the on and off conditions are labelled. For a single pole, single throw switch, the pole may be shown as a short straight line or as an arrow, as in Fig.l(b). There are only two terminals on this type of switch. "Single pole single throw" is usually abbreviated to SPST. SPST switches are probably the most common type of all. They are used in the hundreds of millions (maybe even billions) as on/off switches in all sorts of electronic gear, mains appliances, light switches and power points. Many power switches in appliances have two moving contacts so that both the active and neutral lines can be switched. These switches are referred to as 'double pole single throw'. These are shown on circuits as in Fig.2. Each switch section is distinguished by a label such as Sla and Slb, or perhaps SWla, SWtb. Single pole, double throw Off (b) ----<I" o!!L- ~0-- Ag. 1 SPST SWITCH Fig. 2 OPST SWITCH ~ S10-Flg. 3 SPDT SWITCH ~ s1ao-~ S1b0--Fig. 4 DPDT SWITCH These diagrams show some simple switch configurations, ranging from SPST to DPDT. In the double pole types, the two switching contacts move together. 76 SILICON CHIP A step up in complexity from the single throw switch is the 'single pole double throw' switch, normally referred to in textbooks as an SPDT switch. In this type, the moving pole of the switch can be moved to one contact or the other. In this way, a SPOT switch can be used to connect power to one circuit or another, as shown in Fig.3. Note that an SPDT switch has three terminals, usually with the centre terminal being for the moving contact. SPDT switches are sometimes referred to as 'two way' switches. If you have two-way light switches in your home, you will find that B 0 F G E H There are many different types of toggle switches. Types A, B, C, D and H are DPDT while types E and G are SPDT. SPDT and DPDT switches are also known as changeover switches. Type Fis the simplest, SPST (single pole single throw). each switch is, in fact, an SPDT type. Next step up from the SPDT type is the 'double pole double throw' or DPDT switch. This has two poles and two positions. It is shown in Fig.4. Again, these usually have two sets of three terminals, with t,h e centre ones being for the moving contacts. SPDT and DPDT switches are often referred to as having "changeover" contacts because they change from one set of contacts to another. You'll also come across relays with changeover contacts and these provide essentially the same switch function. Double throw switches also come with more poles than two. You can have 3 pole, double throw (3PDT), 4 pole double throw (4PDT) and so on. Toggles, sliders and rockers Having discussed some basic switch contact arrangements, let's Slider switches are easily recognised. The two on the right of the photo are DPDT types. At the left are two DIP switches. Both contain 10 SPST switches with the one at top being miniature rockers and the one below using sliders. NOVEMBER 1989 77 B Rocker switches are often illuminated. Type A uses a miniature 12V incandescent lamp while type B uses a neon and is therefore suitable only for high voltage switching. Type C is a paddle operated toggle. Toggle switches are also said to have an "over centre" action which refers to the fact that you have to move the lever past the centre point of its travel before the switch contact flicks to the wanted position. In 240VAC mains appliances and also in cars, you will generally find that toggle switches are springloaded more strongly for the 'off' position than for 'on'. You'll also find two conventions for 'on' and 'off'. Equipment designed for the American market generally has toggle switches which are up for 'on'. Most of the rest of the world uses the same convention as in Australia: down for 'on'. You could argue all day as to which is the right or most sensible convention but it's largely a matter of what you've become used to. Rocker switches On a multiwafer switch the wafers can be close together or spaced apart to reduce the capacitive coupling between them. Type A uses a mains-rated switch section at its rear while type B has a shield plate between its two wafers, to further reduce the capacitance between them. Rocker switches are very common and simply use a rocker in place of the lever of the toggle switch. They have the same overcentre snap action so you can operate them by feel. Are they any better than toggle switches? Probably not. The choice of a rocker switch over a toggle or any other switch is more usually a matter of styling than engineering considerations but rockers do have two drawbacks. First, it is not so easy to see whether a rocker switch is up or down (or on or off) - the so-called "visual status". Second, rocker switches usually require a rectangular cut-out to mount them in a panel and often two screws as well. By contrast, a toggle switch only requires a circular mounting hole. Slide switches look at switch types such as toggles and sliders. The toggle is the most common of all and is used by the squillion in all sorts of applications in every field of electrical, electronic and automotive engineering. Superficially, you might think that the word "toggle" merely refers to the stubby lever by which you operate the switch but it goes further than that. A toggle actually consists of two levers connected by a knuckle joint and spring loaded to take up one position or the other. 78 SILICON CHIP When force is applied to the top lever, the sping is extended so that eventually, the bottom lever suddenly flicks over to the other position in a very positive fashion. In a toggle switch, the bottom lever is actually the moving contact. Toggles were among the very first switches to be developed because they provided a means of very positively breaking a circuit. Toggles provide good "tactile feedback" which means that you can operate them by feel. As the name suggests, slide switches operate by sliding from one position to the other. They don't have any spring-loading so they have to be pushed all the way in one direction or the other to make sure they operate properly. Because their switching action is not positive (compared to toggle switches), slide switches are not suitable for many applications. They are not used on mains appliances or in cars. They are generally confined to applications Wafer switches come in all shapes and sizes. Types A, C, D, E and F are all single wafer designs while types B and G have two wafers each. Type F is a lever operated three pole three position switch. All except type D are of open construction. Type D is available with an adjustable stop so that the number of operating positions can be varied. where only very low power has to be switched and where a compact switch is required, such as on calculators. Fig.5a shows the symbol for the simplest slide switch, an SPDT type, but most slide switches you will come across will be of the DPDT type with 6 terminals as shown in Fig.5b. Just to confuse the issue, slide switches are available with three or four positions. These generally have a 'detent' action so that they have slight stops to identify the intermediate positions of the slider. - -- (a) SPDT SLIDE Fig. 5 (b) DPDT SLIDE Slide switches operate by sliding the moving contact(s) from one position to the other. Fig.5a is the symbol for a SPDT type while Fig.5b shows a DPDT type. Fig.6 shows single pole 3-position and 4-position slide switches. While the terminals of DPDT and SPDT slide switches are easy to identify (the moving contacts are the centre terminals), those on 3 position and 4 position switches are in a class by themselves. The only practical way to identify their contacts is to use a multimeter to test for continuity. DIP switches DIP switches are really just a bunch of very small slide or rocker switches in the same size package as a dual in line package integrated circuit. Each switch is a SPST type and they commonly come in packages of 4 (8 pin) or 8 (16 pin), although they are also made in 2-way, 3-way, 5-way, 6-way and up to 12-way. Other variations are available with vertical switch operation with up for 'on' or down for 'on'. You'll also see DIP switches referred to as DIL switches. DIP switches are widely used in fig. 6 These two circuit symbols show a single pole 3-position slide switch (top) and a single pole 4-position slide switch (bottom). computers and printers to program the endless options that this equipment offers. Rotary switches For switches with more than 2 poles and more than 2 positions, the rotary switch is the answer. It can provide lots of poles and lots of settings. Naturally, as the number of poles and positions increase, the cost goes up. Most of the rotary switches you'll NOVEMBER 1989 79 _/F o-S1ao--- S1a 0-0>---- __/F. 0-- S1b 0---- S1b _/;;=- 0-0>---- (a) S1c Fig. 7 (b) 0-- 0---- If lots of switch positions are required, the rotary switch is the answer. Fig.7a shows a 2-pole 5-position switch while Fig. 7h shows a 3-pole 4-position type. come across at parts suppliers will be single wafer designs and they come in the following configurations: 1 pole 12 position, 2 pole 6 position, 3 pole 4 position and 4 pole 3 position. In other words, if you multiply the number of poles by the number of positions, you will always get the figure 12. The switches just described are said to have 30° indexing because there is 30° of rotation between each switch setting. Other indexing figures you'll come across from time to time are 45° , 60° and 90° . Fig. 7 shows a 2 pole 5 position switch and a 3 pole 4 position switch. Note that each pole of each switch is in the same relative position. Multiwafer designs Where larger numbers of poles and positions are required, rotary switches have more than one wafer. For example, with two wafers, you can have 2 poles 12 positions or twice any of the combinations listed above; with three wafers, you can have 3 poles 12 positions and so on. Our photos show a number of multiwafer switches. You can also have differing numbers of poles on each wafer, as you might, for example, on a 5-pole 3 position switch. This could have 3 poles on the first wafer and 2 poles on the second. The wafers usually have spacers When you want a large number of switch poles, nothing can heat a large multiwafer switch. This printer switch has 25 poles and 4 positions. 80 SILICON CHIP between them which makes them easier to wire up but where space is at a premium the wafers may be butted up together. In printer switchers, you can find really massive multiwafer switches, as shown in another of the photos accompanying this article. Multiwafer switches can be fitted with shield plates in between the wafers. This reduces the capacitance between the switch sections and is useful where the coupling between switch wafers must be kept to a minimum. Some single wafer switches are of closed construction. This has the · advantage of keeping dirt out of the switch contacts and probably also reduces corrosion. Another feature you will find on many wafer switches is an adjustable stop. This enables you to change the number of available switch positions. For example, you may have a 2 pole 6 position switch and you only want to use 4 positions. To achieve that, all you do is lift out the washer/stop on the switch bush (the threaded mounting portion of the switch) and set it back for the required number of stops. There are a number of other variations of rotary switches you will come across. One is the lever operated wafer switch shown in a photo accompanying this article. Another is the rotary PCB mounted slide switch. This is a multipole, This photo clearly shows the action of a "make before break" switch. Notice that each of the three poles (or wipers) is at a point between two adjacent switch contacts. Since the adjacent contacts are shorted together at this point, this type of switch is referred to as having "shorting" contacts. Pushbutton switches come in a wide variety of types, some of which are shown here. Most of these are "alternate action" meaning that you push them once to turn them on and then push again to turn them off. The remainder are momentary contact types, meaning that they "make" the circuit only while they are pushed. Type C is an illuminated pushbutton, using a 6V incandescent lamp. Types A, H and N are DPDT alternate action switches. The contacts on one side allow installation on a printed circuit. The contacts on the other side allow conventional wiring. Type M is a pin switch, used as a boot or bonnet switch in car burglar alarms. It grounds or "makes" the circuit when the plunger is released. Type L is a normally on switch, similar in principle to those for refrigerator and car doors. multi-position switch in which the linear actuator is operated by a flat Bowden cable from a rotary knob. Made by the Alps Corporation in Japan, these are widely used in hifi amplifiers and receivers. Incidentally, you will sometimes see the moving contact of a switch referred to as the wiper. This applies particularly in rotary multiposition switches. In fact, most rotary switches are said to have "wiping contacts" a good feature because it keeps the switch contacts clean. Make before break contacts Two terms that you'll occasional- ly come across when dealing with switches is "make before break" and "break before make". This describes the action of contacts in a changeover switch or in a multiposition switch. Let's take "make before break" contacts for example. In a DPDT switch with make-before-break contacts, the moving contacts stay in contact with the first set of contacts while they are being moved over to the second set of contacts. The switch "makes" the second set of contacts before "breaking" the first set of contacts. In some circuits it is most important that 'make before break' swit- ches are used. For example, in an amplifier where a multi-position switch is used to vary the feedback settings, it is most important that the feedback circuit is connected at all times. If the feedback circuit was broken, at the moment of switching from one setting to another, the amplifier would momentarily lose all its feedback and would generate a tremendous voltage spike which could damage it or its load. Another place where 'make before break' switches are important is in the range switch of a multimeter. It is important that the measuring circuit not be broken when switching ranges on the multimeter, particularly when measuring current. 'Make before break' switches are also referred to as having 'shorting' contacts because the two adjacent contacts are shorted together during the act of switching. Break before make switches Inevitably, there have to be NOVEMBER 1989 81 - (a)-----e e- (b)~ Ag. 8 Pushbutton switches can be either normally open (Fig.Ba) or normally closed (Fig.Sb). Pushbutton switches This is an 8-way latching switchbanlc. Pressing one switch cancels and previously pressed switch. Each switch is a DPDT changeover. Numeric keyboards like these use matrix wiring, with four rows and three columns. They need to be used with an encoder IC which continually scans the rows and columns to detect when a particular button has been pressed. While separate buttons are used in these two keyboards, they both use membrane switches. 'break before make' switches, don't there? In fact, all toggle switches are 'break before make' types. Many circuits must use 'break before make' switches otherwise overloads could occur during switching. How do you tell the difference between 'make before break' and 'break before make' switches? On wafer switches with open construction, it is quite easy. Just have a look at the switch wiper (or wipers) and watch their action as the switch is 82 SILICON CHIP slowly switched from one position to the next. Are two adjacent contacts shorted together momentarily at some point in the switching? If so, the switch is a 'make before break' switch. If the switch is of closed construction, you can use your multimeter to test for the shorting of adjacent contacts during switching. Preferably you need to use an analog meter [ie, not a digital meter) for this test because the shorting actions may be very brief. Pushbutton switches generally come in one of two forms: momentary contact and alternate action. "Momentary contact" is easy enough to understand - the switch makes contact only while you push the button down. They are the type used in doorbells and any application where the circuit should be on for a short time. Momentary contact switches are also referred to as having "normally open" (NO) contacts since they are "closed" only when the button is pushed. "Alternate action" is another term for push on, push off. These are often used as the on/off switch on TV sets and hifi equipment. These normally have DPDT contacts. Another type of pushbutton switch is one with normally closed contacts. When you push the button, the contacts are opened. These are widely used as door switches in refrigerators and in cars. These normally have an SPST switch. A variation on these is the pin switch, as used in car burglar alarms for boot and bonnet protection. These connect a single wire to the car chassis. When the switch pin is depressed, the circuit is opened. Switch banks Pushbuttons are often mounted together in banks and they may be latching and/or interlocked. When the buttons are interlocked, you push one button to unlatch a previously pushed button. These are often used as selector and mode switches on high fidelity audio equipment and as mode switches on some multimeters [eg, the Fluke 8060A). • ~ ~ These are microswitches as used in a lot of machinery to "limit" movement. They are also commonly referred to as "limit" switches. You'll find them in the door interlocks on microwave ovens, in photocopiers, garage door openers, juke boxes, ten-pin bowling machines and virtually all electrically powered machinery. Keyboards All keyboards, whether used on calculators, telephones, computers, microwave ovens or whatever, are simply a bank of SPST momentary contact switches arranged in a matrix. Typically, for a numeric keyboard used on a telephone, there are four rows and three columns, giving 12 possible selections. COL 1 COL 3 COL 2 Fig. 9 All keyboards use a bank of SPST momentary contact switches arranged in a matrix. Digital circuitry is used to determine which switch is pressed. When one button is pressed, it connects a row line to a column line. The rows and columns are continually scanned by associated digital circuitry which 'decodes' the row and column selected to determine which button has been pressed. The digital circuitry then generates a code in serial form which can be sent via a pair of wires to the device being controlled. Fig.9 shows a switch matrix for a 12 button numeric keyboard, as used on telephones. By using a matrix arrangement and accompanying decoding circuitry, the number of switch wires is greatly reduced. If a matrix wasn't used, the minimum number of connecting wires for a 12 button numeric switch would be 13 (one wire for each switch plus a common wire). Instead, by using a matrix, the number of wires is only 7. For computer keybards, the advantages of matrix switching are even greater. Instead of having 102 wires lfor a 101 button keyboard), the number is 32 (16 rows, 16 columns) or less. " Ahah" , you might say. " The cable from my computer keyboard uses a 5-pin DIN socket, so it must only have 5 wires. How do you get 32 wires? " The answer is that there are 32 wires, give or take a few, from the keyboard matrix to the microprocessor chip inside the keyboard. Not many people are aware that a separate microprocessor is inside their computer's keyboard but it is. This microprocessor continually scans the keyboard matrix to detect when keys have been pressed and also generates the corresponding ASCII code. This code is fed serially down the keyboard cable to the computer so only 5 wires are needed. This is just one of many circuit techniques used these days to reduce the number of connecting wires. Connecting wires cost money in terms of material and labour cost and they a lso contribute to unreliability in electronic equipment. Hence, designers are continually on the lookout for ways to reduce connecting wires. Switch matrices, as used in digital keyNOVEMBER 1989 83 They may be lever or roller actuated and normally are of the SPDT format. They are often used as limit switches for electrically controlled doors, in juke boxes and as the door interlock switches in microwave ovens. Since they are so often used for limiting the travel of machinery, they are also referred to as "limit" switches. Illuminated switches These switches are an interesting combination. They are PCB mounted multipole slide switches which are operated by a flat Bowden cable by the front panel rotary knobs. The advantage of these over conventional wafer switches is that they allow more flexibility in the design of the control panel and allow the switch elements to be placed where needed on the printed circuit board. boards, offer big advantages in this respect. Membrane switches Membrane switches are commonly used in numeric keyboards where water, dust and dirt is likely to be a problem. The switch contacts are usually in the form of conductive tracks deposited on plastic sheets which are pressed together to make contact. Since they are completely sealed, dust and dirt is not a problem. That is why they are so commonly used 84 SILICON CHIP on the control panel of microwave ovens. They are also used as the keys on credit card sized calculators. Membrane keyboards were also used on early low-cost personal computers such as the Sinclair ZX-80. They are not good for this application though, because they don't have any 'feel'. Micro switches Micro switches are used where a very small switch lever travel is required, hence the term "micro". Many rocker and pushbutton switches are illuminated. When they are turned on, they are illuminated, often by the supply voltage they are being used to switch. The main point to watch when using these switches is the type of illumination. For example, illuminated switches for 240V AC mains employ an internal neon lamp together with a current limiting resistor of lOOkO or thereabouts. Because the neon lamps require at least 70 volts across them to light, these switches are not suitable for low voltage operation; not because they won't switch but because they won't light. Many illuminated switches use a miniature 6.3 volt or 12 volt inGandescent lamp. These usually have separate pins for the lamp supply. Other illuminated switches use light emitting diodes. These have the advantage of very long life but they're not so good in brightly lit areas or in cars, especially when sunlight falls onto the instrument panel. Reed switches These really shouldn't be called switches. More correctly, they are reed relays. They normally have one pair of SPST contacts sealed into a glass tube. The relay is operated when a strong magnetic field is applied, either by a small permanent magnet or by an energising coil fitted over the glass tube. Mercury switches Mercury switches are also referred to as 'tilt' switches. They are basically two contacts sealed into a small glass bulb containing a small blob of mercury. When the switch is tipped one way, the mercury shorts the contacts and is made. When tipped way, the mercury moves the contacts and the broken. the circuit the other away from circuit is Thumbwheel switches These are in a class of their own. Essentially they are an edge mounted rotary switch with 10 positions. Often used on digital equipment, they can be designed to provide a decimal output, BCD output or hexadecimal output. The original thumbwheel switches were operated by the thumb, as the name suggests, but now there are variations which have little buttons to increase (increment) or decrease (decrement) the switch setting. These are sometimes called digital pushbutton switches but they are functionally the same as thumbwheel switches. The means of encoding to provide decimal, BCD or hexadecimal output is mechanical and is achieved by the arrangement of the switch contacts and the internal printed circuit pattern. A BCD thumbwheel This is a 40-position channel selector switch as used in a CB radio. Its operating principle is similar to that used in thumhwheel BCD and hexadecimal switches. By using an output code, it greatly cuts down on the number of switch wires used. While this particular sample has a lot of pins they are duplicates. Only 8 wires are needed to encode the 40 separate channels. switch usually has five terminals, three for the BCD output and two as common. ANTRIM TOROIDAL TRANSFORMERS Interestingly, the 40 position switches used in CB radios use the same operating principle as thumbwheels. They produce an encoded digital output which is decoded by an associated integrated circuit to determine the channel selected. This enables the switch construction to be simplified and the number of connecting wires greatly reduced. In keeping with the trend to making everything smaller, there are now miniature thumbwheel switches designed for mounting directly onto a printed board. As the name suggests they again perform the same function as a · thumbwheel switch except that they are far too small to be operated by anyone's thumb. Instead, they are set by a small screwdriver. Other types There some switches we have not mentioned but after reading this article you will be able to recognise their basic types such as knife switches (DPDT) and key-operated switches (nsually DPDT). ~ General Construction OUTER WINDING QUALITY TOROIDAL POWER TRANSFORMERS, MANUFACTURED IN U.K. NOW AVAILABLE EX-STOCK AT REALISTIC PRICES. TAX PAID PRICES 15VA 30VA 50VA BOVA 120VA 160VA 225VA 300VA 500VA 625VA 10+ 1- 9 32.80 31.70 36.00 35.00 38.50 37.20 41.75 40.35 44.95 43.50 55.70 52.20 62.00 58.20 72.80 68.25 100.00 93.75 112.00 105.00 Enquiries from resellers and OEMs welcome. Quantity prices and data sheets available on request. Distributed in Australia by Harbuch Electronics Pty Ltd, 90 George St., HORNSBY, NSW, 2077 Phone (02)476-5854 Fax (02)476-3231 NOVEMBER 1989 85