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Get a million resistance values with this . . . 6-Decade Resistance Substitution Box By JIM ROWE One of the most common tasks when trying out a new circuit is finetuning the resistance values. This task is made a lot faster, easier and more precise by this 6-decade resistance substitution box. It’s easy to build and gives you the ability to select from thousands of different resistor values between 10Ω and 10MΩ, just by twiddling the switches. When you have found the optimum, just read off the value on the switches. Y ES, WE KNOW about those little “resistor substitution wheel” gadgets, which you can pick up for around $25. Generally they offer a selection of 36 different resistor values, covering a very wide range, usually between 5Ω and 1MΩ. They’re OK but you will usually find that the value you need is not present in that limited range of only 36 values. Then you dive into your resistor 38  Silicon Chip stock and hope that you can find a value that will work. We’ve all been there and know how frustrating it is to find that Murphy’s Law is applicable – there are none left in the drawer concerned. In any case, you tend to end up with a motley collection of resistors on the bench, all of which have to be put back in their drawers afterwards. That’s so boring. Resistor substitution wheels have another drawback which is that their internal resistors are usually only 5% tolerance. So even if one of the 36 nominal values turns out to be suitable for the circuit you’re working on, you still need to check the actual value with your DMM before making your final selection of the value to be used. So what we really need is more like an old-fashioned “decade resistance box”, with a much larger selection siliconchip.com.au T1 S1 S2 0 1M 1 1M 100k 1M 100k 1M 4 100k 1M 100k 1M 100k 6 1M 7 100k 7 1M 8 100k 8 1M SC 2012 100k 9 7 100 8 9 SIX-DECADE RESISTANCE BOX 10 100 10 8 100 1k 10k 9 6 7 8 10 100 1k 10k 8 5 6 7 10 100 1k 10k 7 4 5 6 10 100 1k 10k 6 3 4 5 10 100 1k 10k 5 10 2 3 4 10 100 1k 10k 4 5 6 3 T2 1 2 1k 10k 3 4 5 2 0 100 1 1k 10k S6 0 1k 1 2 3 S5 0 10k 1 2 3 S4 0 100k 1 2 9 S3 0 9 10 9 ALL RESISTORS 1% METAL FILM TYPE (OR 0.1% IF DESIRED) Fig.1: the circuit consists of six 10-position rotary switches (S1-S6) in series with terminals T1 & T2. The associated resistor strings allow each switch to select a unique resistance value ranging from 0-9MΩ (S1) down to 0-90Ω (S6) of closer-tolerance resistance values. But those old decade boxes were big, clunky and expensive. Even the latest models are quite expensive. So why not build your own? We have produced a compact 6-decade resistance box using readily available rotary switches and 1% metal film resistors, all mounted on a PCB to make assembly a cinch. A million resistance values This unit allows you to dial up a million resistance values between 10Ω and 10MΩ, selectable in 10Ω increments. It uses only 54 resistors, so if you use standard 1% metal film resistors they’ll cost you less than $3.50. Add in the cost of a UB1 jiffy box, six standard rotary switches and knobs, a pair of binding post terminals and a PCB and it is still not a large amount – a small fraction of the cost of a comsiliconchip.com.au mercial decade box, in fact. For even higher accuracy, you can use 0.1% metal film resistors instead of the 1% types. These will bump up the total cost to over $100 but it will still be much less than the price of a comparable commercial unit. How it works Fig.1 shows the circuit. Six 10position switches S1-S6 are wired in series, between the two binding post terminals T1 and T2. The resistors are connected in daisy-chain fashion around the six switches. Each click of switch S1 increases the total resistance by 1MΩ, while each click of switch S6 increments it by 10Ω. Since all six switches are connected in series, you can dial up any resistance between 0Ω (all switches set to “0’) and 9.99999MΩ (all switches set to “9’), in increments of 10Ω. Mind you, while we said that you can select a million different resistance values, in practice you would not use all six switches to select each resistance; it’s pointless. It all comes down to the tolerance of the resistors you are using. Even if all 54 resistors are 0.1% tolerance, you will quickly come to realise that if you use three consecutive decade switches to select a value, the 3-digit resolution of the selected value is already equal to the tolerance of ±0.1%. You also need to consider that the minimum resistance of the box with all switches set to “0” is not exactly 0Ω. That’s because the contact resistance of the switches and the resistance of the PCB tracks does introduce a small amount of residual resistance – typically around 0.25Ω, or 250 milliohms. In practice, this doesn’t matter much and merely increases the error of the April 2012  39 1M 5 1k 4 1k 100 1k 1k 6 x100 5 4 T1 100 T2 S6 8 0 7 1 2 x10 6 2 3 100 10k 10k 10k 10k 0 9 5 10 3 4 10 S5 1 100 1k 3 10 7 2 x1k 6 1k 1k 1 7 10k X O B E C NATSISER EDA CED- 6 9 8 0 4 10 S4 10k 10 10 100 9 8 1k 12140140 100 1k 100k 2 3 10 10 2102 © 4 x10k 5 10k 100k 1 6 2 3 0 7 10 1M 5 S3 10k x100k 100k 100k 1 6 9 8 100k 100k 100k 1M 2 3 4 1M S2 7 100k 1M 5 10k 0 100 x1M 6 1M 1 1M 1M 1M 7 9 8 0 100 100k S1 100 9 8 Fig.2: follow this parts layout diagram to build the 6-Decade Resistance Box. Note that the switches must be installed with their anti-rotation spigots orientated as shown. The tops of these spigots must also be removed using side cutters. two lowest settings of S1 (10Ω and 20Ω) beyond the basic ±1% of all other ranges: about +3% for the 10Ω setting and +1.5% for the 20Ω setting. We will discuss some of these points later. For now though, this 6-decade resistance substitution box is a very useful electronics accessory and it is dead-easy to build. Construction All of the switches and resistors are on the PCB which mounts inside a standard UB1 jiffy box. The complete PCB assembly is attached to the box lid, being held there by the mounting nuts of the six switches. The two binding posts are the only components not on the PCB; they are mounted on the lid itself, with their rear connection spigots connecting to the two large pads on the PCB when the latter is attached to the lid. The PCB is single-sided but we strongly recommend that you use a fibreglass PCB which has a solder mask. This will reduce the possibility of leakage paths developing in the future which could reduce the accuracy on the top resistance range. The component overlay is shown in Fig.2. Fit the resistors first. There are only six different values: 10Ω, 100Ω, 1kΩ, 10kΩ, 100kΩ and 1MΩ, with nine of each, making 54 in total. Each value is clustered around its respective switch. Don’t mix up the values. Before fitting the rotary switches, cut the spindle of each switch to about 10mm long or just enough to suit the control knobs you are using. Make sure you remove any burrs from the top end of the spindles with a file, so that their knobs will slip on easily later. All six switches are mounted on the board with the orientation shown in Fig.2. As you can see, the moulded locating spigot on the front of each Table 1: Element14 0.1% Resistors 1083036 1751317 1751444 1751550 1751658 1751718 10Ω metal film resistor 100Ω metal film resistor 1kΩ metal film resistor 10kΩ metal film resistor 100kΩ metal film resistor 1MΩ metal film resistor 40  Silicon Chip 0.1% 0.25W 0.1% 0.25W 0.1% 0.25W 0.1% 0.25W 0.1% 0.25W 0.1% 0.25W $18.60 (2 packs) $15.00 (2 packs) $15.00 (2 packs) $15.00 (2 packs) $14.20 (2 packs) $16.50 (2 packs) switch body is at “1:30”, while each switch’s rotor connection pin (not visible in Fig.2 or Fig.3) is in the “3:00” position as viewed from the top. Once all six switches have been fitted to the board and soldered in, it’s a good idea to make sure that they are all set for a span of 10 positions. To do this, turn the switch spindle fully anticlockwise and then remove its mounting nut, star lockwasher and “stop washer’. Then replace the stop washer with its stop pin passing down through the hole between the numbers “10” and “11” moulded into the switch body. Replace the lockwasher and mounting nut. Then try turning the switch spindle clockwise by hand and you should find that it can be moved through a total of 10 positions (0-9 inclusive). You will also need to use a pair of side cutters to nip the plastic spigot off all the switches. If this is not done, the spigots stop the switches from mounting flush underneath the lid. Do this for all six switches. It is also a good idea to use an old toothbrush and some methylated spirits to scrub off all solder flux residue from the underside of the PCB. This will remove any leakage paths which will otherwise reduce the accuracy of the values selected when you are using siliconchip.com.au Parts List 1 UB1 jiffy box, 158 x 95 x 53mm 1 PCB, code 04104121, 146 x 87mm 1 dress front panel (both PCB and panel available from SILICON CHIP for $20 each + $10 P&P) 6 single pole rotary switches (S1-S6) 6 19mm diameter control knobs, grub-screw fixing 2 black binding posts 4 adhesive fixing rubber feet Resistors (0.25W, 1% or 0.1% metal film) 9 1MΩ 9 1kΩ 9 100kΩ 9 100Ω 9 10kΩ 9 10Ω This is the completed unit before it is mounted on the lid of the case. CONTROL KNOB BINDING POST SWITCH MOUNTING NUT BOX LID STAR WASHER SWITCH SHORTEN PLASTIC SPIGOT BINDING POST MOUNTING NUT Fig.3: the PCB is secured to the back of the lid by resting it on the tops of the switches and doing up the switch nuts. The binding post spigots are then soldered to their pads. PCB the Megohm range switch. The PCB assembly can now be plac­ ed aside while you prepare the box. Preparing the box lid There are eight holes to be drilled and reamed in the box lid. There are six 10mm-diameter holes for the threaded ferrules of the switches, plus two 9mm holes for the binding posts. You can use the front-panel artwork as a drilling template for the lid. This can be obtained in PDF format from the downloads section of the SILICON CHIP website, photocopied and stuck to the lid. Once the eight holes have been drilled and reamed to size, you can either make a dress front panel by laminating another copy of the artwork or you can purchase a PCB front panel from SILICON CHIP. After this you can fit the two bindsiliconchip.com.au ing posts to the panel, using the nuts and washers supplied, as shown in the diagram of Fig.3. The lid can then be lowered down until it’s resting on the lockwashers for the switches. Fit the mounting nuts to each switch ferrule and this will hold everything together. The rear spigots of the binding posts can then be soldered to the matching pads of the PCB. That done, place the lid/PCB assembly into the box itself and fit the four small self-tapping screws supplied, then push in the small rubber bungs to cover each screw head, Finally, fit the control knobs to each switch spindle. curate. These higher-precision resistors are available and are physically very similar to the standard 1% type – so there is no problem making this change. But be warned that there is a significant extra cost involved – the 0.1% resistors will cost you around $1.60 each, compared with the six cents or so for 1% resistors. A set of 60 of these resistors will cost around $95, bringing the total cost of your decade box to around $140. However, we think that the extra cost is well worth it. It is very satisfying to dial up a resistance value with two or three switches and then confirm that it’s smack on the value (or very close to it) with your DMM. It means you can dial in preferred value resistors to a prototype circuit and know that you will get very similar results when you install the same physical resistor. Sourcing 0.1% resistors The 0.1% resistors available from Element 14 (formerly Farnell Components) come in packs of five; you need to buy two packs of each value. Table 1 lists the values. The first number is the Element14 stock number, followed by the description and the cost. Higher precision? Power rating Earlier in this article, we mentioned that 0.1% tolerance metal-film resistors can be substituted for the standard 1% tolerance types, if you want your decade box to be significantly more ac- Finally, note that the power dissipation must not exceed more than 0.25W for the resistance value selected. This can be calculated using the formula SC P = V2/R or P = I2R. April 2012  41