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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that
Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to
submissions to “Ask Silicon Chip”, “Circuit Notebook” and “Serviceman”.
Electronic rust prevention doesn’t work on cars
issue is no exception, especially the letter from Chris Robertson of Sydney describing a visit to the Army Museum
I have noticed a lot of advertising for an electronic rust
of Military Engineering.
prevention device on TV lately. Could you do an article
The vehicle he described seeing is a Ground Elevation
on these devices, what’s in them, how they are supposed
Meter (GEM), formerly used by Army surveyors to calcuto work and why they don’t work?
late spot heights for cartography.
Bruce Pierson,
RAYMING TECHNOLOGY
As an Army telecommunications technician, in 1964 or
Dundathu, Qld.
so I was tasked
to repair the newly-introduced GEM vehiResponse: there are surprisingly
many ads on TV for
prodPCB Manufacturing
and
PCB Assembly
Services
cle
operated
by
the Army survey unit at Enoggera camp,
ucts which can’t work. WeFuyong
don’t understand
why
our
supBao'an Shenzhen China
Brisbane. I had no knowledge of the vehicle nor access to
posed ‘watchdogs’ allow consumers to be taken advantage
0086-0755-27348087
technical manuals, but there was an operator available.
of. Whether it is exercise equipment
which won’t improve
Sales<at>raypcb.com
The problem, as described, was that the GEM was not
your fitness in any measurable
way, or electronics which
measuring distance accurately or consistently.
can’t possibly do what itwww.raypcb.com
claims to do, they keep on getThe GEM was a clever instrument for the time. It comting away with it.
puted spot height by measuring the distances and angles
We have mentioned that electronic rust prevention won’t
as it travelled. It used a pendulum apparatus to measure
work on cars several times before, including March 2000
the angle and a fifth wheel to measure distance. It had
(p107), September 2001 (p100), November 2011 (p101)
four-wheel steering, so the chassis (and pendulum base)
and February 2013 (p90).
remained horizontal when making measurements. For
It works well for boats because the water they are imnormal driving, the rear wheels were locked in the conmersed in forms a path for current to flow. A sacrificial
ventional mode.
anode presents a more attractive route for those electrons;
A constant tyre outside diameter was maintained by a
hence, it oxidises before the rest of the boat. Hopefully,
centralised tyre pressure system which supplied air to all
your car is not permanently immersed in a pool of water!
tyres from a compressor driven by the engine.
Assuming it isn’t, there’s no obvious place to attach a sacWeight distribution was important, so only the driver
rificial anode to allow this current to flowing.
and operator could travel in the vehicle during measureThis is explained in more detail on the following web
ment. Even the comprehensive test equipment and toolpage: https://corrosion-doctors.org/Car/car-electronicboxes were precisely positioned to maintain a horizonrust.htm
tal platform.
I found the fault to be in the 5th wheel assembly. In
Mystery mapping vehicle identified
operation, the wheel was lowered to the ground or road
When my copy of Silicon Chip arrives each month, I
surface. The wheel was connected to a metal disc inside
read the Mailbag pages first, as I often find a gem of inforan enclosure, which was machined with over 900 fine
mation amongst the contributions. The September 2020
RAYMING TECHNOLOGY
Fuyong Bao'an ,Shenzhen, China Tel: 0086-0755-27348087
email: sales<at>raypcb.com web: www.raypcb.com
PCB Manufacturing and PCB Assembly Services
6
Silicon Chip
Australia’s electronics magazine
siliconchip.com.au
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Prices are subjected to change without notice.
Silicon Chip
Obsolete mobile phone networks
As I was part of the team that installed and commissioned the PAMTS (Public Automatic Mobile Telephone
Service) switch infrastructure in Sydney, I enjoyed reading Dr Maddison’s article about the history of mobile
phone networks (September 2020; siliconchip.com.au/
Article/14572).
After spending the following years leading the teams installing the GSM/CDMA networks, I was asked to recover
the PAMTS hardware! It was going to be sold to Vietnam
(with the possibility of me travelling there to reinstall it),
but in the end, they secured a loan from the World Bank
and went ahead with an up-to-date network.
Chris Newton,
Gidginbung, NSW.
Feedback on 78xx Switchmode Replacements
Current and Temperature Data Logger
8
slots around its periphery. It interrupted a light beam between a lamp and photoelectric cell, generating pulses
representing the distance travelled.
The fault was simple but expensive. A grub screw holding the lamp had come out and dropped into the rotating
disc case, damaging some slots, hence the problem with
accuracy and consistency.
Intricate work with a feeler gauge cleared the damage
sufficiently for the GEM to operate correctly, as a test run
to an accurately known height confirmed. I travelled in a
separate vehicle; watching the front and rear wheel sets
turn in opposite directions was odd.
A spare slotted disc was obtained from the USA at enormous cost, in case the problem recurred.
Peter Johnston,
Merimbula, NSW.
I built the 5V and 12V versions of the Switchmode 78xx
series regulator replacements (August 2020; siliconchip.
com.au/Article/14533) and would like to share some observations that may prove useful to others.
Both kits worked straight up driving a 20mA LED load,
with the 5V unit delivering 4.99-5.00V and the 12V version showing 11.5V. I was using external 47µF bypass
capacitors as recommended.
However, when I loaded the 5V unit with a 10W 5W
resistor, the output dropped to 4.44V, and to 4.2V with
a 4.7W load (~ 1A). Increasing the output bypass with
1000µF improved the droop to about 4.74V.
The 12V unit similarly dropped to about 10.8V with a
12W (1A) load. Both ran for about a minute or two before
starting to switch on and off at about 0.5Hz. I noticed the
inductors had become quite warm.
I had been running the input at ~16V (typical for a rectified 12V transformer). When I wound down the supply
to 12V (for the 5V unit), suddenly the output improved
to 4.9V at full load, and only a small bypass capacitor
was needed.
With the higher input voltage, a squeal could be heard
from the inductor – and this was when the 1000µF improved things. I suspect the is going into pulse-skipping
mode at the input higher voltages, and I doubt this is
great for inductor life. The 12V unit showed similar characteristics.
I clamped a TO-220 heatsink to the inductor, using silicone grease and a chopped-down clothes peg. With this
Australia’s electronics magazine
siliconchip.com.au
modification, and using no more than
12V input, the 5V unit drove a 4.7W
resistor at 4.9V for several hours (a little over 1A output).
So I think it is difficult to get more
than about 0.5A continuously from
these devices without a heatsink (probably less with a high ambient temperature), and it is best to have the input
voltage around 2-8V higher than the
target output voltage.
I was surprised to find that the
MCP16311 data sheet made no mention of using different value inductors
for different input voltages, but only
for different output voltages. My experience shows that the recommended inductors will not be suitable up
to the maximum 30V input claimed,
although moving the bypass capacitors closer to the package might help.
Ian Thompson,
Duncraig, WA.
Response: you are right that it is a bit
much to expect a given switchmode
regulator configuration (inductor value, compensation component values
etc) to work well over a wide range of
input and output voltages and currents. We agree that the inductor values recommended in the MCP16311
data sheet are likely only ideal when
the input/output voltage differential
is modest.
It sometimes helps to solder a lowvalue ‘feed forward’ capacitor across
feedback resistor R1 (it can be stacked
on the same set of pads). This reduces
closed-loop gain at higher frequencies
and can eliminate the ‘squeal’, probably also improving regulation. Squeal
can be a sign of subharmonic isolation, which is generally best avoided.
We will go back and analyse our
prototypes, and see if we can come up
with any recommendations to make
these devices less sensitive to the input voltage, and have less voltage sag
at higher currents. The need for heatsinking at higher continuous current
levels probably cannot be avoided (except perhaps with bulkier and more
expensive low-loss inductors).
Insulated mains pins are a good idea
On reading the article on GPOs
(September 2020; siliconchip.com.au/
Article/14573), I was reminded of a
couple of incidents from the past, before the introduction of insulation on
the Active and Neutral pins.
The first occurred while teaching a
secondary school class in a computer
10
Silicon Chip
Australia’s electronics magazine
room. The computers were on desks
around the edges of the room, plugged
into GPOs on the wall under the desks.
A student’s steel ruler got pushed back
and fell between the desk and the wall,
lodging neatly across the pins of the
power plug. The first I knew about it
was all the computers going off.
The second occurred while working at a residential facility for children
with Autism. One of the boys found a
5¢ coin. Thinking it was valuable, he
decided to hide it. The place he chose
was behind his nightlight.
He thought the resulting arc was
pretty cool, so he unplugged the night
light and repeated the process in a different GPO, which happened to be on
a different circuit. Once we worked out
why the power had gone out, we were
able to reset the circuit breakers and
continue preparing dinner. I gave the
boy a good 5¢ in exchange and kept
the melted one.
David Robson,
Goughs Bay, Vic.
DIY wiring is not a significant hazard
The available data does not back up
the Nannies who are trying to stop us
from building our own electrical devices.
“Between 1st July 2000 and 31st
October 2011, there were 321 electrocution deaths reported to Australian
coroners as identified and closed on
the NCIS database. Almost two-thirds
(62.0%) of these deaths were unintentional. Additional NCIS database
searches indicate that there are at least
an additional 39 electrocution deaths
still under coronial investigation.”
(siliconchip.com.au/link/ab2u)
Note that most electrocution data
includes lightning strikes, which comprises up to 20% per year.
This means that we have a large intentional death rate within electrical
deaths, meaning that mental health
is a much bigger problem than worrying about a very few DIY electrical
projects!
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The article goes on to say “Despite this, it is likely that
most of these deaths are still preventable.” This is an understatement, especially concerning mental health!
Looking at the top 20 cause of death from the ABS, electrocutions don’t rate; they are less than 0.1% of all deaths.
“Up until 2010, 15 Australians are killed and 300 hospitalised each year because of preventable electrical accidents in the home.”
The widespread use of safety switches should have now
solved most of these. Preventing people from doing electrical wiring will not change these numbers.
Another ridiculous situation is our certification and qualification system in this country. It persists the idea that to
keep everyone safe, the wiring rules should be hidden behind a very expensive paywall. Public safety information
should be free to all.
The Nannies and trade protectionist fail us miserably
in not blocking unsafe, poor-quality imported electrical
goods. I have tried to report obvious illegal imports, only
to encounter obfuscation.
Where are the nannies and bureaucrats at the docks
stopping this stuff? Hiding behind the piles of ideas about
a few DIY ideas are deadly to the majority of the population. There are orders of magnitudes many more deadly
devices imported every day than a few DIY projects, generally made by people who care about a good job and care
to make it safe!
Neil,
Footscray, Vic.
Comments on AWA radio restoration
I would like to comment on Associate Professor Graham Parslow’s Vintage Radio column on the 1940 AWA
Radiola 501 in the October 2020 issue (siliconchip.com.
au/Article/14613).
The comment about resistor R2 being rather blackened
(on page 89) points to a problem with C5 being excessively
leaky in the past. The longer it was used, the more leakage
there would be and the hotter the capacitor and resistor
would get. On other sets with similar circuits, 20kW 1W
is commonly used for R2.
Mounting the speaker transformer on the end of the chassis does get it away from the power transformer. However,
it is still a good idea to orientate the speaker transformer
laminations at right angles to the power transformer laminations (and often that is adequate, even if they are close
together).
Some sets are found with transformers mounted at rather
odd angles to overcome this problem when the two transformers are near each other.
Changing C22 to 47µF is pushing the 5Y3G a bit hard.
Several data sources I have seen state that this capacitor
should be no more than 10µF. Probably 16µF would be a
sensible maximum as electrolytic capacitors these days
are very close to the stated value.
Early electrolytic capacitors did have quite a wide variation, and finding them to be 50% above the stated values
was not uncommon.
Rodney Champness,
Mooroopna, Vic.
Graham Parslow responds: the 47µF capacitor provides
good filtering after the surge current has passed. However,
I can agree that it is “pushing the 5Y3G a bit hard”. Most
12
Silicon Chip
data sheets give a maximum figure of 10µF or 20µF; I did
see one that said 32µF maximum.
I have a good range of 0.25W and 2W resistors, so naturally, I used a combination of what I had at hand, combining two 10kW 2W resistors to make a replacement for the
burnt-out 20kW resistor. When it comes to power ratings, if
the components fit, I don’t see the harm in a bit of ‘overkill’.
June issue comments
I read the Editorial Viewpoint of the June 2020 issue, and
I sympathise. It beggars belief that an organisation such as
NBN Co could be so poorly organised. I thought that our
companies were being run by people who were educated
at universities in the best management principles, or maybe that is the problem. They were educated by academics
who have never managed a business or a company.
It is interesting that some time in the 1940s or 1950s,
the PMG (the predecessor to Australia Post and Telstra) requested of the then current government that they be permitted to train technicians etc in house. The result was
highly-competent people for the unique jobs of the PMG.
Dr Maddison’s article in the June edition about opensourced ventilators is an eye-opener. It seems that a little
motivation, of the correct kind, can be very productive.
But there was one requirement that was not mentioned.
Many component manufacturers restrict the use of their
components in life support systems where failure could
result in death.
If you check the data sheet of just about any integrated
circuit, you will find a warning that it is not to be used
in life support devices without the prior approval of the
manufacturer.
On a separate note, I received the first three blocks of the
Silicon Chip magazine PDFs on USB and have read many
of them already. I am very impressed by the quality of the
scans. They would have to be some of the best if not the
best scans that I have read.
I was reluctant to part with the money for them, but already, I have found quite a few articles of interest. Even
the old advertisements are interesting.
Finally, I am getting a little annoyed when seeing ads for
wire with a particular current rating (7.5A, 10A etc). Technically, for ordinary hookup wire (not mains use), there is
no such thing as 7.5A etc wire. The size of a wire needs to
be determined by the expected maximum current it will
carry, plus the maximum acceptable loss in voltage, plus
the maximum acceptable operating temperature.
An excellent example of this misleading terminology occurred with one of my neighbours. He has a caravan which
has a 12V-powered water pump for the sink. It wasn’t performing well, so he decided to connect a new pair of wires
to it directly from the battery which was about 5.5m away,
on the tow bar. He bought some dual flex from an autoelectrician mate who told him that the “10 amp” cable
would be sufficient.
The pump’s running current was 5A. The wire seemed
a bit small to me. I calculated the resistance per metre for
1mm2 wire, and the voltage drop at 5A was approximately 1V total (0.5V per wire). It doesn’t seem like much, but
it is an 8.3% loss in the available voltage to the motor results in a significant drop in performance.
I convinced him to buy 4mm2 wire. This reduced the
voltage drop by 75% and provided another 0.75V for the
Australia’s electronics magazine
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pump motor, resulting in a noticeable increase in performance. It would have been better if the pump and battery
were closer; then my neighbour would not have needed
to pay out $38 for the cable.
George Ramsay,
Holland Park. Qld.
Erlang meter clarification #1
Regarding the letter on p109 of your October issue asking about an Erlang meter, these were a standard part of
the Traffic Data Equipment (TDE) installed in PMG (later
Telecom and then Telstra) exchanges from the mid-1960s,
and probably before. I actually have a portable Erlang meter, which I acquired when all the analog measuring equipment in the lab was retired.
The Erlang is a dimensionless unit which represents
the “occupancy” (ie, percentage use) of a circuit or group
of circuits. The meter pictured would have been used for
monitoring up to 200 circuits. Even in the 1960s, these
meters were an anachronism. Initially, a technician would
have manually recorded the meter reading every three minutes, but the TDE performed this function automatically.
The actual meter is nothing special (other than its high
quality) – it is simply an ammeter. It measured the current
fed from the exchange 50V supply via a 100kW 1% resistor for each occupied circuit. Thus the full scale would be
100mA (50V ÷ 100000W × 200).
Ian Binnie,
North Ryde, NSW.
Erlang meter clarification #2
In response to D. D.’s question about an Erlang Meter,
(Ask Silicon Chip, October 2020), these meters were installed in electromechanical telephone exchanges in the
60s and 70s. I first came across a meter like this in a Crossbar Trunk Exchange in the early 1970s. What they really
displayed was a concurrent call count, not Erlangs as the
Erlang has a time component.
Each trunk in a trunk group had a connection that fed
50V DC through a 100kW resistor (0.5mA) when the trunk
was busy. These were all connected in parallel and fed to
the meter. The meter was on a panel in the exchange control room, and it could be switched between trunk groups
or monitor the total call count for the exchange.
Dallas Haggar,
Caddens, NSW.
leaving in 1994. I worked as an OIC for 39 years in SA
and the NT.
Brian Dunn,
Old Noarlunga, SA.
Erlang meter clarification #4
In response to the question asked by D. D. of Berowra Hts
in the October issue (Ask Silicon Chip), during 30+ years
of Telephone Exchange maintenance, I saw and sometimes
used Erlang meters. They were installed in Ericsson crossbar exchanges from 1964 until the 1980s to measure route
and equipment occupancy.
The meter was located in a rack with manual selector
knobs to select between many groups of circuits that could
be measured,
Each device (or trunk) in a group would apply a 100kW
resistor between ground and that group’s traffic measurement lead when in use. So if 40 trunks (two-wire voice
circuits) in a group were in use, the meter would see 40 x
100kW in parallel (0.5mA per circuit or 20mA total). The
resulting meter current would read as 40 Erlangs. The maximum group size was 200 circuits (full-scale on the meter).
In the late 1990s, with the demise of the old analog mechanical exchanges, hundreds of these meters will have
been recycled.
Ian Michie,
Blackburn, Vic.
How times have changed!
I saw this old Philco ad on Facebook. Imagine trying to
sell a portable sound system like that today!
Dr David Maddison,
Toorak, Vic.
SC
Erlang meter clarification #3
In the days of the PMG after World War 2, Erlang meters
were used to measure the use of circuits within the step x
step exchanges and to other exchanges. The relay sets involved had a 100kW resistor fitted; the resistors were connected together and were fed into a rotary switch and then
to the Erlang meter.
When the relay set was in use, a positive (Earth) was put
onto this 100kW resistor, and as the Erlang meter had -50V
behind it, a reading of one Erlang would be shown. If 15
relay sets were in use, it would read 15 Erlangs.
As each exchange had several junctions each to other exchanges, the Erlang meter was able to indicate at a
glance what the traffic was in that exchange, both external
and internal, and was used to calculate further expansion.
I joined the PMG in 1950 as a Technician in Training,
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Australia’s electronics magazine
November 2020 13
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