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Wide-Range hmMeter This Wide-Range Ohmmeter is more useful than a milliohm meter. It measures very low resistances, down to around 1mΩ, but it can also measure up to 20MΩ with an accuracy of around ±0.1%. That makes it handy in any electronics lab, and it's easy to use; just connect a device and read off its value. Having described how it works last month, we now move on to building it. C onstruction is relatively straightforward as most parts mount on a single modestly-sized PCB. The four binding posts/ banana terminals mount on the case’s front panel and are wired up via two figure-eight leads and two-way locking header plugs. The six-AA battery holder is stuck to the base of the case and hard-wired to the on/off switch, with power going to the PCB via another header plug. The rest of the parts are on the PCB, which mounts behind the front panel of the case. Several of these parts are only available in SMD packages, so some surface-mount soldering is inevitably involved. Still, we have tried to make it relatively easy. You need the right tools, including a temperature-controlled iron, a syringe of flux paste, solder wick, a good light and a magnifier. It’s also essential to exercise patience; it’s easier to make mistakes if you rush into soldering these devices. A little practice soldering fine-pitched SMDs also wouldn’t go astray (using our SMD Trainer from the December 2022 issue). Don’t feel daunted; we believe most constructors with modest soldering experience can build the Wide-Range Ohmmeter without too much difficulty. So let’s start the assembly process. 34 Construction The Wide-Range Ohmmeter is built on a double-sided PCB coded 04109221, measuring 90.5 × 117.5mm and available from the PE PCB Service. Fig.6 is the overlay diagram, which shows which parts go where. Start by checking the PCB, checking that you have all the required parts and tools. Commence by mounting the SMDs. The usual advice for soldering these goes: use plenty of flux, take your time, use a loupe or good handheld magnifier to check, then double-check for bridges between tracks and when you find them, use solder wick to remove them. Oh, and leave the quadruple espresso coffee until after you are finished. One of the most important things to do, and we can’t stress this enough, is to check that you have the right part in each location and that it is orientated correctly before you solder more than one or two pins. While it is possible to remove an SMD IC that has been fully soldered – without damaging it or the board, then cleaning up the board to re-solder it – it is a lot of work! Some MAX11XXX ADCs have a chamfer along the pin 1 side and no dot to indicate pin 1. So if you can’t find the dot, look at the IC edge-on under magnification; hopefully, you Part 2 by Phil Prosser can spot the chamfered edge. Pin 1 is on that side. It’s also an excellent idea to use your magnifier to check carefully that all of an IC’s pins are correctly located over its pads after soldering one pin in each corner, before soldering the rest. It’s easy for an SMD IC to shift slightly if you just tack one pin, and very hard to fix the alignment after soldering more than a few. Besides most ICs and regulators on the board being SMDs, there are also a handful of surface-mounted bypass capacitors and resistors, but they are much larger and easier to solder. It’s generally best to start with the finepitch ICs; that way, you have the best view and access to their leads. So, fit IC1, IC2 and IC4 first (remember what we said about checking their pin 1 markings first!), then MOSFETs Q2 and Q4, followed by IC3, REG2 and REG3 (don’t get the different types mixed up). Follow with the five smaller 100nF SMD ceramics, the remaining 10µF SMD ceramics and then all the SMD resistors. Clean off any gross flux residue (using a special-purpose flux cleaner or pure alcohol), then, under good light, check every pin on the SMDs for bridges. Some phone cameras can zoom in for a really close-up photo; if yours Practical Electronics | September | 2023 Fig.6: most of the components are mounted on the top side of the PCB. The only part on the underside is the 16×2 LCD. Take care to orient the ICs, diodes, electrolytic capacitors, relays and TO-220 devices correctly and note how the relay footprints support two common styles of signal relay. Regardless of relay style, the striped (coil) end faces to the left. offers that facility, take a picture or two and check them well. We have a reasonably inexpensive binocular microscope in our lab which is brilliant for finding pesky shorts. While you’re at it, also check that all the device pins and leads have a proper fillet from the lead down to the PCB pad. It’s relatively easy to get the solder to stick to a pin but not flow onto the pad, or vice versa, especially if you don’t use enough flux during soldering. If you find any problems, fix them up. You can fix bad joints by adding a dab of flux paste and then touching the tip of your iron to the junction of the device lead and PCB pad. Some small solder bridges can be solved in the same way, although it can be better (and is usually advisable) to follow up the flux paste with some solder wick (if it’s saturated with solder, cut the end off and use a fresh section). Note that there are a few unoccupied pairs of SMD pads for optional parts that we determined aren’t required. Through-hole parts Move on to mounting all the remaining resistors. The 47W resistor in series with the LCD backlight can be reduced in value for more brightness, Practical Electronics | September | 2023 but that will reduce LED life. Or, for maximum battery life, select a higher value that provides acceptable brightness. Use good-quality resistors in the current source and references. We have provided some recommendations in the parts list. Ensure that the high-precision 10kW resistor goes in the indicated location and not in place of one of the regular 10kW resistors. If you don’t have a 205W resistor, you can use 220W instead and replace the two parallel resistors (marked as 47kW and 1.5MW) with two 5.6kW resistors to get reasonably close to the required values. Next, fit the diodes, making sure that the cathode stripes face as shown in each case. Start with the 1N4148s, then the BAT85. Be careful here – a BAT85 looks a lot like a 1N4148, but they are very different. Then install the 1N4004 and 1N5819 diodes. They are similar sizes, so don’t mix these up either. Now is a good time to mount the NE555 IC. It doesn’t need a socket, and once again, watch its pin 1 orientation. Follow with the two tactile switches, then all the through-hole ceramic and plastic film capacitors, which are not polarised. In case you’re wondering, two of the 10nF capacitors are PPS types (adjacent to S1 in Fig.6) rather than ceramic because these need to be low-leakage types. If you can’t get PPS capacitors, use the best film capacitors you can and check that they don’t adversely affect high resistance readings. Install all the headers now. Remember that for programming and SPI monitoring, headers CON4 and CON6 need to be fitted. If you are using a pre-programmed PIC, then you can fit a wire link in place of JP1. If fitting JP1, simply place the jumper on it after soldering and, unless you need to reprogram the PIC, you can leave it on permanently. Next, fit the four BC547 (or BC546, BC548 or BC549) transistors, as well as the LM336. These are all in the same packages, so don’t mix them up. Follow with the two 10kW trimpots, orienting VR1 as shown in Fig.6. Then install all the electrolytic capacitors, with the longer positive leads going to the pads marked with a ‘+’ sign on the PCB. The two near the top need to be laid over as shown. This is a good time to install the relays, for which we have provided two options. One is available from 35 Altronics, while the narrower type is commonly available from major suppliers such as Mouser, Digi-Key and element14. The two different outlines are shown on the silkscreening; regardless of which type you use, ensure that the striped end faces to the left as shown. The LCD mounts via a header on the back of the board. Choose the right location for the LCD type you have. It is necessary to mount the LCD quite close to the PCB, but not so close that it touches the solder joints on the main board. We left about a 2mm gap and put a couple of dabs of neutral cure silicone under the screen to keep it from moving. Once set, the silicone will hold everything tight. Reducing leakage paths At this point, the PCB should have all the parts on it. If you have a special-purpose flux cleaner such as our favourite, Kleanium Deflux-It G2, it’s a good idea to start cleaning by spraying the board with that. Let it dissolve the flux, then dab it dry with a lint-free cloth before scrubbing it with alcohol. That will remove a lot of the residue in one easy pass, making the next step easier. Now get some isopropyl alcohol and a good scrubbing brush to clean the PCB (we used an old toothbrush). Thoroughly clean around the reference resistors, ADC and the input buffer, taking particular care to scrub away any residual flux around the ADC. After scrubbing, wet it again with alcohol and then dab it clean with a lint-free cloth to soak up any residue. Once you’re sure the board’s critical areas are clean, liberally coat the ADC and reference resistor area with a clear, protective lacquer, being careful not to spray the headers. Ideally, you should use a purpose-­designed PCB conformal coating (the solder-through type is great in case you find a problem later). We want all sensitive parts of the PCB clean and sealed from moisture. Testing The first test is to apply power and check that the regulator outputs are Troubleshooting It is normal on the first power up for a message stating that default calibration values are being loaded. If the Meter is not working at all, check the following: ● The solder joints on all SMDs – check for improperly formed joints or solder bridges. ● The battery voltage (you should have checked this earlier). ● The regulator output voltages (ditto). If the LCD is not displaying text: ● Can you adjust VR2 to get anything on the display? ● Is there about –2.2V at the anode of D10? If not, check around the 555 for faults. ● Check for activity on the LCD RS, RW, E and D7, 6, 5 and 4 lines (the rest are not used) on the LCD header. If these are not active, check the soldering on the microcontroller and verify that it has been programmed. ● If there is a problem with the ADC, there will be a message on the LCD telling you that. In this case, check the soldering on the ADC chip. Also check the SPI lines with a scope for activity. You should see activity on the CS, MCLK, SDI and SDO lines. The absence of activity suggests a short or similar problem. If it appears to be working, but the measurements are wrong: ● The connections for Sense+, Sense−, Force+ and Force−. If you have these swapped, the Meter will not make sensible measurements. ● Are the relays clicking? If not, look at the ADC connections again. Look at the four digital output lines and also make sure you have used proper BC54x transistors and the pinouts are correct. We have heard about some parts labelled BC54x that use the wrong pinout. ● Have you used relays with 5V DC coils? ● Are the reference resistors the correct values? ● Connect an ammeter on its 200mA range or similar from pin 3 of IC3, the LT3092 (the one closest to the top of the board) to the anode of D3, with the sense lines shorted (eg, using a jumper). You should measure very close to 50mA, then if you remove the short on the sense lines, it should drop to 0.5mA. ● Check that the 2.5V reference voltage is right; you should have checked this while adjusting it. ● Check that you put those push buttons in the right way around; if you rotated them by 90°, they would be shorted ‘on’ and you are probably stuck in calibration mode and keep getting calibration messages, but the buttons won't work! 36 correct. Prepare the battery of six AA cells. There are many options for this, but the parts list specifies two 3-cell holders, and you just need to connect them in series, negative to positive. Also cut and mount the side switch in the box, as shown in Fig.7. The switch can be mounted at any convenient location on one side of the case; the photo overleaf shows where we placed ours. Use masking tape to mark the drill holes for the screws; 2mm holes are a good start. Also mark and drill two holes that define the ends of the slot. These are 5mm in diameter, and once you have drilled them, use a small file to join them into a slot. Mount the switch and then, ensuring the switch is off, wire up the battery to it (insulating any exposed joints with heatshrink tubing). Next, crimp and solder the two remaining wires into the plug housing that will go to the PCB. Don’t make the leads too short; ensure there is sufficient wire length to assemble and calibrate the instrument conveniently. Double-check the polarity as there is reverse polarity protection on the PCB, but it’s a bit brutal; if wired backwards, the battery will be shunted by a 1N4004 diode. Leave the PCB on the bench so you can make measurements easily, then plug in the battery/switch combination to the header and switch it on. Using a multimeter set to measure low DC voltage, measure between the ground test point right at the top of the PCB, and the output tabs of REG2 (3.45-3.75V) and REG3 (4.5-5.2V). If either reading is wrong, check the input voltage at the cathode of D9, in the lower left-hand corner of the board. This should be around 8-9V. If something is getting hot, switch off and figure out why. If one voltage is low, carefully check the soldering of the regulator Fig.7: the on/off slide switch can be placed along any convenient edge of the case. Apply this template, drill the two mounting holes plus 5mm holes at either end of the slot outline, then file away the material between those holes. You can download it from the September 2023 page of the PE website at: https://bit.ly/pe-downloads Practical Electronics | September | 2023 Front and rear shots of the Ohmmeter PCB. At the rear, two different types of 16x2 LCD modules can be fitted, as the ones found online typically come in one of two sizes. and its surrounding components and verify that the components are the right types and oriented correctly. Verify that you have not put the LT3092 in place of a regulator. Assuming they check out, verify that the LCD backlight is on, then adjust 10kW trimpot VR2 until text shows on the screen. Now it is time to calibrate the 2.5V reference, which also optimises its stability. Monitor the voltage across TP1 and TP2, in between the holes for the test terminals on the PCB. Adjust 10kW trimpot VR1 to get a reading as close to 2.50V as possible. This does not need to be super precise, but get it close. At this point, all the adjustments on the PCB are finished, and when you switch it on, the relays should click, and a message saying ‘Over Range, Check Sense Conn’ should come up on the screen. You will find that the Meter is now working but not fully calibrated. Mounting it in the case The PCB is designed to fit into the Altronics H0401 instrument case. The front panel drilling and cutouts are in Fig.8. You will have already mounted the slide switch. There are four holes for the Kelvin probes binding posts/banana sockets. The specified binding posts include standard 3mm banana sockets. These holes line up with the large holes in the PCB, allowing the wiring to run straight through. There are also four countersunk holes for M3 screws used to mount the PCB. The front panel covers the PCB mounting holes, so we were careful to countersink the screw heads to be flush with the front panel. The smaller LCD cut-out shown matches the LCD we used. An alternative cut-out is shown for another common type. Before cutting, check which hole suits your LCD module. There could be a third option, in which case you’ll have to figure out the location and size of this cut-out. Internally, the case preparation is simple. By keeping the LCD mounted close to the PCB, the LCD will sit neatly behind the clear opening in the laminated label. Fix the cell holders inside the base with either a dab of neutral cure silicone sealant or double-sided tape. To allow the PCB to fit, we cut off the two standoffs at the top of the base so we could line up the battery holders along the top, as shown in the photo published last month. There is minimal wiring involved in preparing the case. The power, Practical Electronics | September | 2023 Force and Sense connections all use pluggable headers. Start with two pairs of red/black wires 150mm long, and crimp these to the pins that match the polarised header plugs. Note that the + and – pins are swapped between the Force and Sense headers. The easiest solution is to simply insert these in the plastic blocks last, ensuring they line up 37 Left: this shows where we mounted the on/off slide switch on our prototype. Above: measuring a 3.3W enclosed wirewound ceramic core resistor. Reproduced by arrangement with SILICON CHIP magazine 2023. www.siliconchip.com.au Fig.8: these drilling/cutting templates fit on the inside of the case front panel. Select the one which lines up with your LCD screen. You can download the template from the September 2023 page of the PE website at: https://bit.ly/pe-downloads Print them out, then cut them up and stick them onto the panel so you can accurately mark the locations of the holes. 38 Practical Electronics | September | 2023 with the silkscreened markings on the PCB. We printed the front panel label onto thick paper and cut out the hole for the LCD. You can download the artwork as a PDF from the September 2023 page of the PE website at: https://bit.ly/pe-downloads There are two versions to suit the display window locations for two common types of compatible LCD screens, as shown in Fig.9. We then laminated this and used a sharp knife to cut out the holes for the banana plugs. The laminate makes a simple and effective window for the LCD. After that, we stuck it onto the front of the case with a very thin layer of neutral cure silicone sealant. Calibration The calibration procedure has been deliberately kept simple. There is one adjustment per range, which is stored in flash memory and loaded on powerup. As you need access to pushbutton switches S1 and S2 for calibration, it can only be done with the case open. Start calibration by pressing the ENTER key (S2) on the PCB until a calibration message comes up. The button press detection for the user interface is not terribly fast; buttons are checked after each ADC measurement, or about four times a second. Keep that in mind while calibrating the unit. The calibration process generates a correction for each range independently of all other ranges. Start by connecting a calibration resistor to the Meter as if you were measuring its value. The values used should ideally be close to the top of each range (as specified in the parts list last month and in Table 1). Once the resistor is connected, you adjust the calibration up/down until the Meter reads the correct value of the calibration resistor. You then accept the calibration value for that range. Once all ranges have been calibrated, the data is saved, and the Meter reverts to normal operation. WIDE-RANGE OHMMETER FORCE - + - + SENSE Fig.9: while the instrument is simple enough that you might get away without a front panel label, it does make it look quite a bit nicer. Once again, select the one that matches your LCD panel position. Cutting out the LCD rectangle before laminating it produces a protective window for the LCD screen. The Meter has five ranges, shown in Table 1, along with the recommended calibration resistors. All but the 10MW types have ±0.1% tolerances, and most are less than 50p. If you’re going to use different calibration resistors, they should ideally have tolerances of ±0.1% or better and temperature coefficients no higher than 50ppm/°C. On each range, the Meter will prompt you for a calibration resistor. Once you clip the resistor onto the Table 1 – ranges and calibration Range Calibration resistor Suitable test resistor Notes 0-30W YR1B27R4CC (27.4W ±0.1%) YR1B10RCC (10W ±0.1%) A few test resistors in the 20mW220mW range would be handy 30W-3kW YR1B2K94CC (2.94kW ±0.1%) YR1B1K0CC (1kW ±0.1%) 3kW-100kW YR1B97K6CC (97.6kW ±0.1%) YR1B100KCC (100kW ±0.1%) 100kW-1MW YR1B976KCC (976kW ±0.1%) YR1B1M0CC (1MW ±0.1%) 1MW-20MW MF0204FTE52-10M (10MW ±1%) Practical Electronics | September | 2023 High-precision resistors in this range are very expensive 39 Meter, it will present readings. Make adjustments as follows: 1. If no button is pressed, the Meter will continually update the measured resistances. 2. When the SELECT button (S1) is pressed: a  You will see either a < or > symbol to the right of the measured value. b The > indicates you will increase the calibration factor and the presented value. c Similarly, < indicates you will reduce the calibration factor. d  To reverse the direction, hold down the SELECT button and then press ENTER (S2) briefly at the same time. e  Pressing SELECT changes the calibration factor and thus the displayed value in the direction shown. f The longer you hold the SELECT button, the faster the calibration corrections change. To slow the rate of change down, release the SELECT button for a second. There are three speeds – the slowest will allow tiny corrections, while medium and fast speeds let you get to the required value quicker. 3. If the ENTER button is pressed alone, it will accept the current calibration value and move to the next range. 4. After all adjustments are completed, the calibration data is saved, and the Meter goes back to normal. Accuracy and precision Our tests show that the precision of this Meter between about 10mW and 10MW is entirely defined by the calibration precision. We calibrated the prototype using the recommended reference resistors and achieved precision close to ±0.1% across most of the range. The better calibration you can give it, the better performance you will achieve. Repeatability across our five prototype meters is excellent, indicating good linearity of the ADCs. We have gone to great lengths to ensure stability over time and temperature, so it should remain stable once calibrated. You will notice that the meter displays more significant digits than the precision would indicate. The Meter is very stable and, in most ranges, provides noise-free measurement to a resolution of much better than 0.1%. While the accuracy is limited to about 0.1%, the resolution and short-term repeatability are much better than this. So if you want to match resistors to a high precision, the Meter provides the extra resolution you need for that. Using it It’s just a matter of switching it on, connecting the device to be measured and reading off the value. At start-up, it shows the firmware revision and the measured battery voltage. If the battery falls below 6.5V, it will ask for a new set. Try not to leave the Meter on for hours at a time, since it does draw some current, especially in the low range. Aside from this, we believe this will become a handy tool for your workbench. We do not expect the Meter to need calibrating all that often. We went to a fair bit of bother to make sure things should stay stable. Still, keep those calibration resistors and clip them on once a year or so. If you are making a critical measurement, a quick check will only take you a second or two. When measuring low-value resistances, on the order of a few milliohms, component lead resistance can become significant. So connect the test clips as close to the body of the device as possible. ESR Electronic Components Ltd Your best bet since MAPLIN Chock-a-Block with Stock Visit: www.cricklewoodelectronics.com Or phone our friendly knowledgeable staff on 020 8452 0161 All of our stock is RoHS compliant and CE approved. Visit our well stocked shop for all of your requirements or order on-line. We can help and advise with your enquiry, from design to construction. 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