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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
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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
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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
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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
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