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ASK SILICON CHIP Got a technical problem? Can’t understand a piece of jargon or some technical principle? Drop us a line and we’ll answer your question. Send your email to silicon<at>siliconchip.com.au Using Battery Balancer with supercaps Can I use the High-current Four Battery/Cell Balancer (March & April 2021; siliconchip.com.au/Series/358) to balance a “battery” of capacitors? I have a bank of six supercaps (2,7V each) connected in series, which I am using instead of a lead-acid storage battery. So I wonder if the Battery Balancer can be used to keep these supercaps in balance. (C. B., Bonville, NSW) • It should work with capacitors, given that their fully charged voltages are not that far from something like LiFePO4 cells. However, keep in mind that capacitors can discharge to a much lower voltage than batteries; once the total voltage is low, the device will cease operation. If you’re only concerned about it balancing when the capacitor bank is mostly full, it should work. Incorrect resistor colour code given In the article on the Arduino-based Power Supply from February 2021 (siliconchip.com.au/Article/14741), for the 51kW resistors in the parts list, it lists colour bands of “green brown black orange brown”, which is 510kW. In the PCB photos, it looks like you have used 510kW and not 51kW. Which one is correct? (R. S., Epping, Vic) • There is a typo in the parts list; it should read “green brown black red brown”. It correctly gives “green brown orange brown” as the four-band code, matching the 51kW value shown in the circuit and parts list. The photos show a four-band 5% resistor with an orange multiplier band, which gives 51kW. 510kW would be too high a value in that divider. Recommended software for 3D printing I would appreciate an article on software for drawing objects to print with a 3D printer. Lots of software seems to 108 Silicon Chip be available, but as I inspect the products I find, I find lots of haystacks, but no needles. AutoCAD is an obvious candidate, but at the ‘open heart surgery’ end of the spectrum. I found a product from the AutoDesk stable targeting schools, but it seemed to be more for play than work. I could do lots of fun things, but when I tried to define the gadget I wanted to build, it seemed offended and to be telling me I should migrate to AutoCAD. A friend recommended DraftSight. He seems to love it, but when I grizzle about the difficulties I’m having, he describes the commands that flummox me in an enthusiastic tone. I recently found its user manual, which is helping, even though I’m only up to page 70 of 180+ pages. Do you have any better suggestions? (K. A., Kingston, Tas) • We generally use OpenSCAD (https://openscad.org/), which has great precision for engineering-type designs. We have also used FreeCAD (www.freecadweb.org/) with some success, although not specifically for 3D printing. It does some things in non-obvious ways but seems quite capable. Blender (www.blender.org/) is another very popular software package; among its other capabilities, it can create 3D models from orthographic (plan/elevation view) drawings. USB SuperCodec oscillator frequency Thanks for publishing what looks like a great piece of audio test gear in the USB SuperCodec (August-October 2020; siliconchip.com.au/Series/349). I have a question regarding the oscillator frequency that clocks the ASRCs (asynchronous sample rate converters). Why have you used a 25MHz oscillator when 24.576MHz (192kHz × 2 7 ) is available? The 24.576MHz crystal would allow the ASRC to do its interpolation much more ideally mathematically when Australia’s electronics magazine upsampling from a standard 48/96kHz sampling rate. Also, when recording, you would want to set the sample rate to say 96 or 192kHz, which evenly divides 24.576MHz but not 25MHz. Am I missing something? Also, how is the hardware sampling rate set when recording? Finally, years ago, when I was doing high-speed digital design, I learned that controlling transmission line effects on the PCB can be important. So I expected to see 22-47W series resistors in the clock lines between the chips, or some other method to minimise reflections by controlling line impedances. How did you get away without them? (I. B., Armidale, NSW) • Phil Prosser responds: You are correct that 24.576MHz is a standard crystal frequency for audio applications because it is a power-of-two integer multiple of several common sampling rates, including 48, 96 & 192kHz. But the ASRCs work a bit differently than your standard ADC or DAC. The ASRC chips require the clock frequency to be at least 130 times the master input/output clock rates. 25MHz achieves this nicely, as 192kHz × 130 = 24.96MHz. On the surface, it might seem that selecting a closer sampling rate to the actual audio clock would be better. But when you look at how the ASRC IC operates, all calculations are made with 32-bit resolution. The ‘digital domain’ THD + noise level is between -148dB and -173dB, depending on the ratio of converted sampling rates. So the impact of the digital calculations on the overall THD + N figure of the device is vanishingly small. Our test results of the analog performance are representative of the native performance of the ADC and DAC ICs themselves, which are shown in the article to be outstanding. The sampling rate for the ADC (ie, recording) is always 195.3125ks/s, irrespective of the sampling rate the PC operates at. That is why there are siliconchip.com.au two ASRC chips on the board, and not just the one for output. One converts the lower input sampling rate from the MiniDSP up to 195ks/s for the DAC, and the other converts 195ks/s from the ADC down to a lower sampling rate to feed to the MiniDSP. The driver software on your computer configures the MiniDSP’s clock rate. This will never be more than 192ks/s, so the ASRC is always downsampling the data from the ADC to feed it to the computer. As stated above, that does not reduce the quality in any measurable way. The use of the ASRCs allows us to drive both the ADC and DAC with a single clean, jitter-free clock source. The ASRC interfaces between this ‘clock domain’ to the lowerfrequency MiniDSP MCHStreamser ‘clock domain’, avoiding potential jitter problems from the XMOS processor used for the USB Interface on the MiniDSP card. As for the high-speed signals, the rise and fall times are what gets you in high-speed digital design. This can be mitigated with good layout techniques, including controlled impedance in the routing. There is termination on the MCLK line, which I included as a ‘belts-and-braces’ measure. I have designed quite a few circuits involving digital audio interfaces, and found them quite tolerant of ‘average’ routing practice. I did check the MCLK signals for bad behaviour. With the kit I have, I could only measure a very nice looking waveform. That said, a better test kit (say a 300MHz scope, now I have an excuse to buy one!) might have shown more. If you look at the top and bottom layers around the digital signals routed from the clock IC and the ASRC to the ADC and DAC, you will see that the bottom layer is an almost complete ground plane, with only one break that I could not avoid. I spent hours trying to get rid of that interruption! I believe this has helped keep the digital signals clean by minimising the size of current loops. Trouble calibrating Ultrasonic Cleaner I am having trouble with the HighPower Ultrasonic Cleaner (September & October 2020; siliconchip.com. au/Series/350). I tested the transducer siliconchip.com.au from the earlier version (August 2010; siliconchip.com.au/Article/244) and the one I bought recently from your Online Shop. Both transducers work fine on the older unit. Using the re-calibration method described in the October 2020 issue with 3L of water, the resonance climbs to 56kHz. If I reduce the water volume to 1.2L, the resonance after re-calibration is 19kHz. I have tried winding a different number of turns on the transformer secondary in steps of upwards of 10 each time, and I have tried it with as many as 75 turns. The results are similar. Using the diagnosis mode, with 57 turns and 1.2L of water, I get 2.09V (maximum) at TP1 at 38.73kHz, with 240V peak-to-peak at the transducer. With 75 turns and 1.2L of water, I get 2.09V (maximum) at TP1 at 38.73kHz, with 300V peak-to-peak at the transducer. I was able to achieve 4.3V at TP1 with about 500mL of water at 38kHz with 75 turns in diagnosis mode. As soon as I shut down and return to re-calibrate with different water levels, I end up with a resonance frequency that is either way too low or way too high. The supply voltage is correct. Any thoughts and help would be appreciated. (P. J., Adelaide, SA) • It seems that the current measurement is not working correctly, as the higher voltage applied to the transducer (300Vp-p) compared to 240Vp-p at the same frequency and the same amount of water does not change the current reading. Check the current reading section of the circuit, including IC2 and its associated parts. Check the 100nF capacitor at pin 5 of IC2. Also, check the windings on the transformer. The voltage output with 57 turns should be closer to 100V RMS. It is possible that the coupling to the water basin is damping resonance, depending on what is used to attach the transducer and what the basin is made from. Altering RGB Xmas Star bypass capacitors I have built the RGB version of the November 2020 RGB Christmas Star (siliconchip.com.au/Article/14638) and have mounted all components except for the 100μF electrolytic capacitors. Australia’s electronics magazine In place of the five 100μF electrolytic capacitors, is it acceptable to use 47μF tantalum capacitors? (K. J., Campbelltown, NSW) • That should be fine. Arguably, 47μF tantalum capacitors are superior to 100μF electrolytics. That project went a bit overboard on bypassing; probably only one capacitor per board is really necessary, or perhaps two. So reducing the capacitor values by half should not cause any problems, especially since your tantalum capacitors will likely have reasonably low ESR values. Shunt resistor values to use for audio pots Back in 2019, I built the Silicon Chip Ultra Low Noise Remote Controlled Stereo Preamplifier (March & April 2019; siliconchip.com.au/Series/333), but I had problems with the VR1b section of the motorised pot. The pot track’s ground end wasn’t connected properly to the solder tag, causing an open/high resistance circuit, which resulted in the right channel having a higher volume. I managed to use a pair of blunt cutters to squeeze the rivet together, restoring continuity. The preamp works fine; my only quibble is that the bass/treble pots are too close together, only allowing the use of tiny plastic knobs. Last year, I was given a 1RU rackmount case to put the preamp in, and decided to desolder all the preamp pots to space them apart further by using shielded cables, allowing the use of 32mm knobs. When the preamp was fired up in the 1RU case, the left channel had low audio volume intermittently, and the right channel had high volume intermittently. The VR1b ground end solder lug was going open circuit again, and the VR1a connection was also going open-circuit intermittently. I pulled the pot off the gearbox to allow better access to V1b’s track crimp. This restored the balance, but unfortunately, I think I left out/broke a part in the plastic clutch. The motor runs forward/back OK, but the pot shaft only occasionally moves now. Last week, the audio level problems recurred (Rotorua’s H2S levels don’t help either). VR1a’s ground end track rivet isn’t responding to recrimping. A new pot is the only solution. Altronics have the Cat R1998 motorised pot on June 2021  109 backorder, with no indication of when it will be available again. The March 2019 article stated that a dual 20kW log pot could be used instead, with a 4.7kW resistor shunting the wiper. But I can’t find any supplier that has a dual 20kW log pot. I can get 10kW or 50kW standard pots that I could use while waiting for a new motorised pot. What shunt resistor values should I use with those? (D. M. C., Rotorua, NZ) • You can use 10kW log pots without adding the 4.7kW ohm resistors (or any other value), although if you do want to add resistors to those, use 10kW. For the 50kW pots, use the 4.7kW resistors specified. These resistor values are not critical. They are there to lower the impedance when higher-valued potentiometers (compared to the 5kW specified values) are used. The resistors will alter the law of the log pot, so the values are a compromise between reducing noise, where low values are better, and obtaining a reasonable volume control resistance law, where higher values are better. Choosing a coil for Multi-Spark CDI Thanks for your answer to my questions on a CDI system for Kawasaki jet skis in the January 2021 issue (p110). Digging deeper into the articles and designs, I concluded the multi-spark was not suitable for twin-cylinder engines that use a wasted spark, firing both plugs every time. I can’t find any kits available for the High-Energy Multi-Spark CDI (December 2014 & January 2015; siliconchip. com.au/Series/279), so now I have ordered the PCB from you and am getting the parts from Jaycar today. I still have one design conundrum: how do I choose an appropriate coil that it will drive comfortably? Most coils don’t come with specifications, and automotive stores (Repco, Autobarn, Supercheap Auto etc) can’t give me specifications on the coils they sell either. For a 650 twin-cylinder two-stroke jet ski with CDI, the original coil specifications give a primary resistance of 92mW ±15% and a secondary resistance of 4.1kW ±15%. This is the lowest primary impedance that I can find. From the article, the primary is getting 350V from the CDI, not 12V, and 110 Silicon Chip there is no mention in the article about a suitable impedance. How can I tell if I am about to blow up the coil? I plan to replace the CDI and coil as a matching package. (L. C., Donvale, Vic) • The ignition coil specifications are not critical; the CDI unit should work with the coil you plan to use. Because it is a capacitor discharge type that applies a pulse to the coil, rather than charging the coil conventionally and releasing the charge to fire the coil, most coils will work. There is no coil saturation current to be concerned about. Problems compiling NTP time source code I am having problems programming the Internet Based Time Source (The Clayton’s “GPS” time signal generator, April 2018; siliconchip.com.au/ Article/11039). The ESP8266 code fails to compile with the following error code: Arduino: 1.8.13 (Windows 7), Board: “WeMos D1 R1, 80 MHz, Flash, Legacy (new can return nullptr), All SSL ciphers (most compatible), 4MB (FS:2MB OTA:~1019KB), v2 Lower Memory, Disabled, None, Only Sketch, 57600” ... Using library ESP8266WiFi at version 1.0 in folder: C:\ Users\Home\AppData\Local\ Arduino15\packages\esp8266\ hardware\esp8266\2.7.4\ libraries\ESP8266WiFi Using library ESP8266HTTPClient at version 1.2 in folder: C:\ Users\Home\AppData\Local\ Arduino15\packages\esp8266\ hardware\esp8266\2.7.4\ libraries\ESP8266HTTPClient exit status -1073741502 Error compiling for board WeMos D1 R1. (J. R., United Kingdom) • We tried installing the same versions of the software (Arduino IDE 1.8.13 and ESP8266 board files 2.7.4) on Windows 10, but can’t recreate your error. Since the error occurs at the ESP8266HTTPClient library, we suspect that you have a problem with the files installed for that library, or one of the other libraries that it depends on. We have read reports of similar problems when other (unrelated, but Australia’s electronics magazine incompatible) WiFi libraries are present, causing the compiler to become confused. More detailed (!) error messages can be set under the Preferences page (File → Preferences); tick “Show verbose output during compilation”. That could point to another library causing issues with the ESP8266HTTPClient library. Transformer choice for SC200 amp Your articles on the SC200 Audio Amplifier module (January & February 2017; siliconchip.com.au/Series/308) mention using a 30-0-30 160VA transformer for the lower-power version. Do you mean one transformer for each module in a stereo setup, or one transformer shared between both modules? (T. B., Bumberrah, Vic) • Unless you need to deliver full power continuously (unlikely with any sort of program material), one transformer shared between two modules should be fine. A 160VA toroidal type would be a reasonable choice for a lower-power version of the SC200. A 300VA transformer would probably be overkill, but it would allow both modules to deliver full power on a sustained basis. Converting mechanical speedo to electronic I have a rear-engined car. The speedo is a mechanical drive via a long flexible shaft that is prone to failure. I am also conscious of the load on the plastic gears in the car gearbox speedo drive. The speedometer instrument is part of a set on the dashboard, also containing the odometer and trip odometer. I wanted to adapt a small gear motor to drive the speedometer and control its speed with a PWM kit to overcome the mechanical unreliability. A quick test shows that 400RPM gives about 100km/h on the speedometer. What do you suggest as an electronic connection between the gearbox output and the motor on the mechanical speedometer? (G. T., Londonderry, NSW) • A standard speedometer sensor from a vehicle with electronic speedometer connections could be adapted to fit into the gearbox cable attachment. continued on page 112 siliconchip.com.au That signal could then be applied to the motor drive circuit. The motor drive circuit would need to convert the speedometer sensor signal to a voltage drive for the motor driving the mechanical speedometer. The required circuitry would depend on the components used. A simple voltage-controlled PWM driver might not be effective, as it will only provide open-loop control. Some form of motor speed feedback is required to ensure the motor rotates at the correct speed despite the varying battery voltage and temperature. You might be able to convert the speed sensor signal to a voltage using a simple RC filter that is then used to control a PWM motor controller such as our DC Motor Speed Controller Mk.2 (June 2011; siliconchip.com.au/ Article/1035). Alternatively, a vehicle instrument specialist can supply many of the parts you need. See www.attspeedautoinstruments. com 2-layer PCBs supplied for older designs I am currently building the Electrolytic Capacitor Reformer (August & September 2010; siliconchip.com. au/Series/10). I have received the PCB from you and am very impressed with the quality. The instructions say to solder in 11 wire links. However, the supplied board is double-sided with plated through-holes, and I assume it has printed wire links on the component side. Can I skip fitting the wire links? (K. C., Strathfield, NSW) • When we supply boards that were single-sided designs these days, we tend to place the links in the top layer as it costs very little to do so. That includes the board you have. You can check one or two of the links using a continuity meter, pressing the probes into the vias/throughholes at either end of where the link is supposed to be. That will verify the presence of those top-layer tracks. Rarely would we get boards made based on old designs without adding links to the top layer. Higher supply voltage for SC480 amp Can I use a 60V centre-tapped transformer to power SC480 Audio Amplifier modules (January & February 2003; siliconchip.com.au/Series/109) instead of a 56V centre-tapped transformer? (J. A., via email) • No SOA curves were published for the SC480, so it’s hard to evaluate the effect of changing the supply voltages. Given that you’re only talking about a couple of extra volts per rail, if you plan to drive 8W or 6W speakers, it should be OK. Still, we suggest changing the BC557s to BC556s for a bit of extra safety margin. It would help to know the VA rating of the transformer, and it would also be helpful to measure the actual voltage, as it can vary quite a bit from the nominal voltage. The DC supply rails are given as ±40V, so if you build the supply and get unloaded readings of around ±42V or ±43V, that would not be particularly worrisome. ±45V or higher might cause problems, though. SC Advertising Index Altronics...............................87-90 Ampec Technologies................... 9 Dave Thompson...................... 111 Digi-Key Electronics.................... 3 Emona Instruments................. IBC Hare & Forbes....................... OBC Jaycar............................ IFC,53-60 Keith Rippon Kit Assembly...... 111 LD Electronics......................... 111 LEDsales................................. 111 Microchip Technology.................. 5 Ocean Controls........................... 8 PMD Way................................ 111 Premier Batteries...................... 37 SC Vintage Radio Collection..... 63 Silicon Chip Shop.................... 97 Switchmode Power Supplies....... 7 The Loudspeaker Kit.com......... 93 Tronixlabs................................ 111 Vintage Radio Repairs............ 111 Wagner Electronics................... 10 Notes & Errata Programmable Hybrid Lab Supply with WiFi, May 2021: in the parts list on page 36, the item at the top of the right-hand column should have read VXO7805-500 (5V) rather than VXO7803-500 (3V). The circuit should still work even with the 3V part fitted. Also, the MCP4725 DAC specified comes in several versions; MCP4725A0T-E/CH is the required part. Arduino-based Power Supply, February 2021: the 51kW resistor’s five-band colour code is incorrect. It should read “green brown black red brown”. DIY Reflow Oven Controller, April & May 2020: in the May 2020 issue on page 90, Fig.11 shows the 20-wire ribbon cable between the control board and LCD screen connected incorrectly. It is shown correctly in the photo at the top of p89, with the red stripe going to pin 1 on both boards. Deluxe Touchscreen eFuse, July 2017: The HEX file we have been providing has not had the AUTORUN flag set, meaning eFuses built with a preprogrammed chip or using the HEX file from the Silicon Chip website will not work without being run manually from MMBasic. We’ve updated the HEX and MMBasic files to fix this and also to fix a bug that may cause the Micromite to crash and reset if the screen timeout was set to certain values. The July 2021 issue is due on sale in newsagents by Monday, June 28th. Expect postal delivery of subscription copies in Australia between June 25th and July 9th. 112 Silicon Chip Australia’s electronics magazine siliconchip.com.au