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Part 2 by Phil Prosser POWER LCR METER We introduced this new device last month. It isn’t just another LC meter; it can deliver a range of currents up to 30A to determine how an inductor behaves as its core starts to saturate. This tester can also measure very high capacitances and very low resistances. This article covers its assembly, testing, calibration and use. T he Power LCR Meter has two basic modes: it either applies a fixed current or a fixed voltage to the device under test (DUT) and samples the voltage across it and current through it many times over a short period. It then examines those samples to determine either its resistance, capacitance or inductance. Because it can control the current used for the test, for power inductors, it can step through a range of currents and calculate the inductances, allowing you to see how it changes. For a typical inductor with a ferrite, iron or mu-metal core, the inductance will remain relatively steady until a certain current level is reached, then it will fall off as the core saturates. Having this information is invaluable as it allows you to determine whether the inductor will be suitable for applications that demand a certain inductance up to a certain current level, like a loudspeaker crossover or switch-mode power supply. Construction The Power LCR Meter is built on a double-sided 156 × 118mm PCB coded 82 Silicon Chip 04103251. It mostly uses through-hole parts, but there are a few SMDs, which should be fitted first. During assembly, refer to the component overlay diagrams, Figs.10 & 11, to see which parts go where. You can see in the photos that we didn’t have a 5W 0.39W resistor, so we used two smaller resistors in series. We only installed one 47,000μF capacitor on this prototype, which was enough for the test inductors used. Fit both if you want to test large, low-­resistance inductors. You will also see that we have used 1μF & 10μF SMD tantalum capacitors, while the final parts list suggests ceramic capacitors instead. You can use either, but the specific ceramic capacitors should be cheaper, more reliable and perform better. If you use tantalums, make sure you orientate them with the positive stripes as shown on the PCB and in the photos. We always like to fit all the power supply parts before the remaining active semiconductors to make testing easier. So start by mounting all the parts in the power supply section, which is everything to the left of the Australia's electronics magazine white vertical line on the silkscreen (the black line in Fig.10, including the parts in the lower-left corner). It’s easiest to start with low-profile components like resistors and then work your way up to the taller ones, ending with the bulky and heavy inductors. Watch the orientations of the diodes, electrolytic capacitors, regulators and transistor. For the regulators and transistor, pay attention to which side the metal tab goes (REG3 & REG5) or flat face (the others) so that they match Fig.10. There is space for a heatsink for the LM2576 (REG5), but it is not required. The average dissipation is low enough that it will be fine without it. With all the power supply components installed, you can connect a 12-20V DC power supply to CON4 (with the positive lead nearest the fuse) and check the following: 1. Check the 10V filtered rail is 9-11V; our four prototypes all measured about 9.8V. You can measure this on the DUT+ terminals. There is a GND test point just next to the power switch; we found it convenient to siliconchip.com.au 100nF Spare S4 NO S1 siliconchip.com.au (S4 SPARE) 4.7kW 100W 1kW 33nF 4.7kW 100W Q5 TIP121 Q1 0 BC558 100nF 470W 1W 1W CON11 TRIGGER 470W 100nF IC7 TLC072 100nF 47kW 47kW 4.7kW 470W S5 Power DUT− 4.7kW 100nF BAT85 S3 NO DUT+ 4.7kW + 4.7kW (S1 ENTER) 470W BAT85 100nF TP3 NC S2 NO 4.7kW 4.7kW BAT85 Fig.10: we recommend you fit the power supply components first (the whole leftmost section) so you can verify that is all working before adding the rest of the parts. Be very careful to orientate IC1 correctly, with its pin 1 dot at upper left, before soldering it. Also watch the orientations of the other ICs, diodes, electros, and transistors (including the Mosfets). Q9 BC548 Fig.11: there aren’t many parts on the back of the PCB; just the four or five switches. The main measurement terminals pass through the two large holes near the middle. Down NC NO BAT85 D9 Up Enter NC 4.7kW IPP013N04NF2S 4.7kW 4.7kW 4.7kW 4.7kW 4013B IC3 D7 IC8 INA281B1 1m F V1.2 SILICON CHIP Power LCR Tester NC (S2 UP) T P5 TP4 BAT85 + 47,000mF IC4 LM393 100nF D6 Q2 4.7kW 4.7kW BC548 IC6 INA281B1 CON1 + KELVIN SUP70101EL 12V ZD12 CON5 10W 10W + CON6 100nF − Q4 1m F 47,000mF IC2 MCP4822 Q7 Q6 4.7kW 1mF Q8 BC548 330W 1W D5 JP8 10kW 18pF ZD11 12V 56 0 W 33 0 W 1 0m F 1kW 220pF D8 REG3 100nF LM337 1 00 m F 10mF 4.7kW (S3 DOWN) 4.7kW 10kW 4.7kW 100nF 100nF 100nF 10mF 100nF 4.7kW Q3 BC548 10mF 18pF 100nF 8MHz D10 TP6 100nF IMON 2.7kW CON7 X1 470W 0.005W 0.39W 5W + TP8 +3.3VA CON3 + L2 330 m H 1 00 m F REG2 LM2950-33 + −3.3V 100W 1W 1 4148 1kW 100W 100nF + +3.3V REG1 LM2950-33 100nF GND 100mF 100nF RAIL IC1 SENSE 100nF 1 TP7 TP2 100nF VR1 20kW D3 4148 PIC3MK0128MCA048 10mF + + JP10 4.7kW Q1 BC558 33kW 100mF 10mF 1 00 m F 100nF IC5 25AA256 100nF JP9 4148 D2 10mF 16 1000mF D1 4148 10 m F +10V GND CON2 100nF D4 L1 330mH 4.7kW 4.7kW POWER SUPPLY 10 0 W 58 22 100nF REG5 LM2576 1000mF RS E CON4 POWER 1000mF (S5 POWER) F1 1A + + + DUT− DUT+ Australia's electronics magazine April 2025  83 solder a piece of tinned copper wire into this to clip onto. 2. Check the +3.3VD, +3.VA and -3.3V voltages. Test points for these are just above the circular cutouts for the DUT connectors. We expect the two positive rails to be within 100mV; note that in normal operation running from 12V, these regulators get quite warm. If any of these are off significantly, or something gets hot, check the orientation of all capacitors and diodes. We tried to keep all capacitors orientated the same way, but because switch-mode power supplies have exacting layout requirements, the diode placement in that area is not so consistent. The 330W resistor just above the 47,000μF capacitors is there to put a sufficient load on the switch-mode power supply that it runs continuously. We need this to generate the -3.3V supply. If your -3.3V supply does not come up properly, but everything else looks OK, check it. The following surface mount parts can go on next. With the power supplies behaving, it is time to get the fiddly bits on while there is still room. That includes: ● The PIC32MK0128MCA048 (IC1). ● The two 10μF surface-mount capacitors. ● The eight 100nF surface-mount bypass capacitors, which are mostly around IC1. ● The two 18pF SMD capacitors near the crystal oscillator. ● The three 1μF SMD capacitors, which are next to the INA281s and across the DUT terminals. ● The 25AA256-I/SN serial EEPROM. ● 470W series resistor for the crystal oscillator. ● The 10kW and 1kW resistors next to the reset header. ● The two 10W resistors for the Kelvin connection option. ● The two INA281B1 devices (IC6 & IC8). The INA281 devices are in SOT23-5 packages, which are a little on the small side. However, if you approach 13 15 – A them with some care, they are not too difficult to solder. The PIC microcontroller is in a 48-pin thin quad flat pack (TQFP), which has a 0.5mm lead spacing. This was the most easily soldered IC in the series we could find, alternative devices being in leadless packages, which are daunting to solder. We have provided soldering guides for TQFP and SOT-23 packages in the past. Our key tip is to use plenty of flux paste and to use a magnifying loupe to check for bridges between pins when you’ve finished. Use solder wick to remove any bridges you find. If the joint on a pin looks a little dry, resolder it before it causes you trouble later. When you’ve finished construction and apply power, if the LCD does not fire up immediately, come back and double check those pins for shorts. We have had to fix plenty of solder bridges ourselves in the past; the PIC microcontrollers are very tolerant of shorted pins and we have not managed to blow one up yet from a solder bridge (but it’s still better to clear them before applying power). Pro tip: after soldering all the SMDs, you will probably have flux residue that gets in the way of a proper inspection. Clean it off using a flux solvent (or isopropyl alcohol or methylated spirits if that’s all you have) and it will be much easier to spot any problems. Your board will also look a lot nicer and be less sticky! Mounting the LCD We want to connect the 16×2 LCD to the main PCB with a 16-way ribbon cable. To fit neatly in the case, we directly soldered the ribbon cable to the 14 through-holes on the LCD. This was a nuisance, but there was not room in the case for the IDC header we wanted to use. We say 14 and not 16 because the backlight connections are at the other end of the LCD. We show how we connected this in the photo below. Ensure that the red wire on your ribbon cable goes to pin 1 at both ends. Also make sure that once crimped, the IDC cable comes out in the right direction. The total length of ribbon cable we used was 300mm, with about 200mm between the IDC header and LCD board, leaving that extra length to connect to the backlight on the LCD board. Pins 1-14 of the ribbon cable are connected to the same pin number on the LCD. Note that the pins alternate between the two columns on the LCD. For the two remaining wires on pin 15 and 16 from the main board, strip the end of these and solder them to the anode and cathode backlight pads. Importantly, for the Altronics screen, you must place jumpers horizontally on JP9 and JP10 on the main board as shown in Fig.10. This applies 3.3V to Vdd (pin 2) on the LCD and grounds pin 1. If you are using a different display, check its data sheet, as these pins are sometimes swapped between manufacturers. If this is the case, you can install JP9 and JP10 vertically, which will swap the rails. Getting the microcontroller working At this point, we can install the remaining parts in the microcontroller section. That is the section at upperright bordered by a solid vertical line on the left and a broken horizontal line below. The four pushbutton switches mount to the rear of the PCB (S4 is not needed). For these, it is important that you rotate them so the normally open (NO) pins are at the bottom. Double-check this using a continuity meter; if on startup the system always goes into calibration mode, you almost certainly have the switches in the wrong way around. Also watch the orientations of the BAT85 protection diodes as they are not consistent. We also note that you can save quite a bit purchasing these from the larger online suppliers. We have used a lot of 4.7kW resistors to make it easier to purchase and manage the parts for this project. However, there are some 470W resistors as well, which will have similar colour 14 16 – K 1 2 This shows how to solder the ribbon cable to the Altronics 16×2 LCD. We tried to use an Altronics P5162A 14-way IDC-toPCB adaptor, but it wouldn’t fit in the space available. If you are installing it in a larger case, you may be able to use it. 84 Silicon Chip Australia's electronics magazine siliconchip.com.au codes, so take care not to mix them up. Mount all the 4.7kW resistors at once and you can be confident you won’t confuse them. Plug the LCD onto the main PCB, making sure that you get the pin 1 ends right at both the PCB and display end. We can now test this part of the board. Apply power and check the power rail voltages again. The voltages should be about the same; if any are very low, look for things getting hot or capacitors in the wrong way around. You should see the LCD backlight come on. If not, check the connections on the LCD from the header to the backlight LED and check that the headers are plugged in the right way around. You now need to adjust trimpot VR1, which controls the LCD contrast. Start at one end and turn it until you get good contrast on the display. There should be legible text if everything is fine, but if the LCD has not fully booted, you will still see lines of boxes or characters. If you get no display at all, double-check your LCD data sheet to make sure JP9 and JP10 are in the right locations. If the LCD is not displaying anything at all, check the soldering on the microcontroller and your cabling. If this all looks good, you probably want to check for activity on the LCD RS and E lines with an oscilloscope (if you have one). We put test points on the PCB for these – although we didn’t have to use them, as the 16×2 LCDs seem to mostly just work. If you still think nothing is happening and the display is blank, check the crystal oscillator drive on its associated 470W resistor. There must be an 8MHz sinewave here; if it is missing, double-check the microcontroller solder joints. You should now have a screen with text on it. Installing the measurement section You can solder all the remaining parts in place now. The only heatsink that you need to attach is on Q5, as shown in Fig.10. The other devices don’t dissipate enough power to warrant heatsinks, even though we have space for them on the board. With all parts mounted, you should be able to fire the meter up and get a screen saying “Resistance < 300R, Enter to Meas” and similar for siliconchip.com.au You can see how we wired the sockets to the PCB, all via polarised plugs or screw terminals. Capacitance, Inductance and Inductance Saturation. If you press the Enter/OK button, the meter will display “Measuring Resistance”, “Measuring Capacitance”, “Measuring Inductance” or “Measuring Inductance Sat’n” respectively and go off and measure the value. Note on our case we labelled “Enter” as “OK”. The standard measurements take about a second, while the inductor saturation tests need to perform quite a lot of measurements and take longer. Because we are dealing with inductors carrying a lot of current, we also need to provide a decent charge and decay time. So the inductance saturation test can take a few seconds, depending on the value of the inductor under test. Australia's electronics magazine The results are displayed on the screen and, once presented, you can press Enter/OK to repeat the measurement. If you want to change between resistance, capacitance and inductance measurements, press the up/down keys to cycle through the options, then press Enter/OK to measure. After a saturation current measurement is complete, you can cycle through the 10 inductance values across the range the meter can provide. The maximum current the meter will test to is 30A, plus readings from 5% to 90% of the maximum current. We have selected this range to ensure that noise at the start of the measurement does not grossly affect results (although it may still affect it if April 2025  85 the inductor rings badly). By pressing up and down, you can review: • The current at which the measurement is made. • The percentage of the inductance value of the second inductance measurement, which is considered 100%. We chose the second measurement, as this was always ‘clean’ in our tests. • The value of inductance at the displayed current. Calibration If you don’t calibrate the meter, it will load defaults, which will work but definitely compromise accuracy. To calibrate the meter, apply power and hold both the up and down buttons continuously. The meter will present the question “Calibrate meter?”, “Y/N, Up/Dn”. Press the up button, and a series of help screens will walk you through the process. As you will see in operation, inductance values are ‘all over the shop’ with current, so we have kept calibration focused on the few key parameters. We can calibrate critical parameters, but we do not seek to create a ‘lab standard’; this is more of a working measurement system for power devices where a few percent precision is sufficient. There are five steps to calibration: Fig.12: drill the holes in the lid as shown here. It’s best to start with pilot holes and then enlarge them to size. For the rectangular cutout, you could use something like a jigsaw, but you can also drill lots of small holes within the outline, knock the centre out, then file it to shape. It doesn’t have to be perfect as the bezel will cover minor imperfections. 86 Silicon Chip siliconchip.com.au #1: 10mA constant current test The current measurements in steps 1-3 are important for resistance and capacitance tests. Connect a milliammeter across the DUT terminals. The Meter will drive a 10mA current. Measure this and use the up/down buttons to enter your measured value. Get this to within 0.1mA of your meter reading. prototypes, the minimum measurable capacitance was around 20nF, and we achieved reasonable performance for values of 100nF and above. This is a power device tester, and does not seek to measure low-value capacitors. Once this is all done, it stores the new calibration factors in EEPROM, and you are ready to start testing! #2: 100mA constant current test This is the same as step 1 but at 100mA. We housed our tester in an Altronics H0310 ABS box. The board mounts on the lid, with onboard buttons and switches passing through holes in that lid. The specified switches all have the same height, so provided you make holes in the lid that all align with the switches, this provides a very neat mounting arrangement. We have always struggled with mounting 16×2 LCDs as they don’t generally come with a bezel. Therefore, we designed a bezel that can be 3D-printed to match the Altronics Z7018 LCD. You can download the STL file from siliconchip.au/ Shop/6/605 If you use a different LCD screen, you might want to design a similar bezel to match yours, as it makes assembly easier and neater. Fig.12 shows the front panel/lid cutouts and drilling details, while Fig.13 (overleaf) shows the drilling required for the side of the case. The Kelvin probe connectors mount on the side; we used banana sockets, allowing us #3: 1A constant current test The meter pulses the current on for two seconds, then off for about eight. This reduces heating in the constant current sink. Make sure your meter is not on a low-current range when you connect it. Adjust the value displayed until it is within 1mA of your meter’s reading. #4: Measure 3.3VA This voltage defines the full-scale value for the ADC and affects all measurements. Measure the voltage between ground and the 3.3VA rail at TP8. Enter this into the meter using the up and down buttons. #5: Null capacitance Leave the DUT terminals open circuit for this stage. This measures the internal minimum capacitance and uses it to correct low readings. In our Putting it in the case to use Kelvin probes when we want to measure really low resistances. You don’t need to use them for normal inductor and capacitor tests. We also installed BNC connectors so that we could use an oscilloscope to monitor the current waveform – see the photo below. These are optional. You do need to mount a power socket. This meter needs a minimum of 12V. We selected a socket that matched our power supply; there are many options. We chose a convenient spot on the side of the enclosure for this, as shown. The arrangement of holes and connectors on the side is what we recommend, but you can customise this to your needs. Ensure that all holes are centred in the lower half of the case so the connectors will not interfere with the PCB. Fit the LCD bezel to the LCD now. Test-fit it before gluing anything in place, as we have seen 16×2 LCDs in so many configurations. Make sure that yours will fit before committing to glue. If you use the Altronics screen and our 3D-printed bezel, it should be fine. The bezel is a tight fit, so expect to jiggle the display to get it on. If necessary, you can use a knife to scratch/trim the printed bezel. Those who have used a 3D printer will be used to this fettling process. Glue the bezel in place with a few drops of superglue on the inside of This shows how we arranged the connectors on the side of the case. You can also see our snazzy Dymo labels. At least we’ll be remember what everything does when we come back to it in six months! On this side, everything but the power socket is optional. Still, if you want to measure low resistances, the Kelvin connectors are required. siliconchip.com.au April 2025  87 the enclosure. Then install the LCD in the bezel and glue that in place after double-­ checking that you have the LCD the right way up. The DUT screw terminals affix to the front panel and project through two matching holes in the PCB. Mount them and do them up tight; we will wire them up later. Mount the four 10mm standoffs to the PCB using machine screw and shakeproof washers, then jiggle the PCB to get the pushbuttons through the holes in the front panel. Make sure the back of the LCD is clear of your PCB. The LCD ribbon cable comes out to the side of the PCB and will reach the header. The PCB mounts to the front panel as shown in the adjacent photo. Now you can install 80mm of 7.5A or 10A rated wire between the DUT+ and DUT– terminals on the PCB and the red and black screw terminals. Fig.13: this is how we arranged the connectors on the side of the case. You might decide to leave some of these out so verify which connectors you actually need before drilling the holes. The front and side panels are shown opposite. The front panel is shown at 40% actual size, while the side panel is at full size. You can download both of them from siliconchip.com.au/ Shop/11/1832 We soldered ours directly to the PCB to minimise resistance, but the board accepts 6.3mm spade terminals and you could crimp 6.3mm spade lugs to these wires. If doing that, make sure the connections are nice and tight. We need to make provision for Kelvin connections required for measuring low resistances accurately. These connect to the PCB via CON1. We simply ran two 150mm wires to banana sockets on the side of the case. For monitoring the operation via an oscilloscope, we recommend mounting two BNC sockets. One connects to CON11 and provides a trigger signal, while the other goes to CON7 for current monitoring. We used 150mm ribbon cable offcuts to wires these up. We put these on the side of our case next to the Kelvin connectors as we don’t use them much and that was where there is room. These oscilloscope connections are optional but present some interesting data. You need a digital ‘scope set to single-shot mode to capture the data. Set the trigger level to 1V. The vertical scale of the current curve is 100mV per amp. Most pulses are pretty short; for low-value inductors, they are in the 10s of microseconds. Large inductors can be tested over a few milliseconds. If you look at the waveforms presented last month, you will see that inductor current curves are almost never straight. Where there is a reasonably high DC resistance but no saturation, they curve downwards, while if the inductor saturates, they curve upwards. Usage hints Never use this meter to test components in circuit. The currents it drives may destroy something. Never connect this meter to powered circuitry. We have protection for inductor back-EMF, but if the input is driven above the 10V rail, you will damage the Meter. Always discharge capacitors before connecting them – if they hold a charge above 10V, you might damage the Meter. You don’t need to use the Kelvin connections for anything but low resistances. If you want to measure resistances below a few ohms, you really should use them. With these, you can measure right down into the milliohm region. SC 88 Silicon Chip Australia's electronics magazine siliconchip.com.au ENTER Inductance | Capacitance | Resistance DUT+ DUT− UP DOWN POWER Our assembled board; the one below uses two large capacitors, as recommended, but it can be used with one. The heatsink shown here on REG5 is not necessary. ENTER Inductance | Capacitance | Resistance OWER LCR METER This shows the PCB mounted to the inside of the case lid, with the LCD ribbon cable in place. siliconchip.com.au Australia's electronics magazine + − POWER KELVIN DUT− DOWN 12V DC DUT+ UP TRIG MONITOR POWER LCR METER The lid artwork & connector labels – see the Fig.13 caption for details. April 2025  89