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MAILBAG your feedback 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 Helping to put you in Control Ethernet Digital IO, Voltage, Temperature, Humidity Alarm and Control The TCW241 is an Ethernet control unit with 4 digital inputs, 4 relay outputs, 4 analogue inputs and a 1-Wire interface for up to 8 x 1-Wire sensors. SKU: TCC-025 Price: $313.55 ea + GST Modbus RS-485 Red Indicator Easy to mount the SMI2 fits into a standard 22.5 mm borehole for signal lamps. The SMI2 is a universal display unit for monitoring industrial processes. 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SKU: NOS-030 Price: $59.95 ea + GST 4-20mA Loop Current Simulator/ Generator The KTA-366 is a 4-20mA loop powered current calibrator designed to make testing and measuring 4-20mA loops simple, it is suitable for testing 2 and 3 wire transducers; it has two operating modes, Simulate and Generate mode. SKU: KTA-366 Price: $145.00 ea + GST For Wholesale prices Contact Ocean Controls Ph: (03) 9708 2390 oceancontrols.com.au 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! siliconchip.com.au Cable Assembly & Box Build Assembly Metal Work Label and Wire Marker CNC Engraving and Machining Functional Test and Logistic Service Electrical box assembly <at>Ampec we specialise in manufacturing of custom design cable assemblies as well as turnkey electronic and electric product assemblies. Fully automatic cut, strip and crimp machines High mix low volume and quick turnaround +61 2 8741 5000 e sales<at>ampec.com.au w www.ampec.com.au 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 siliconchip.com.au 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, siliconchip.com.au Australia’s electronics magazine November 2020  13