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ELECTRONl~S FQR RlrnRYO'.NE Potentiometers are widely used in electronic equipment. They are the variable controls you use on amplifiers, tape decks, TV sets and other electronic gear. In effect, they are resistors with a moveable contact. PT.4: POTENTIOMETERS & TRIMPOTS By LEO SIMPSON Broadly speaking, potentiometers come in two types. The most common is the rotary control which is used in the volume, bass, treble and balance controls in your amplifier or car radio and in all other sound equipment. Rotary pots ("pot" is short for potentiometer) are also used for the rotary knobs on power supplies. In fact, these days almost all the rotary controls on electronic equipment are pots. The few exceptions are variable capacitors, as used in some oscillators and the tuning controls on radios. Other places you will find rotary potentiometers are in joysticks as used in electronic games and radio remote controls. Each joystick uses two potentiometers connected so that they rotate through a maximum of 90°. The other type of pot is the linear slider which is commonly used in audio mixers and graphic equalisers. A potentiometer always has at least three connections to its resistance element. There is a connection for each end of the resistance element and one for the moving contact. Fig.1 shows the most commonly used circuit symbol for a potentiometer. As you can see, it is a variation on the zigzag symbol 6 SILICON CHIP for a resistor but with the addition of an arrow to show the moving contact. The moving contact, which is also referred to as the wiper, can move anywhere up and down the resistance element. Fig.2 shows shows another common symbol for a potentiometer and again, this is a variation on the rectangular box symbol for a resistor but with the moving contact shown as an arrow. Most of the rotary potentiometers you come across will be carbon types. Their circular resistance element is made by depositing a carbon mixture onto a thin insulating material which is generally paper based phenolic laminate (similar to printed circuit board material). Some elements may be moulded integrally with the base. Fig.1 shows the most commonly used circuit symbol for a potentiometer while Fig.2 shows a common alternative. The moving contact generally takes the form of a number of springy fingers plated with silver. One of the photos in this article shows the usual construction. Carbon potentiometers can dissipate only a small amount of power, usually less than half a watt. Where the circuit requirement calls for higher power dissipation, a wirewound potentiometer is the answer. As the name suggests, these have a circular wirewound resistance element. Depending on their size, wirewound potentiometers can dissipate a lot of power. One of the units pictured in this article can dissipate about 200 watts. Why are they called potentiometers, anyway? Why not some easier to understand name such as "variable resistor"? Well, pots are variable resistors but the name potentiometer relates to a timehonored voltage measuring technique involving variable resistors and a galvanometer in a bridge circuit. The particular virtue of the potentiometer method of measuring voltage is that no current is drawn from the circuit being measured. In fact, the name "potentiometer" is a coined word made up from "potential" (meaning voltage) This photo shows a collection of typical rotary potentiometers. They are all carbon-track single gang types with the exception of the unit at bottom right which is a dual-gang type. Rotary pots come in a range of values. and "meter" (meaning: to measure). Because variable resistors were (and sometimes still are) used in potentiometer circuits, they became known by the general name potentiometer and the name has stuck, long after the measuring technique has ceased to be relevant. Rheostats OK, so what's a rheostat? This is another olde-worlde term you may occasionally come across and it refers to a variable wirewound resistor which is usually rated for high power dissipation. So when you see the term rheostat, just think "variable high power resistor" and you'll be right. Ganged and concentric pots Pots come in a wide variety of types and it is helpful to be able to put a name to some of the common types when you are shopping for parts. One very common type is the dual ganged pot which is widely used in stereo amplifiers. In effect, a These are linear slider potentiometers. This type of pot is commonly used in audio mixers and graphic equalisers. ganged pot is two pot sections controlled by a single shaft. A stereo amplifier needs a gang- ed pot for its volume control so that when you alter the volume setting in one channel the other channel is SEPTEMBER 1989 7 ~~ L ___ ___ _j FIG.3 Fig.3: the circuit symbol for a dual-ganged pot. The dotted line indicates that the two wipers mo've together. n,r-: Dual-ganged pots are commonly used in stereo amplifiers for the volume, bass, treble and balance controls. In effect, a ganged pot consists of two pot sections controlled by a single shaft. "t: Figs.4 & 5: the common circuit symbols for trimpots. and so dispense with the need for dual ganged pots for each control function. Incidentally, the faders used for balance between right and left and front and back in car sound systems are also variants of potentiometers. Faders used in public address systems are also based on conventional wirewound potentiometers. Loudness taps These two pots have had their dust covers removed to reveal the 'works". The circular resistance element consists of a carbon mixture and is connected to the two outer terminals. The moving contact is connected to the centre terminal and consists of several springy metal fingers. altered by exactly the same amount. Similarly, ganged pots are used for the bass, treble and balance controls in stereo amplifiers. Fig.3 shows shows how a dual ganged pot is shown on a circuit diagram. A dotted line connects the two wipers to show that they move together. Another type of common dual pot is the dual concentric type. These are often used on car radios where front panel space is strictly limited. A dual concentric pot is a combination of two pot elements but each is operated by a separate shaft, both concentric. These pots use knobs that are split. For example, the front portion of the knob might operate the volume while the back 8 SILICON CHIP section operates the tone control. For stereo car radios the concentric pots can actually be dual ganged concentric pots because the volume and tone controls need to use ganged pots. In some stereo amplifiers you may also come across volume controls that are quadruple ganged types. These are used to obtain the lowest possible circuit noise. Another variation you may come across is the triple concentric potentiometer which is used in some of the latest stereo car radios. The relevant control may be used to alter the volume, tone and balance (between left and right or front and back]. In these car radios the three pot sections are used to vary a DC signal to both channels in the radio On some dual gang pots you will find that each pot section has four terminal lugs instead of three. Usually, the extra lug is at about the 40% rotation of the shaft. This provides for the loudness boost circuit in stereo amplifiers. Switchpots As the name suggests, switchpots are a combination of a potentiometer and a switch. They are frequently used as the combination volume control and on/off switch for TV sets and radios. The switch section is mounted on the back of the pot and is switched off when the pot shaft is rotated fully anticlockwise. You may also come across switchpots in which the switch is pushon, push-off or pull-on, push-off. These are preferred by some designers because the switch can be operated without rotating the pot and so there is less wear on the resistance element. On oscilloscopes too, you will find combination switches and pots, usually for the amplifier and sweep controls. These are not conventional switchpots but are combinations of multiwafer switches with a pot section mounted at the back and operated by concentric shafts. Resistance characteristics Talking about resistance characteristics of potentiometers is really opening a "can of worms" since many manufacturers do not use the same coding. Fortunately, you will normally only come across three different resistance characteristics. They are linear, log (short for logarithmic) and antilog. The linear pot is easy enough to understand. The resistance between one of the outside terminals and the moving contact varies in a linear fashion; ie, directly proportional to the shaft rotation. You can check this by setting a linear pot so that the shaft (or knob) is set to exactly half rotation. Then if you measure the resistance between the centre (moving contact) terminal and the outside terminals you should get a result which is close to half the total resistance. Linear pots are often (but not always) designated by an "A" stamped on the case. Some manufacturers use "B" instead of Both the resistance element and the moving contact can be clearly seen in this very large wirewound potentiometer. It can dissipate over 200 watts. "A". Logarithmic pots are normally used for volume controls on audio equipment and on TV sets. What happens with a log pot is that the resistance increases more rapidly for the first half of the shaft rotation and then more gradually for the remainder. This is done to make the volume control seem audibly progressive and to compensate for the more or less logarithmic response of our ears. Log pots are usually designated by a "C" stamped on the case. And as you might expect, an antilog pot is the reverse of a log pot. Combination log and antilog pots ganged to,1Iether are often used as the balance pot in stereo amplifiers. Multiturn pots The normal potentiometer has a rotation range of less than a full cir- Switchpots combine a potentiometer and a switch in the one body. The switch contacts are mounted on the hack of the pot and are opened when the pot shaft is rotated fully anticlockwise. cle; normally it is close to 300°. These are perfectly satisfactory for most applications but where precision setting of a pot is required, designers often specify multiturn potentiometers. Typically, these have 10 turns of rotation from one extreme to the other and allow for very easy adjustment. Trimmers and trimpots Trimmers and trimpots are the same thing. They are miniature versions of potentiometers which are usually adjusted by means of a screwdriver. They are normally set once and then not touched again for the life of the equipment they are in. They are used in a host of applications where once-only or infrequent adjustments are required to set levels, calibrate signals, set quiescent currents or voltages and so on. Trimpots are depicted on circuits as shown in Fig.4. This is a variation on the pot symbol shown in Fig.1 except that the wiper is shown as a slider instead of an arrow. SEPTEMBER 1989 9 As the name suggests, wirewound potentiometers have a circular wirewound resistance element hut the principle is the same as for carbon pots. The advantage of wirewound pots is that they can dissipate more power than carbon types. Another variation is shown in Fig.5 which is based on the rectangular symbol for a resistor. As a further indication that a pot is a trimpot, they will often be labelled on the circuit diagram as RVl, RV2 and so on or perhaps VRl, VR2 etc. Wide variety Trimmers are available in a huge variety of shapes and sizes and resistance elements. As well as the common carbon track and wire wound types there are moulded track and conductive plastic varieties and one you will often come across: cermet. "Cermet" is a contraction of ceramic metallisation and refers to a resistance track which is made with fine metal particles in a glass base fused to a ceramic former. The resulting element can dissipate relatively high power within a small area. The disadvantage is that the wiper wears rapidly so cermet trimpots can not be adjusted very many times before they start to give trouble. Most carbon trimmers look like skeletal versions of pots (ie, no outer case) but there are also sealed or totally enclosed versions 10 SILICON CHIP Trimpots come in a wide variety of shapes, sizes & resistance elements. They are used where once-only or infrequent adjustments are required to set voltage or current levels or to calibrate equipment. which are generally a little more expensive but more reliable in the long run. Trimpots are also available in multiturn versions, usually around 10 or 15 turns , and these have the advantage of easy and very precise adjustment. ~