Fullscreen HTML5Med Res (??MB)HTML4

Resistor-Capacitor Substitution Box with parallel and series RC output As any engineer, technician or advanced hobbyist will tell you, a resistance substitution box can save a lot of tears and angst. Same comments apply to a capacitance substitution box. Here’s one that combines both resistance and capacitance in one box – and you can choose either resistance, capacitance or a combination of both – and that combination can be in series or parallel. I t often seems to be the case that you can never lay your hands on the particular resistor or capacitor you need. You may be developing a new circuit, repairing an old one, tuning or tweaking equipment, testing test gear . . . whatever you’re doing, circumstances will conspire to ensure that the one component you need is the one that you don’t have. That’s when a resistance substitution box or capacitance substitution box can get you out of trouble. Of course, it’s not a permanent ‘fix’ – it’s one that tells you what you need to buy at your next available opportunity. The beauty of using a true resistance or capacitance substitution box is that the good ones give you a far greater choice of R or C than even discrete components do. So if your circuit needs, say, a 3,480Ω resistor, you can provide it. You can also tell if a 3.3kΩ would do the job or if you need to go to a tighter tolerance. (Incidentally, you can get 3,480Ω in the E48 series or above). In our April 2012 issue, Jim Rowe described a very handy Resistance Substitution Box, capable of ‘dialling up’ any one of a million resistance 76  Silicon Chip values between 10Ω and 10MΩ. A couple of months later, in July 2012, Nicholas Vinen presented a Capacitance Substitution Box, which similarly allowed you to dial up virtually any capacitance between about 30pF and 6F. Altronics have taken this concept one step further again, with a combined Resistance AND Capacitance substution box. With a range of 1Ω to 999,999Ω and 100pF to 9.99999µF, it covers the vast majority of resistors and capacitors that you’d normally need in any service, development or troubleshooting work. Both the resistance and capacitance sections of the box can be used independently via their own pairs of terminals but can also be connected in series or parallel by means of a 3-position slide switch. The combined RC network is again brought out to another pair of terminals. The result is a versatile RC box that is more useful than two separate boxes. It’s also smaller than our previous substitution boxes by dint of the use of a pair of six-way, ten-position thumbwheel switches to select the R or C value required. It’s mounted in a sealed ABS enclosure with an overall size of 145 x 105 x 65 (d) mm, with the top-mounted binding posts adding another 16mm. Residual capacitance You may be wondering why the minimum capacitance setting in this new box is 100pF when it’s easy to get lower values, down to 1pF. The reason is simple: residual capacitance. When everything is installed on the PCB, even with all care taken to minimise stray capacitance on the PCB, connecting wires, switches and terminals, the residual capacitance is bound to be a lot more than 1pF. Hence, the residual capacitance in the box is about 20pF. You will need to mentally add this value to any low value of capacitance you select, up to about 500pF; above that, the difference is likely to be swamped by the 10% tolerance of the switched capacitors. Residual resistance Similarly, although the lowest selectable resistance value is 1Ω, the residual resistance in the switches, terminals, PCB tracks and interconnecting wiring amounts to about 1.3Ω. siliconchip.com.au Decade Article By ROSS TESTER If that sounds like a lot, consider that there are six thumbwheel switches, one slide switch and umpteen solder connections to the wiring in the Resistance Selection and you can see that just a few milliohms in each connection can easily add up to one ohm or more. So again, when you are selecting low resistance values, you will need to mentally add 1.3Ω to any value below about 100Ω. Above that value, the 1% tolerance of the switched resistors becomes a dominant factor in the actual resistance value. The circuit The full circuit of this Resistance & Capacitance Substitution Box is shown in Fig.1 overleaf. It basically consists of six switched banks of resistors and capacitors. The resistance and capacitance sides of the box are independent of each other until specifically connected together by the 3-position slide switch S1. First of all, we’ll look at the resistance side. The box works by switching resistors in series. Each switch position adds in another resistor. Because there are ten positions on each thumbwheel switch, they’re siliconchip.com.au called ‘decade’ switches – they switch in the sequence 1, 2, 3, 4, 5 etc. So on switch one, position one you’d have one ohm between the resistance terminals; position two switches in another one ohm resistor for two ohms, position three yet another for three ohms, and so on. This is repeated with the other five switches which, in turn, work with 10Ω, 100Ω, 1kΩ, 10kΩ and 100kΩ resistors. So with all switches in position ‘9’, you would have 9 x 100kΩ (900kΩ) plus 9 x 10kΩ (90kΩ) plus 9 x 1kΩ (9kΩ) plus 9 x 100Ω (900Ω) plus 9 x 10Ω (90Ω) and 9 x 1Ω (9Ω), all in series. Add those all up and you have This truth table shows how the binary-codeddecimal switch brings in the capacitors connected to the 1, 2, 4 & 8 terminals. Position 5, for example, connects the capacitors on terminals 1 and 4. DEC 0 1 2 3 4 5 6 7 8 9 8 0 0 0 0 0 0 0 0 1 1 4 0 0 0 0 1 1 1 1 0 0 2 0 0 1 1 0 0 1 1 0 0 1 0 1 0 1 0 1 0 1 0 1 999,999Ω (plus the 1.3Ω of residual resistance, of course). The resistance set by the thumbwheel switches is made available at the top set of red and black terminals. Capacitance Switching Capacitance selection is done a little differently, using binary-coded decimal (BCD) switches to achieve a similar result with fewer components, saving both space and money (larger capacitors tend to cost more!). And remember that we are switching capacitors in parallel (not series, as with resistors) to obtain larger and larger capacitances. Connected to the 1, 2, 4 & 8 terminals of the BCD switches are a combination of parallel-connected capacitors. Looking at the ‘100pF’ switch, a 100pF connects to the ‘1’ terminal, a pair of 100pF (ie, 200pF) connect to the ‘2’ terminal, a 180pF and 220pF (ie, 400pF) connect to the ‘4’ terminal while a 330pF and 470pF (ie, 800pF) connect to the ‘8’ terminal. Now the BCD coding comes into play. Have a look at the BCD ‘truth August 2014  77 9 x 100k BINDING POSTS 9 x 10k Sr6 x100k 1 9 8 7 6 5 4 3 2 1 0 2 3 R 1 2 3 R Sr5 x10k 9 8 7 6 5 4 3 2 1 0 DECADE THUMB SWITCH COM DECADE THUMB SWITCH COM S1 1 2 3 C 1 2 2x 10F 10F 8x 1F 2x 1F 4x 10F 8x 10F 1F 4x 1F 3 C 1: R & C IN PARALLEL 1 2 4 8 2: R & C IN SERIES BCD THUMB SWITCH RC 3: USE R OR C INDEPENDENTLY Sc6 x10F 1 2 4 8 Sc5 x1F COM BCD THUMB SWITCH COM RC SC 2014 RESISTOR – CAPACITOR SUBSTITUTION BOX table’ above. In this, ‘0’ means no connection while ‘1’ means a connection. This is all arranged by switch contacts within the BCD switch. Remember that capacitors in parallel add together, so with the ‘100pF’ switch in positions 1 or 2, you get 100pF and 200pF, respectively. In position 3, the switch connects terminals 1 and 2 together, to give you 300pF. In position 4, you get 400pF, position 5 connects terminals 4 and 1 together to get 500pF, position 6 connects terminals 4 and 2 together (600pF) while 7 connects 4, 2 and 1 together (700pF). There are two sets of six thumbwheel switches, one set of BCD switches for the capacitors, the other a decade set for the resistors. The six switches click together and are held in position by end plates, as shown here. 78  Silicon Chip NOTE: THIS SUBSTITUTION BOX MUST NOT BE USED ON ANY CIRCUIT WHERE THE VOLTAGE RATING OF CAPACITORS (50V), OR THE VOLTAGE AND/OR WATTAGE (0.6W) RATINGS OF RESISTORS MAY BE EXCEEDED Position 8 has only the 800pF connected to it while position 9 connects 8 and 1 to give 900pF. The second, or x1nF switch, has slightly different values but they equate to the same thing – 1nF on terminal 1, 2nF on terminal 2, 4nF on terminal 4 and 8nF on terminal 8. Similarly, the third, or x10nF switch, with the 1, 2, 4 & 8 units. The end result is the same – a maximum of 9.99999F at the Capacitance (centre) terminals when all capacitance switches are in the ‘9’ position (not forgetting the residual capacitance Here’s how to tell the switches apart: on the decade switch PCB, each switch position has a single track brought out to the rear connector. The BCD switch has a more intricate PCB track pattern. that we mentioned). Series/parallel RC The 3-position slide switch S1 connects the resistance and capacitance sections in series or parallel and the resultant RC network is connected to the third set of terminals, coloured green and yellow to distinguish them from the R and C terminals. So if you’re working on a project (or perhaps repairing a device) which uses an RC time constant (such as a timer, frequency generator, filter or even a radio circuit) you can easily The six BCD switches (for the capacitors) each have a 9-way header socket attached (only five pins are actually used). The capacitor PCBs plug into these sockets. siliconchip.com.au 9 x 1k 9 8 7 6 5 4 3 2 1 0 100nF Sr3 x100 Sr2 x10 DECADE THUMB SWITCH 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 COM 470nF 10nF 100nF 100nF Sc4 x100nF 9 x 1 Sr4 x1k 330nF 100nF 9 x 10 9 x 100 2x 150nF 10nF DECADE THUMB SWITCH COM 10nF 33nF 18nF 22nF 47nF 1nF 1nF 1nF DECADE THUMB SWITCH COM 3.3nF 2x 1.5nF 1nF 1 2 4 8 1 2 4 8 1 2 4 8 BCD THUMB SWITCH BCD THUMB SWITCH BCD THUMB SWITCH COM Sc3 x10nF Sc2 x1nF COM Sr1 x1 9 8 7 6 5 4 3 2 1 0 4.7nF 100pF 100pF DECADE THUMB SWITCH COM 100pF 330pF 180pF 220pF 470pF 1 2 4 8 Sc1 x100pF COM BCD THUMB SWITCH COM Fig.1: the circuit consists of the various thumbwheel switches bringing resistors and capacitors into circuit. At left, a 3-position slide switch allows series, parallel or independent connection. achieve this by setting the R and C to their appropriate values and moving the slider switch to either the series or parallel position, depending on the circuit requirements. Here’s where one of the really handy features of this RC box emerges: if the time constant or frequency is not exactly what you’re after, it’s simply a matter of turning the thumbwheel switches to achieve the desired result. No more unsoldering and resoldering components . . . just dial up and go! When you have got exactly what you need, simply read the values of R and C from the switches, select the same value components and finish/repair/ calibrate/etc your project! As you can see, an RC box is a pretty handy device to keep on your workbench or service toolbox – and this one is the handiest we’ve seen. First step is to assemble the two thumbwheel switch sets. They look quite similar, so make sure they’re not mixed up – the BCD switches have five terminals, while the decade switches have ten. There are seven small PCBs used in this project, six of which hold the various capacitors and attach to the back of the BCD switch bank. Four of these seven are identical and hold the through-hole capacitors. The other two boards, also identical, hold the 1F and 10F capacitors which are all surface-mount devices (SMDs). If you’re wondering why SMDs were used on these boards, it’s because through-hole versions simply wouldn’t fit – apart from the fact that they cost more! The final board is basically a termi- There are two SMD boards which hold the larger value capacitors. All of the capacitors are identical on their respective PCBs. Four PCBs hold the through-hole capacitors and are mounted sideby-side. Use this photo as a guide to capacitor placement. And here’s the view from the opposite side, showing the six header pin sets underneath which plug into the BCD thumbwheel switches. siliconchip.com.au Construction August 2014  79 Sr1-6: RESISTOR THUMBWHEEL SWITCHES (DECADE) Sc1-6: CAPACITOR THUMBWHEEL SWITCHES (BCD) CONNECTIONS SHOWN AS INDIVIDUAL WIRES FOR CLARITY – PROTOTYPE USED MOSTLY MINI FIG.-8 THERE ARE NO POLARISED COMPONENTS R+ R– C+ C– REAR OF SWITCH Sr1, 9 x 1 RESISTORS REAR OF SWITCH Sr2, 9 x 10 RESISTORS REAR OF SWITCH Sr3, 9 x 100 RESISTORS REAR OF SWITCH Sr4, 9 x 1k RESISTORS REAR OF SWITCH Sr5, 9 x 10k RESISTORS REAR OF SWITCH Sr6, 9 x 100k RESISTORS RC+ RC– ON Sc6 8 4 C0257.K B ON Sc5 x8 8 4 x4 <at> <at> <at> <at> <at> <at> 1 COM <at> A x1 BINARY ARRAY 2 2 <at> <at> <at> <at> COM # A BINARY ARRAY 2 1 COM x1 4 2 x2 A x1 100nF <at> ALL 1F SMD x2 # # <at> <at> <at> <at> 1 4 x4 # # # # # ALL F 1 SMD C0257.K B 8 # # # # x2 x8 # # # # x4 150nF 150nF 100nF 100nF 100nF 1 COM A x1 10nF ON Sc4 B B0257.K x8 ON Sc3 8 B B0257.K x2 22nF 18nF 10nF 10nF 1 COM A x1 1nF 470nF 330nF x8 47nF 33nF x4 8 2 4 x4 8 4 COM x1 A 100pF ON Sc2 B B0257.K x8 B0257.K x2 C+ C– (Cap B.Posts) 220pF 180pF 100pF 100pF 4.7nF 3.3nF 1.5nF 1.5nF 1nF 1nF 1nF 1 (Cap Box) K.7520A CB+ CB– S1 MOUNTS ON TOP (IE OPPOSITE) SIDE OF PCB 2 S1 (UNDER) x8 470pF 330pF RC– RC+ x4 R– x2 R+ RB+ RB– B VIEWING UNDERSIDE OF PCB (Res B.Posts) ON Sc1 ALL RESISTORS SOLDER DIRECTLY TO THEIR RESPECTIVE THUMBWHEEL DECADE SWITCH TERMINALS (Res Box) ALL CAPACITOR BOARDS MOUNT ON THEIR RESPECTIVE THUMBWHEEL BCD SWITCHES VIA HEADER PIN SETS ATTACHED TO COM, 1, 2, 4 & 8 Fig.2: the component layout shows how the resistors and capacitors are mounted – follow this, in conjunction with the photographs, when assembling your Resistance/Capacitance Substitution Box. All resistors mount on the back of the thumbwheel switches in series, with the switches themselves also connected in series thence back to the 3-way switch and output terminals. 80  Silicon Chip nation point for the slider switch pins (which mounts on it) plus the various flying leads to the other PCBs and to the six terminals. The resistors (and there are 54 of them!) all mount directly to the terminals of the decade switch bank (these terminals are actually small PCBs but we haven’t counted them as they are part of the switches). Nine 1Ω resistors mount on the first switch, nine 10Ω on the second and so on up to the nine 100kΩ on the sixth bank. This is quite fiddly work as the nine resistors all solder in a tight parallel arrangement, with one lead soldered to the switch contact and its other lead crossing over to the next switch contact. The wrinkle here is that the next resistor in the string also has one lead soldered to the same pad, so you have to ensure that you don’t unsolder one as you solder the other! Our close-up photo at left shows the resistor thumbwheel completely assembled so you can see what we mean. Once you get the hang of it, it’s not that difficult – just tedious. One down, 53 to go. Two down, 52 to go. . . These boards are all connected in series: each of the six ‘finish’ terminals connects, via a short length of hookup wire, to the ‘start’ terminal on the next switch. The ‘start’ terminal of switch one and the ‘finish’ terminal of switch six connect back to the main termination PCB mentioned earlier (and which we’ll come to shortly). Capacitors As we mentioned earlier, two different types of PCBs hold the capacitors. There are four which secure to the BCD switches 1-4 (100pF, 1nF, 10nF, 100nF) and hold traditional (ie, through hole) capacitors from 100pF to 470nF. The final two boards (1F and 10F) are for SMD (surface-mount device) 1F and 10F capacitors. The four boards mount horizontally while the other two (ie, the 10F and 1F boards) mount vertically. The main reason that different boards are used for the larger-value capacitors is that through-hole components over 1F (and especially the 10F) are too large to mount on the boards so they can fit on the switches. Once again, assembly isn’t too difficult but is complicated a little by the use of SMDs. However, these devices are being used more and more these siliconchip.com.au days (in fact, many components are no longer available in through-hole) so you’d better get used to them! For more detail on the use and soldering of SMDs, refer to the articles on the subject in the March 2008 and December 2010 issues – both available online at siliconchip.com.au Fortunately, all SMDs on each board are identical – there are 15 1F capacitors on the 1F switch board and 15 10F capacitors on the 10F switch board. Just don’t get the 1F and 10F types mixed up because they do look similar although the 10F capacitors are somewhat larger. SMD capacitors normally do not come with any markings. Speaking of mixups, the other four boards are not quite so simple because there is some difference in the component position, not to mention that the component values are all different. Take your time and refer to both the photographs and to the component overlay diagrams. Unlike the resistance PCBs, all six of the capacitance PCBs connect in parallel – all the ‘A’ terminals are connected together, as are all the ‘B’ terminals. The four horizontal boards are connected with short loops of tinned copper wire – the offcuts from the resistor leads are ideal. They should be butted up to each other. The two vertical-mounting boards have short lengths of tinned copper wire which connect the two boards together (A to A and B to B) and then ‘jump across’ to join onto the A and B positions on the horizontal boards. The close-up photo will show this more clearly. All six boards ‘plug in’ to header sockets which in turn plug in to mating pins on their respective BCD rotary thumbswitches – connecting COM to Parts List – Resistor-Capacitor Substitution Box 1 Termination/Switch PCB, Coded K7520A, 28 x 35mm (Altronics) 4 Through-hole capacitor PCBs, Coded K7520B, 35 x 8mm (Altronics) 2 SMD Capacitor PCBs, Coded K7520C, 35 x 16mm (Altronics) 1 ABS Case, 145 x 195 x 65mm, punched and printed (Altronics Cat H0307/K7520) 6 Thumbwheel decade switches (0-9) (Altronics Cat S3302) 6 Thumbwheel BCD switches (0-9) (Altronics Cat S3300) 2 Pairs end caps for thumbwheel switches (Altronics Cat S3305) 1 4-pole, 3-position slider switch (Altronics Cat S2033) 2 40-way pin headers (Altronics Cat P5430) 2 Header pin sockets, 40 pin, 90° (Altronics Cat P5392) 8 Machine screws, M3 x 6mm 4 M3 threaded stand-offs, 12mm 1m hookup wire (or mini fig-8) Tinned copper wire (if required) 2 short lengths (~50mm) ribbon cable Capacitors CODES: µF Value 15 10F 50V SMD 10F 15 1F 50V SMD 1F 1 470nF 100V MKT 0.47 1 330nF 100V MKT 0.33 2 150nF 100V MKT 0.15 4 100nF 100V MKT 0.1 1 47nF 100V MKT 0.047 1 33nF 100V MKT 0.033 1 22nF 100V MKT 0.022 1 18nF 100V MKT 0.018 3 10nF 100V MKT 0.010 1 4.7nF 100V MKT 0.0047 1 3.3nF 100V MKT 0.0033 2 1.5nF 100V MKT 0.0015 4 1nF 100V MKT 0.001 1 470pF 50V ceramic – 1 330pF 50V ceramic – 1 220pF 50V ceramic – 1 180pF 50V ceramic – 3 100pF 50V ceramic – Resistors (1% metal film, 0.6W) 9 100kΩ (Code brown black black orange brown) 9 10kΩ (Code brown black black red brown) 9 1kΩ (Code brown black black brown brown) 9 100Ω (Code brown black black black brown) 9 10Ω (Code brown black black gold brown) 9 1Ω (Code brown black black silver brown) COM, 1 to 1, 2 to 2, 4 to 4 and 8 to 8. Termination Board The only “component” on the terminal board is the 3-way switch. All other points connect to the thumbwheels or terminals. siliconchip.com.au This PCB not only provides an anchor point for the wires coming from the resistance and capacitance board assemblies and going to the six binding posts (terminals), it also provides a mounting point for the two-way, three-position switch which selects between isolated R & C, series R & C or parallel R & C The switch mounts on the conven- IEC Code EIA Code 10 106 10 105 470n 474 330n 334 150n 154 100n 104 47n 473 33n 333 22n 223 18n 183 10n 103 4n7 472 3n3 332 1n5 152 1n0 102 470p 471 330p 331 220p 221 180p 181 100p 101 NOTE: only 1% (5 band) or better resistors should be used for this project to avoid errors. tional side of the board (it will only go in one way) and the board then mounts upside-down on four 12mm pillars via 6mm M3 screws. This method enables the switch actuator to poke through the front panel at the right height. The various wires (ten of them, or five lengths of Fig.8) solder to the exposed copper side of the PCB. Using the photos as a length guide, cut the wires to appropriate lengths, bare and tin both ends and solder the August 2014  81 Finally, here’s the completed project, all mounted inside the lid of the case. It has the capacitor switching at top left, resistor switching at lower left, through/parallel/series switch on its PCB at top right and the terminals down the right side. six solder lugs (which came with the binding posts) to one end. Fit the binding posts to their respective wires. The opposite ends are now soldered to the PCB – make sure you get the right ones in the right place. The remaining four wires (or two Figure-8s) solder to the ‘A’ and ‘B’ positions on the resistance and capacitance boards, as per the layout diagram and photos. The case If you’re putting this together from the Altronics kit (K7520) it will come with the case already punched and drilled for the thumbwheel switches, MaxiMite miniMaximite or MicroMite Which one do you want? They’re the beginner’s computers that the experts love, because they’re so versatile! And they’ve started a cult following around the world from Afghanistan to Zanzibar! Very low cost, easy to program, easy to use – the Maximite, miniMaximite and the Micromite are the perfect D-I-Y computers for every level. Read the articles – and you’ll be convinced . . . You’ll find the articles at: siliconchip.com.au/project/mite Maximite: Mar, Apr, May 2011 miniMaximite: Nov 2011 Colour MaxiMite: Sept, Oct 2012 MicroMite: May, Jun, Aug 2014 plus loads of Circuit Notebook ideas! PCBs & Micros available from PartShop 82  Silicon Chip parallel/series switch, binding posts and screws – and the top of the case will also be printed, as per our photos. Checking it out Give your project the once-over, checking for bad solder joints, misplaced components, etc. Checking the individual ‘R’ and ‘C’ functions is delightfully easy: switch the series/parallel switch to ‘off’ (ie, fully left) and connect your multimeter on the appropriate range (R or C) to the appropriate substitution box terminals (R or C) and switch through the ranges with the thumbwheels. Apart from the ‘000000’ settings (or even very low ohms or capacitance), you should find the multimeter reads the same, or at least quite close to, as what your thumbwheels say – otherwise, you’ve got a problem! If you get no reading at all, it’s almost certainly an open circuit/dry joint in your soldering; if you get strange readings, it’s more than likely mixed-up components. As mentoned earlier, with all switches set to zero (on both R & C) it is normal to obtain very low readings – perhaps an ohm or so on resistance and maybe 20pF or so on capacitance. Residual C and R should always be taken into account when working with low settings. This applies to all RC substitution boxes, certainly not just this one! Checking the series or parallel RC is not quite so simple – probably the easiest way is to use a moving coil multimeter, set the RC Box to parallel and with your multimeter already connected to the binding posts and on its lowest DC value, switch the RC box to the highest R&C settings. You should see the voltage rise fairly quickly as the multimeter itself charges the capacitor. Change the box resistance to a much lower value and the voltage should rise much more quickly. If it does, you can be fairly confident that it’s working as it should. Of course, advanced hobbyists, technicians or engineers would have much better ways to check this function but if you don’t have advanced equipment, you don’t have it! SC Where from, how much? This project was designed by Altronics Distributors, who retain the copyright on the PCBs. Complete kits are available from Altronics stores, resellers and via www.altronics.com.au for $119.95 plus p&p. (Cat K7520) This includes the pre-printed and punched case. siliconchip.com.au