Defence technicians not all valve jockeys

I must take you to task for allowing publication of the snide remarks by Keith Walters in his article on "The Start of Colour TV in Australia". Maybe he found it necessary to spice up his otherwise very interesting nostalgic article by denigrating technicians from the war-time armed forces by including them amongst his "infamous valve jockeys . . . often there were a lot of these somewhat pathetic individuals (often ex-military with no real theoretical background)" then continuing on with his smug comments right through to "I suppose as long as there was a competent workshop to back them up, they could usually be relied upon to put in a reasonable day’s field work".

I really take exception to these comments as I was an RAAF WW2 Wireless Maintenance Mechanic trained by the RAAF through the Melbourne Technical College by completing the college’s normal 4-year radio mechanics’ course (I still have my course certificate issued by the college).

I am not sure where the AIF wireless mechanics were trained or to what standard but as far as I am aware, all of the RAAF and RAN wireless maintenance mechanics did the above extensive training through the Melbourne Technical College (now RMIT). As for the RAAF, after completing this course, we were split up to progress into either wireless or RDF (now radar) maintenance.

We then undertook months of practical training in, both HF and VHF, aircraft and ground-based receivers, transmitters and antennas. Some of us were also trained in telephony. Now if Mr Walters as a "bright-eyed 19-year old raised on a diet of EA/ETI magazines" considered himself better theoretically trained, at the time, in radio theory than those of us who went through the Melbourne Technical College’s training course and then spent years in the armed services maintaining radio equipment, he must have been a very smart fellow. How would he shape up these days faced with servicing one of the latest hi-tech amateur transceivers? Not quite as easy to service as a TV.

As I was involved for many years in TV servicing I am quite aware that, at first, there were many of the so-called "valve jockeys" as Mr Walters points out but at the time someone had to fill the gap until TV technicians could be trained. They soon passed into history. He should have let it go at that and refrained from trying to include or denigrate radio trained ex-servicemen in his slur. Yes many of them would have been struggling to come to terms with the arrival of the transistor age but training courses were available for those interested enough to upgrade their skills.

Ron Mills,
VK5XW, via email.

Tsunami warning systems a problem

There is no doubt that the recent tsunami in the Indian Ocean was an enormous tragedy. However, I would like to comment on Mr Simpson’s Publisher’s Letter in the February 2005 issue.

In deciding how best to reduce the impact of tsunamis in the future, the economic state of the area must be taken into account. As Mr Simpson pointed out, there are tsunami warning systems currently in place in the Pacific Ocean. However, the countries with these systems (USA, Japan, etc) have the money to maintain these systems.

Having been involved with the design and installation of dam flood forecasting systems in The Philippines and in North West Sumatra, I am aware of the harshness of life in these areas. The cost of the installation and maintenance of these systems is very high relative to the average person’s income. What makes these systems feasible is that they are associated with hydroelectric power stations which are a source of income for the maintenance technicians.

If elaborate warning systems for tsunamis are to be considered, the problem of expensive maintenance must be considered. In times of hardship, sometimes a permanent condition, it is very probable that the maintenance of this equipment will be neglected in many areas. The attitude would probably be that "such a disaster couldn’t possibly happen to us again!"

I still think warning systems should be considered but I believe that simpler solutions should be implemented first. After the 1998 tsunami in Papua New Guinea, posters were placed around the towns, explaining how to identify an incoming tsunami, and what to do when it came – eg, "climb a coconut tree". If people were better educated on how to identify a tsunami, people may be less likely to go and pick up fish on the beach, and more likely to run away. Posters are very cheap to maintain!

Remember that Australia, a first world country and not so far from the New Zealand fault line, has no warning system in place because it has been considered difficult to justify. So let us proceed with tsunami forecasting but maybe delay a little to consider all the options.

Peter Johnston,
Coffs Harbour, NSW

Comment: if the reaction of the populace to more recent severe earthquakes is any guide, it will be many years before complacency about tsunamis ever sets in. These regions will be very keen to have any warning systems available.

Matching the loop antenna to old radios

At the end of the article on the Loop Antenna published in March 2005, you mentioned impedance difficulties with direct connection to a radio. I used a Little Nipper I recently acquired and repaired (it’s the same as in Vintage Radio for September 2004 but with all the original knobs). It has no ferrite rod and presumably a very high impedance input. The loop performed very poorly.

I found out that after connecting one end of the variable capacitor directly to the aerial input of the valve radio and not connecting the other end, I had a good signal with a strong peak when the loop is tuned. The radio is earthed through the mains, after following your advice about using a cheap extension lead.

The signal was quadrupled when I connected about a metre of wire as a short aerial to the other end of the capacitor, without losing too much selectivity. The Little Nipper can now pick up well over 50 stations at night-time that way, although some suffer from fading. Before I had perhaps five without disturbance. AM radio listening is now fun.

Peter Mendelson,
via email.

Photocopiers are a treasure trove I have found that discarded photocopiers are a treasure trove for hobby robot builders. They contain power supplies that can be boxed up and used as is and one or more PC boards that contain reusable power transistors, hybrid module stepper motor controllers, switches, LEDs, etc. They also contain large quantities of mechanical parts, loads of plain and ball bearings, gears and cogs, toothed drive belts, electromagnetic clutches, DC motors, stepper motors, reduction gear-boxes, many shafts and rollers and a multitude of solenoids. They also make use of a large quantity of infrared vane detectors and a smaller number of infrared proximity detectors. Most of the proximity detectors only have a range of a few millimetres (to detect if paper is present); only really good for line-following robots. However, if you are lucky, you find ones that have a range of about 50mm, ideal for short-range collision avoidance. Photocopier accessories such as automatic collators contain mainly mechanical parts and are a good source of drive belts. Lids with document feeders always contain a good motor and a PC board with the drive transistors (nearly always an H-bridge) that go with the motor. A bonus for building robots is when you find two identical motors and gear-boxes, as this is ideal for building a skid steer base. Often, big companies will upgrade all of their photocopiers at the same time, meaning that if you are lucky you can get two or more identical machines to dismantle, always a good scrounging result! The most useful tool to have when reusing cogs and shafts is a small lathe. I have the Jaycar lathe (Cat TL-4000) which is ideal for modifying the parts. Almost all motors in photocopiers run on 24V DC. They are assembled almost exclusively with M2 and M3 screws. Obtaining taps for these allows them to be reused. I hope this is useful to readers, Todd Noyce, RNZAF Whenuapai, NZ.

Current transformers revisited

I feel I must contribute my tuppence worth to the debate in Mailbag on current transformers. I am 50 years old and have been interested in electronics since I was a teenager. I have learned much from magazines like yours over the years. I have frequently found myself dissatisfied when circuit explanations avoided the finer points because analysis would be considered too complicated for the average reader.

I am especially disappointed when the novice reader is left with a confusing and inaccurate analysis when there exists a relatively intuitive explanation, which is not very complicated.

The problem with the three responses published in the April issue is that their arguments rely on the idealised transformer model which has infinite inductance and unity coupling between primary and secondary. They then try to explain away the inconsistencies this creates when looking at the transformer used in current mode.

David Millist in the March issue correctly calculated that the secondary load resistor "R" in his current monitoring transformer will appear as a primary load of "R/a²" where "a" is the turns ratio. The problem with this model is that when the secondary is open-circuit, this predicts an infinite resistance, even when reflected to primary side, and consequently no current can flow through the transformer or the primary load which is placed in series with it.

Mr Denniss in the April issue brings up this exact point but fails to analyse it further. Mr Spencer in the same issue explains that current transformers operate in a different mode to voltage transformers, a notion I totally reject. After introducing the concept of magnetic flux induced by primary current, he goes on to say that the absence of secondary current means that the normal transformer rules cannot apply.

If this was the case, what would one expect the secondary voltage to be on a voltage transformer, when the secondary is open-circuit? I have no doubt that it would be the primary voltage multiplied by the turns ratio and that the primary current could be calculated from the primary voltage, its waveform and the primary inductance. Assuming a sinewave primary voltage, i = v/wL; where w = 2p x frequency

One gets a much better feel for the behaviour of transformers if you use a 4-port network model which is composed of an ideal transformer with the appropriate turns ratio and an inductance in parallel with the primary which has the same inductance as the real-world transformer primary.

This model assumes that the real-world transformer has a coupling coefficient of unity and ignores other effects such as winding resistance, inter and intra-winding capacitance, losses due to induced currents in the core and core saturation. This model is developed in many text books on circuit theory (Hugh Skilling – "Electric Networks" and Lawrence Huelsman – "Basic Circuit Theory" are two examples).

If one looks at the open secondary current transformer using this model, it is immediately apparent that the secondary voltage is equal to the voltage drop across the primary inductance multiplied by the turns ratio. It should be noted however, that this voltage is dependent on the rate of change of primary current and so harmonics and transients from such things as electric motors may produce secondary voltages which are higher than expected. A short circuit test load would leave the full mains voltage across the current transformer primary until the fuse blew. I would have expected this to be destructive, despite fuse protection.

A secondary resistance R will have the same effect on the primary circuit as R/a² in parallel with the primary inductance. If this is small compared with the inductance at the frequency of interest, it will have the benefit of providing a resistive current monitor rather than an inductive one.

In summary: use a secondary resistance as this makes your current measurement insensitive to frequency. You could equally put it in the primary but it will have to be a lower value and will carry more current, perhaps leading to early failure.

Peter King,
East Doncaster, Vic.

Powering devices from DC plugpacks

Thomas Scarborough’s use of a diode bridge in his plugpack checker (SILICON CHIP, April 2005) made me realise that this configuration could be used to power equipment from DC plugpacks of either polarity. In this respect, the arrangement is better than the protection diode included in the DC line in many designs.

The small voltage drop across the bridge (about 1.25V at low currents) could also be useful, given the poor regulation of many plugpacks. The use of a diode bridge in the DC line seems obvious once one sees it but is not an arrangement which comes readily to mind.

I wish the designers of my old Panasonic cordless phone had included some protection against opposite polarity voltages in their design. Some time ago, I inadvertently connected the DC plugpack supply from my broadband router into the phone base. A loud splat and a bright flash told me immediately that I had done something wrong.

The two pieces of equipment use DC plugpacks with the same voltage and current ratings but opposite polarity. Logic says there should be an agreed standard but apparently not, as I have seen a lot of commercial equipment where the central pin of the plugpack connector is negative rather than the more usual positive.

With my Panasonic phone, I did manage to obtain a circuit and found that there is a transistor connected as a voltage regulator in the DC input line. Why a modern design would not use a voltage regulator with built-in protection is beyond me but that is another issue.

I did hope that the regulator transistor may have blown before the following components were damaged. Replacing the transistor restored all the voltages to those shown on the circuit but the phone was still dead. At this point, I realised I was fighting a losing battle and bought a new phone. A simple diode or diode bridge in series with the DC line could have avoided $100 down the drain.

My only consolation is that I did not connect the phone plugpack into the broadband router; that could have been even more expensive.

Brian Knight,
via email.

Endorsement of High Energy Ignition

This is a note of thanks to John Clarke for his Universal High Energy Ignition System published in SILICON CHIP, June 1998.

My car is a Datsun 120Y vintage (almost) 1974 and during the winter of 1998 it chewed up a battery, a set of plugs, distributor points, rotor, distributor cap, distributor capacitor and a set of high tension leads. None of this fixed its stubbornness to start on a cold morning and its consumption was climbing to about a $1.00 a kilometre. While I was declaring to my wife, through a haze of blue air, that this car should be given a pension, I read John’s article.

I immediately purchased a kit from Dick Smith Electronics – anything was worth a try to stop the dollar haemorrhage. It was easy to build and install and it still functions reliably today.

The only tune-up since this time, aside from spark plugs, has been replacement of a set of points whose rubbing block wore out.

It is a pleasure not to be adjusting points every three months and driving a lively Datsun that has stopped chewing dollars. Thank you John! I’m a grateful fan,

Bob Hammond,
via email.

Radiator fan running after engine turn off

After reading the above letter, I was moved to offer my advice as a qualified mechanic. In modern cars, thermo-siphoning just isn’t applicable. That technology hasn’t been in a workable automotive application since about the 1920s (Ford model T & A models) and maybe a few vintage tractors and stationary engines since.

Further, aside from high performance applications, thermo fans and auxiliary fans are just a "band-aid" solution. My advice is for a cooling system overhaul which is really quite simple: full radiator and engine flush (until rusty water becomes clear), thermostat check, fan operation (consider viscous coupling, if fitted). If the radiator is easily dismantled, core check and clean, otherwise consider a new core or radiator. Also don’t forget your heater circuit.

Paul M.,
Ararat, Vic.

Basic theory of transformers

I want to comment on the discussion about current transformers in the March & April issues. Talk about making transformers complicated; let’s get back to basics.

Start by assuming a perfect transformer – no magnetising current, no core saturation, no winding resistance, etc. Now we can apply two simple rules to such a transformer: (a) the voltage on one winding always appears on the other windings, scaled according to the turns ratio; and (b) the current into all windings sums to zero, after applying turns-ratio scaling.

Both rules always apply so that a transformer is ALWAYS a current transformer AND a voltage transformer. What differentiates the two in the real world is the intended application and therefore the design compromises.

In a CT with an open-circuit secondary, the voltage on the secondary winding will increase until one of the following events occur: (a) an alternative conductive path is found in the secondary winding – usually due to high-voltage-induced insulation breakdown; (b) the full (primary source) voltage appears across the primary winding; or (c) the core saturates and the transformer ceases to behave as a transformer.

Because of the way a CT is designed, (a) is far more likely to occur than either (b) or (c).

Dale Rebgetz,
Belgrave, Vic.