I have successfully built the 40V
Hybrid Switchmode/Linear Bench
Power Supply (April-June 2014 issues;
siliconchip.au/Series/241), a really
excellent and compact, fully adjustable supply to modernise my workbench.
As I was testing it, before enclosing
it in the case, I noticed the heat dissipation of the two linear regulators
was quite high for an input of 19.5V.
This design accepts up to 24V at the
input, meaning the dissipation could
be higher than in my instance. Based
on my finger test, these internal 7805
and LM2940 regulators appear to run
above 50°C, and the case has no ventilation.
I was pondering the lack of ventilation and whether I should drill holes
in the case but concluded that would
detract from its looks. I then recalled
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Dave Thompson........................ 111
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Emona Instruments.................. IBC
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Keith Rippon Kit Assembly....... 111
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SC Christmas Ornaments........ 109
SC Breadboard PSU...................... 8
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Silicon Chip Subscriptions........ 97
The Loudspeaker Kit.com............ 6
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112
Silicon Chip
another Silicon Chip project and wondered if the quiescent heat dissipation
could be improved in this project by
replacing the 7805 regulator with your
78xx Replacement project from August
2020 (siliconchip.au/Article/14533).
It looks possible to replace the
LM2940 12V regulator as well, using
the same approach.
Do you have any advice on whether
this would introduce any problems
or if it would require modifications
beyond the 78xx Replacement substitution? (B. R., Eaglemont, Vic)
● Both linear regulators are provided with fairly generous heatsinks
and contact with PCB copper, so they
should run well within their specifications, even if they get a bit warm.
50-60°C might seem hot, but their
maximum junction temperature ratings are 150°C.
Still, we understand the desire to
reduce power consumption and keep
the case cooler. Using thermal paste
between the regulator and heatsink,
and heatsink and PCB, could reduce
the junction temperature, as could
using slightly larger heatsinks. However, the same total power would still
be dissipated within the case.
Your idea of using a switch-mode
regulator is a good one. All that the
12V regulator (REG1) powers is a 7555
timer (IC2) driving charge pumps to
generate some auxiliary rails (-5V and
VBOOST) and 7805 regulator REG2,
which delivers the +5V rail.
None of those sections should be
bothered by the extra noise expected
from a step-down/buck regulator, such
as the one we published in August
2020.
By all means, try the substitution;
just verify that the supply doesn’t have
any odd behaviour after you swap the
regulator over. If it does (which seems
unlikely), you might need to add an
Errata & Sale Date for the Next Issue
Switchmode substitute
for warm regulators
RC or LC low-pass filter between the
output of that regulator module and
the rest of the circuitry.
You could probably also replace
REG2 (7805) with a 5V buck module
but we’d be a little more cautious with
that one. It drives the panel meters,
which should not be a problem, but
it also provides a reference voltage for
the voltage and current adjusting pots
and trimpots.
The safest thing to do would be to
leave REG2 as a 7805 but disconnect
the 5V rails going to the two panel
meters and run them from the output
of a separate 5V buck converter. We
don’t think that would cause any problems and would substantially improve
efficiency.
Identifying a kit sold by
a third party
I need your help to find the firmware
for a PIC16F84 chip. I purchased a Big
Clock kit from Quasar Electronics in
England many years ago. The kit was
a Big Clock model AS3073.
My old but very exact clock suffered damage to the PIC16F84 microcontroller and does not work anymore. I know it will be difficult to find
this old program, but I would greatly
appreciate it if you could help me. (R.
C., via email)
● We can’t find any mention of
“Quasar” or “AS3073” in any of our
magazines. Perhaps they took one of
our designs and turned it into a kit
without our knowledge.
We have published many clock
designs, but the one that seems most
likely to be a match is the Big-Digit
12/24-Hour Clock (March 2001 issue;
siliconchip.au/Article/4235). The software for that project is here (PCBs are
also still available): siliconchip.au/
Shop/6/2171
SC
Watering System Controller, August 2023: the original V1.2 version
software had two serious faults. It was not driving the correct I/O pins
as shown in the circuit diagram, and a calculation error could cause it
to water on the wrong day. V1.3 fixes those and adds a new SMTP relay
service for sending emails (SMTP2GO), as some users have had difficulty
opening a free account with SendGrid. Several minor changes were also
made to improve the web pages generated by the firmware. The new
firmware is available for free download from our website. If upgrading an
existing installation, you can just overwrite the four files in the WebMite’s
internal file system, then type RUN “RETIC.BAS” and press Enter. The
“settings.dat” file will automatically be upgraded.
Next Issue: the December 2023 issue is due on sale in newsagents by
Monday, November 27th. Expect postal delivery of subscription copies in
Australia between November 24th and December 12th.
Australia's electronics magazine
siliconchip.com.au
