This is only a preview of the August 2022 issue of Silicon Chip. You can view 0 of the 104 pages in the full issue. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Wide-Range Ohmmeter, Part 1":
Items relevant to "isoundBar with Built-in Woofer":
Items relevant to "SPY-DER: a 3D-printed Robot":
Items relevant to "Secure Remote Mains Switch, Part 2":
Purchase a printed copy of this issue for $8.50. |
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
How to convert 12V AC
to 24V DC
I want to add low-voltage LED retro
festoon string lights to an existing
garden lighting system. All the units
that look good and are robust require
24V DC, whereas the garden lighting
is 12V AC.
Additional cabling would require
lifting up pavers & digging up an established garden. So I am keen to know
whether I can use a small 230:115V
AC transformer in reverse to step up
the voltage and then regulate it to
DC. The 12V transformer has plenty
of spare capacity as originally sized
for 12V halogens which have been
replaced with 12V LED units. (T. H.,
Batehaven, NSW)
● It’s possible that the step-up transformer would work; it depends on the
details of the transformer, but likely it
would give you double the AC voltage.
You would still have to convert that
to DC. However, there are simpler/
easier ways.
To start with, a simple full-wave
voltage doubler feeding a pair of
4700µF 16V electrolytic capacitors
will give you pulsating DC averaging
around 25V DC with a 2A load. Using
pairs of diodes in series would drop
that to be very close to 24V DC, as
shown in Fig.1. The capacitors need
to be low-ESR types with a high ripple current rating, ideally at least 2A.
There will be a few volts of ripple
across the LEDs (6V peak-to-peak
according to our simulation). We suspect that won’t bother them, but it
depends on their exact design.
The output voltage of that circuit
is somewhat load-dependent; for a
1A load (say), you would just need to
reduce the filter capacitors to 2200µF
and eliminate the extra series diodes
to get much the same output voltage.
If you need the DC supply to be
ripple-free, there are a few ways to
achieve that. Fig.2 shows a similar circuit with a very basic linear regulator
based on an NPN transistor and zener
diode. Simulation shows it delivers
a smooth ~24V DC output. The transistor should be a type with decent
gain up to a few amps (such as the
KSC2334Y shown).
Q1 will dissipate close to 10W, so it
will need a decent heatsink; an IP65
sealed metal case could be used to
house the circuit and also as a heatsink, with an additional finned heatsink bolted to the outside directly
opposite the transistor. The output
ripple with this version is a fraction
of a volt.
With a linear design like this, it’s
hard to avoid dissipating a few watts
if you need a mostly ripple-free output. For a more efficient approach,
try using a rectifier/filter circuit like
those shown here with Tim’s Blythman’s Buck/Boost LED Driver (June
2022; siliconchip.au/Article/15340) to
convert the pulsating DC to a smooth,
regulated DC. We have a kit for that
project (siliconchip.au/Shop/20/6292).
Because that board can deal with
an input voltage above or below the
output, the exact voltage being fed
to it isn’t critical. However, it’s better to arrange for the input voltage to
be above, or at least close to, the output voltage. That will give maximum
efficiency. It can deliver around 5A
in this configuration, given sufficient
filter capacitance on the output of the
rectifier.
Dimming for Buck/
Boost LED Driver
I ordered the Buck-Boost LED Driver
kit (SC6292; June 2022, siliconchip.
au/Article/15340) and LED panel from
you last week, and the package arrived
promptly early this week; thank you.
It’s now assembled and running nicely.
Compliments to whoever laid
the PCB out, as it’s obviously been
designed for ease of assembly. Having
most of the smaller components lined
up across the board’s edges with large
pads made it quite easy to place and
solder them.
One comment about the assembly
instructions: nowhere did I see any
mention of soldering the ground pad
of the LM5118 to the PCB. I heated the
via on the underside of the PCB with
a hot iron and fed a quantity of solder
into the via. It seemed to wick in, so
hopefully that worked.
Fig.1 (left): the full-wave voltage doubler will give close to 24V DC from 12V
AC, but with substantial ripple.
Fig.2 (below): adding a very basic linear regulator applies a smooth 24V DC
to the load, but with about 10W dissipation. Using the Buck/Boost LED Driver
instead would result in much lower losses.
100
Silicon Chip
Australia's electronics magazine
siliconchip.com.au
Getting to the point of my email,
Can you suggest a modification to add
a dimmer control? I want to make a
work light with the LED, but it’s very
bright. I’d thought the current limit
adjustment pot might work, but it only
reduces the current to 1.5A, which
is still very bright. (D. S., East Melbourne, Vic)
● Thanks for the kind words about
the layout. We must admit that the
LM5118 is well suited to a convenient
layout. You are correct that we should
have mentioned the thermal pad on
IC1 – we soldered it on our prototype. We have published an erratum
to make readers aware of this. With
hand soldering, the process you used
is about the only practical way to solder such a pad.
Unfortunately, the current control
does not lend itself well to a minimum value near zero as it depends
on the current rising high enough to
overcome the diode threshold above
the 1.23V reference voltage. If you
are comfortable with a proportionally
lower maximum current, substituting a higher-value shunt resistor than
15mW (the one between TP4 and TP6
only) should work.
Another option is to apply a PWM
signal across JP1, which should effectively PWM the output. We haven’t tested this; you might need to
remove (or reduce) the 100nF capacitor labelled C14 attaching to IC1’s pin
7 (7th component from the left along
the bottom) to cut out the soft-start
ramping. We think a relatively low
PWM frequency would give the most
linear response.
Finally, you could simply add an
external voltage control pot. It won’t
give perfect control but should let you
cut the brightness way down, while
the current limiting will prevent any
damage to the LED at the upper end
of its range.
With the current limit at 4A, the
voltage pot range from off to full will
be about 700W to 400W. So a 500W pot
with a 750W parallel resistor between
the wiper and one end, plus a 390W
resistor in series with both, should give
a suitable adjustment range. This combination can be wired up in place of
the onboard 5kW multi-turn trimpot.
Solar PV (photovoltaic)
water heating
What is the strategy to connect solar
panels (in series or parallel) to a water
heater with a resistive element, say
about 2.4kW? I expect you need to
interface with maybe an inverter and
batteries. Perhaps you have covered
this previously in the magazine. (F.
C., Maroubra, NSW)
● We have covered this previously
in the magazine on several occasions.
There are various ways to do what you
are asking, all of which have problems.
For example, see:
• September 2013, pages 98 & 99
• September 2014, pages 98 & 99
• October 2017, page 96
• December 2017, pages 4 & 5
The bottom line is that if you’re
going to use solar power to heat water,
you ideally want an electric water
heater with dual elements (a main
element and a ‘booster’) so that you
can power one from the mains and
the other from solar power. That way,
you’ll always have hot water.
The problem with feeding solar
power to an inverter with the output of the inverter driving one of the
water heater elements is that it likely
won’t deliver any power unless there is
enough solar power available to drive
the element at its full power rating (eg,
2.4kW). So that approach is generally
not going to work well.
That leaves the idea of arranging the
panels so that they produce around
240V DC in peak sun under load and
driving the heater element with DC.
This has the advantage that it will provide whatever power is available, even
if your panels cannot deliver enough
power to achieve the full rated element power. It’s also going to be the
most efficient method.
However, using DC will promote
corrosion, so it is necessary to arrange
for a contactor or similar to reverse the
polarity periodically (eg, every 12 or 24
hours). Also, it is unclear whether the
thermostat will last long if it’s switching DC rather than AC.
Finally, you would probably need to
get a licensed electrician to do the wiring if the heater is also mains-powered,
and it’s unclear whether your average
electrician would want to do this sort
of work.
In conclusion, you’re probably better off installing a standard solar hot
water system as they are designed for
the job.
Help to find a
Coilcraft part
I am having trouble finding one of
the Coilcraft items for the Precision
AM/FM DDS Signal Generator (May
2022; siliconchip.au/Article/15306).
Please verify the part number for the
Coilcraft 1206CS-121XJEC 120nH chip
inductor. It does not appear to exist on
either the Coilcraft site or Tricomponents site. (J. S., Avondale, Qld)
● It seems like a valid part number.
Here it is on Coilcraft’s website:
siliconchip.au/link/abff
We don’t think that using that particular part is especially critical;
Raspberry Pi Pico BackPack
With the Raspberry Pi Pico at its core, and fitted with a 3.5inch touchscreen. It's easy-to-build and can be programmed in
BASIC, C or MicroPython. There's also room to fit a real-time
clock IC, making it a good general-purpose computer.
This kit comes with everything needed to build a Pico BackPack module, including
components for the optional microSD card, IR receiver and stereo audio output.
$80 + Postage ∎ Complete Kit (SC6075)
siliconchip.com.au/Shop/20/6075
The circuit and assembly instructions were published in the March 2022 issue: siliconchip.au/Article/15236
Australia's electronics magazine
August 2022 101
many manufacturers have 120nH chip
inductors that could be used instead
and might be easier to get.
Note that element14 stocks the
XGLC and XJLC versions of those chip
inductors, and they would be fine in
this application.
Yet another version of
the R80 Receiver kit
I refer to the review of the R80
Receiver kit in the November 2021
issue (siliconchip.au/Article/15101).
I have built this unit but am having
problems with the modifications on
page 43. The BC548 emitter is connected to pin 9 of the display PCB
plug (GND), while the collector is
connected to the junction of R18 and
D3, not the junction of D2 and CP5
as shown.
The BC548 is mounted with the flat
towards the board, not away from it as
shown in the article. I have checked
for errata in later editions but could
find nothing on this. Was this a later
modification? (J. P., via email)
● Andrew Woodfield replies: your kit
appears to be an earlier (!) version than
the one we reviewed, most likely V6
(the review was of V7). Most readers
who reported discrepancies between
their kits and our review had later versions of the kit.
We aren’t sure if you need to make
the squelch modifications to the V6 kit.
It may help, but it will have to be done
differently as the design is obviously
not the same. It would help if you had
a circuit for your version of the kit to
compare to the one for V7 posted on
the Silicon Chip website (siliconchip.
au/Shop/6/5950).
Flight level is based on
pressure, not altitude
I am reading the “Advanced GPS
Computer” by Tim Blythman (June &
July 2021; siliconchip.au/Series/366),
and I hope you’ll indulge me in clearing up an uncertainty. The Computer
can show flight levels (FL) as well as
altitude, but I missed seeing how barometric pressure is measured (which is
necessary to convert altitude to FL).
Many thanks for pointing this out to
me; I hope I haven’t missed the obvious. (G. M., aircraft museum curator,
London, UK).
● The Advanced GPS Computer uses
the altitude data from the GPS receiver
102
Silicon Chip
module, so it does not calculate an
altitude or flight level based on barometric pressure. So it is more correct
to say that it can display the altitude
in the same units as flight level, but
does not display a true flight level
based on pressure.
We’re considering an update to the
project which adds a barometric pressure sensor like the BMP280 so that it
can show the proper flight level.
Design for density
altitude meter wanted
Have you published a design for
a density altitude meter? (L. B., via
email)
● We have not, but we have published
hardware designs that could be used as
a density altitude meter with some relatively simple changes to the software.
For example, the Touchscreen Altimeter and Weather Station (December
2017; siliconchip.au/Article/10898).
It has an onboard barometric pressure sensor and temperature sensor.
Those provide the two values that you
need to compute the density altitude.
The BASIC software is available online
(siliconchip.au/Shop/?article=10898),
so it should be relatively simple to
modify the software to calculate and
show the density altitude, then upload
that to the BackPack.
Transducer power for
Ultrasonic Cleaner
I am interested in building the
“Large Ultrasonic Cleaner” featured
in the August 2010 issue (siliconchip.
com.au/Article/244) but with some
modifications.
First of all, I am wondering if it’s
possible to use a 35W transducer
instead of 50W as the larger unit is
somewhat hard to get at the moment,
and the smaller unit can be had for a
very good price on sale. If so, I imagine that some changes may need to
be made to the drive circuit to avoid
over-driving the transducer.
Secondly, to compensate for the
lower-power transducer, I was thinking of running two in parallel. I imagine this achieved not by simply running both transducers from one circuit,
but by duplicating the drive circuitry
(Mosfets and transformer), running
both sets from one microcontroller.
Is that feasible? Or would two transducers on the same cleaning tank
Australia's electronics magazine
interact negatively with each other?
(A. C., Auckland, NZ)
● You can use the 35W transducer at a
reduced power level. We don’t recommend using more than one transducer,
even if the driver section is duplicated.
That’s because each transducer needs
to operate at the correct frequency for
the selected output power. Individual
transducers will have slightly different
resonances, so one transducer will be
delivering the majority of the power
output if two are used.
Note that we published a revised
version of that project in September
& October 2020 (High Power Ultrasonic Cleaner, siliconchip.com.au/
Series/350). Altronics sell a kit for the
newer design, Cat K6022.
BWD power supply
circuit diagram wanted
In the Serviceman’s log column in
November 2010, you detailed a repair
to a BWD 207B power supply and mentioned that you had obtained several
versions of the service manual. I have
tried various methods to try and obtain
a service manual for this power supply but to no avail. Are those manuals
available on your website, and if so,
where? (P. A. S., via email)
● We haven’t uploaded the BWD 207B
manual or circuits to our website but
we can supply them upon request.
2010 DAB+ Tuner has
limitations
I have had this DAB+/FM Tuner
(October-December 2010; siliconchip.
au/Series/13) running for many years
now (I built it from the Jaycar kit) without any problems.
The ABC has recently updated
(reassigned bandwidth) to their DAB+
transmissions. They recommended
re-scanning the stations, so I did, but
the scan did not complete.
I turned off the unit and found that it
only found 62 stations of the 72 available in Melbourne. I did this at least
three times with the same result each
time. I did notice that some but not
all of the ABC stations were scanned.
Once, it left out ABC Melbourne.
I have the latest firmware of 7.71. Do
you know why it is not picking up all
the channels? (A. L., Watsonia, Vic)
● It turns out that the 2010 DAB+/
FM Tuner firmware has a hard-coded
continued on page 104
siliconchip.com.au
limit of 64 stations in total that it can
store. We are unsure of the reason for
this limitation as the designer of that
project left Silicon Chip many years
ago. We assume it is due to the limited amount of flash memory or RAM
available.
It’s possible that could be changed,
but we no longer have a prototype to
test new firmware, nor are we confident that compiling the source code
with a current compiler will necessarily produce working code without
being able to test it.
Our much more recent Touchscreen DAB+/FM/AM Tuner design
(January-March 2019; siliconchip.au/
Series/330) does not suffer from this
problem. It must have an upper bound
on the number of stations it can tune,
Advertising Index
Altronics.................................43-46
Dave Thompson........................ 103
Digi-Key Electronics...................... 3
element14................................... 11
Emona Instruments.................. IBC
Hare & Forbes............................. 13
Jaycar.......................... IFC, 5, 7, 21,
..............................24-25, 77, 79, 99
Keith Rippon Kit Assembly....... 103
LD Electronics........................... 103
LEDsales................................... 103
Microchip Technology.................. 9
Mouser Electronics..................OBC
Ocean Controls............................. 8
SC Pico BackPack.................... 101
but nobody has run into it yet. Also,
the source code is available (it’s written in BASIC), so it could easily be
fixed if such a limitation existed.
While the parts for that project are
somewhat hard to come by, we do
have a handful of Si4689 ICs on hand
for anyone who wishes to build one,
and the PCB, Explore 100 kit and other
associated parts are still available – see
siliconchip.au/Shop/?article=11369
Consider, though, that this newer
project requires some fairly small
SMDs to be soldered. But we think
that is a better situation than the old
tuner, which needed a module that
wasn’t available to purchase (it only
came as part of the now-discontinued
Jaycar kit).
Replacement pot for
Class-A amp
My 20W Class-A amplifier (MaySeptember 2007; siliconchip.com.au/
Series/58) is in need of a new volume
pot as it has gone all scratchy.
It is an Alpha dual-gang 20kW log
motorised pot, previously sold by
Altronics as Cat R2000 but no longer
available. Do you know of a suitable
replacement? Is it possible to retrofit
one from another brand? (N. M., Sunbury, Vic)
● We suggest you try spraying some
contact cleaner into the wiper assemblies first as that might resolve your
scratchiness with much less expense
and hassle than replacing the pot
(eg, try Jaycar Cat NA1012). If you do
need to replace it, the pot value is not
so critical (eg, you could use a 10kW
or even 5kW dual-gang log pot). The
main concern is it fitting on the existing preamp PCB.
Bourns Pro Audio PRM162-K420K103A1 is a 10kW dual gang logarithmic
pot with dimensions very similar to
the Alpha unit Altronics sold. The PCB
SC SMD Test Tweezers.............. 63
Silicon Chip Shop.................70-71
Silicon Chip Subscriptions........ 23
Silvertone...................................... 6
The Loudspeaker Kit.com.......... 10
Tronixlabs.................................. 103
Wagner Electronics..................... 73
104
Silicon Chip
Errata and Next Issue
Silicon Chip Binders................ 103
probably would need some slight modifications to make it fit (due to slightly
different motor mounting posts) but
we think it would not be too hard to
retrofit. Unfortunately, it is somewhat
hard to find anyone with this in stock.
Verical list 142 in stock at the time
of writing and we have purchased
from them before, so we think that is
a reasonable option (siliconchip.au/
link/abdh).
Master Electronics also say they
have 147 in stock but we have no experience with them – see siliconchip.au/
link/abdi
Stereo Compressor kit
wanted
Is the Stereo Compressor kit still
available? It used to be sold by Jaycar,
but they no longer supply it; I hope
you can help. (A. B., Christchurch, NZ)
● We designed two different Stereo
Compressors that were made as Jaycar
kits, one in June 2000 (siliconchip.au/
Article/4328; Jaycar Cat KC5291) and
one in January 2012 (siliconchip.au/
Article/809; Jaycar Cat KC5507). Jaycar makes their kits based on our articles; we do not make them for Jaycar.
It does look like both Jaycar kits are
no longer available.
You might be able to get a kit for the
January 2012 design from Altronics
(Cat K5526). It is still listed on their
website; it looks like they are very low
on stock, but they have kits in a few
stores. Perhaps if you contact them,
they can get one back to their warehouse and deliver it to you.
Failing that, we can supply PCBs
and panels for the January 2012 design,
see siliconchip.au/Shop/?article=809
You would need to get the rest of
the parts yourself. It looks like they
are still available; the critical part is
the SA571 IC (in DIP), and they are for
sale on eBay.
SC
Spectral Sound MIDI Synthesiser, June 2022: the orientation of diode D2
in Fig.9 is incorrect. Install it with the cathode stripe facing to the right, as
shown on the PCB silkscreen.
Digital FX (Effects) Pedal, April & May 2021: Fig.2 in the April issue
shows incorrect connections for op amp IC3b. Its pins 5 & 6 are swapped.
Pin 6 (−) should be at the top, connected to the 4.7μF capacitors, while pin
5 (+) should be at the bottom, connected to Vcc ÷ 2. The PCB has the right
connections.
Next Issue: the September 2022 issue is due on sale in newsagents by
Monday, August 29th. Expect postal delivery of subscription copies in
Australia between August 29th and September 16th.
Australia's electronics magazine
siliconchip.com.au
|