Model train controller

I have built the Model Train Controller from the November 1995 issue of "Electronics Australia" and have a question regarding its operation.

The controller works OK on a resistive load, with the voltage slowly rising and falling as designed but on a train motor, the back-EMF seems to be triggering the SCR continually. The train goes into "cruise control" and the throttle (VR1) no longer controls the speed.

Can you please suggest a fix for this? Are there any more recent or more advanced train controller designs published by SILICON CHIP? (J. Y., via email).

In our experience, SCR controllers do not do a good job with model locomotive motors. Having said that, check diode D7 and capacitor C3 as they should be filtering hash on the lines.

24V LED Drivers For Underwater Lights I have built six underwater lights using the 3W Star LEDs (Jaycar ZD-0526). I am now looking for a suitable 24V driver for them. The Jaycar catalog lists two units: AA-0580 rated at 1W, 11-30V and KC-5389 rated at 1-5W, 12V. Will the AA-0580 drive the 3W LEDs? Alternatively, would the KC-5389 be a better choice if it can be modified to run at 30V? If the KC-5389 could be modified, should I use one driver for each LED or put two LEDs in series? The LEDs are heatsinked and enclosed in a stainless steel housing. (J. N., via email). The Jaycar AA-0580 is suitable for driving 1W LEDs only. As described in the May 2004 issue of SILICON CHIP, the Luxeon LED Driver (Jaycar KC-5389) was designed for operation from a 12V DC supply. However, it can be modified to operate from 24-28V DC and drive two 3W stars in series with the following changes: (1). Replace the 24V/5W zener diode (ZD1) with a 33V/5W type (available from www.wiltronics.com.au or www.iinet.net.au/~worcom) (2). Install a 470pF ceramic capacitor for C1. As described in the article, resistor R1 should be 0.15Ω for 700mA of drive current (recommended) or 0.1Ω for 1000mA. (3). If you don’t need the low battery cutout function, then remove D2 from the circuit to disable it completely. Conversely, if you do need this function, then replace the following parts in the circuit: 750Ω with 2.7kΩ 18kΩ with 33kΩ and 9.1kΩ with 6.8kΩ. For a really smooth and realistic controller, have a look at any of SILICON CHIP’s switchmode transistor designs such as the "L’il Pulser" from the February 2001 issue.

Spark generator wanted

I am looking for a spark generator circuit that will produce a nice spark at the press of a switch; battery-powered would be preferable. Has SILICON CHIP done such a circuit? (Y. M., via email).

Have a look at the Jacobs Ladder project described in the September 1995 issue. It uses an ignition coil to generate a healthy column of fat sparks.

Digital clocks do not keep good time

Can you explain why it is that identical radio clocks gain time at vastly differing rates when they all use ICs clocked by AC from a common household supply? Is it possible for control tones to cause false triggering of the counters, perhaps to a different degree? (H. M., via email).

Are you sure that the clocks are controlled by the 50Hz mains? They could be crystal-controlled, which would explain the differing time gains. If they are crystal controlled, there is sometimes a trimmer, which can be adjusted for better time-keeping.

We would not expect mains control tones to affect time-keeping since they are reduced in amplitude compared to the mains voltage and so should not trigger the counter inputs.

Damping factor of class-A amplifier

I am in the process of putting together a stereo set-up of the 15W class-A amplifier and would like to know how much better the damping factor is compared to most top brands. I’m also curious as to how its damping compares to your other amplifiers, especially the 100W class-AB Ultra-LD amplifier. By the way, would you know the gain dB of the 15W class-A unit? (N. N., via email).

If you have the articles for these amplifiers, you will see the damping factor quoted in the specification panels. For the 15W unit, it is greater than 200 at 100Hz & 1kHz, while for the Ultra-LD it is better than 170 for the same frequencies.

In other words, both amplifiers are so good that the damping factor is academic. The voltage gain for both amplifiers is also identical, as set by the 18kΩ and 1.2kΩ feedback resistors. The voltage gain is x16 or +24dB.

Further mods to Battery Zapper

I recently built the "Deluxe Lead-Acid Battery Zapper Mk.2" featured in the May 2006 issue and uncovered several problems:

(1) When connected to a 12V battery, the Zapper circuit would usually operate as intended with an audible 1kHz tone but every now and then would refuse to start properly. No tone would be audible and the 3A fuse would open after a few seconds.

This problem was rare at first, but after a week of continuous operation it occurred much more frequently. Further investigation revealed the supply to IC1 was oscillating wildly at around 200kHz. This was the result of the LC power filter interacting with the output of the zapping circuit. Replacing RFC1 with a link cured this problem without adversely affecting the regulation of IC1’s supply.

(2) When zapping a 24V chain of batteries while connected to a 2-stage smart charger, the battery voltage held steady at 28.5V. This caused 2W of power to be dissipated by the 1W-rated ZD1. This also applies to the checker regulator diode ZD4.

While this may not matter for short periods of time, the Zapper is expected to run continuously for days. This will result in an unacceptable load on the zener regulators. There are several possible solutions. I chose to replace the two 100Ω 5W resistors with 180Ω parts and replaced both ZD1 and ZD4 with two 6.8V 1W zeners in series.

(3) When zapping 24V batteries, the circuit generates a large amount of heat. I decided to drill a large quantity of ventilation holes in the sides and lid in the top half of the case before sealing the lid, but even then the circuit board became discoloured and the plastic formers of L1 & L2 deformed.

Perhaps most importantly, D3 be-
comes extremely hot. Even with a small heatsink fitted, it managed to char the insulation of a wire that happened to come into contact with it. I recommend that all constructors increase the ventilation and fit a heatsink to D3 if they intend to operate the zapper at 24V.

(4) The condition checker circuit would work fine at 12V but would destroy a Mosfet or two, as well as Q8, D10, D11 and the three 0.22Ω resistors, whenever an attempt was made to check a 24V battery. The Mosfets appeared to be failing short, taking out the other components due to the over-current condition.

I managed to reproduce the problem non-destructively by replacing the three 0.22Ω resistors with one 0.5Ω. This revealed that the current limiting would work perfectly at 12V but would break out into wild multi-MHz oscillations when connected to 24V.

After eliminating all other possibilities, I guessed that the MOS parasitic G-D and G-S capacitances were forming a feedback loop within the source-follower circuit configuration. This was confirmed when I inserted four 0.5Ω 5W resistors in parallel between the MOS drains and the battery positive, at which point the oscillations ceased.

I trust that you will test and publish these modifications, as they have caused me several days of headaches and I would not want others to have to go through the same process.(R. F., via email).

After further testing, we too have discovered the instability problem in the Zapper circuitry, which seems to occur only with a proportion of STP60NF06 Mosfets. As noted in Notes & Errata in the October 2006 issue, we recommend fitting a 100Ω stopper resistor in series with the gate of Q1, which prevents this instability without degrading the Zapper’s performance.

We have not encountered the other problems you describe when checking the condition of 24V batteries. However your modifications would be very applicable for anyone who expects to use the project with 24V batteries. Thanks for providing the details.

6V to 12V radio conversion

M. S. asks in the September 2006 issue about running a 6V car radio on 12V in a 1953 Pontiac he is restoring. Perhaps he could salvage one of the old 6V headlamp bulbs and put that in series with the radio to act as a dropping resistor?

A 55W bulb would draw 9A at 6V, so it should be able to handle the current drawn by the radio. He could perhaps fit it inside the radio casing. (P. C., via email).

That is an interesting suggestion but it could lead to damage in the radio, even if you did match the headlight bulb current with the radio current. Initially, the valves will not draw any HT current and so the HT voltage will be much higher than it should be. This could damage the electros and the vibrator itself. It might also blow the valve heaters.

The idea of installing a 55W bulb inside the radio casing would not work either – headlight bulbs get extremely hot.

DC-DC converter for vintage radio HT

I refer to the 12AX7 valve audio preamplifier published in the November 2003 issue. I wish to incorporate the DC-DC converter from this project into a vintage valve car radio, using it to supply HT to five valves instead on one. The current on the HT rail will need to be about 50mA.

Your article quotes the 12AX7 valve as needing only 2mA from the HT rail. How much current can the DC-DC converter provide? (C. S., via email).

The unit should comfortably deliver 50mA. However, its hash may interfere with radio reception unless it is well shielded.

How to connect ultrasonic transducers

I have recently purchased a couple of Jaycar AU-5550 ultrasonic transducers. Although there is a data sheet on the Jaycar website, it doesn’t give any other information. There are only two pins but how to use them? Is the body the earth?

The manufacturer’s part number is T/R40-16B but I cannot find any practical information on the web. Can you assist me please? (T. U., via email).

In essence, these piezoelectric transducers are capacitors. There is no polarity and you don’t connect the case (although note that some transducers like the Murata MA40 series have one pin connected to the metal housing).

Have a look at the Ultrasonic Parking Radar in our February 2000 issue. It shows how to use them.

Additional outputs for remote control extender

I am looking at building your IR Remote Control Extender from the October 2006 issue. I was wondering what modifications, if any, would be required to add extra LEDs at the output so it could be used to control more than one device.(R. S., Ballarat, Vic).

The circuit can drive more than one LED if you duplicate the 220Ω resistor and LED section for each additional output. In other words, use an additional 220Ω resistor and LED in series between the collector of Q1 and ground (0V).

Valve preamp loads power supply

I have assembled the valve preamp kit (November 2003) and the Jaycar KC5347 power supply kit intended for the SC480 amplifier. I am having trouble with certain voltages. With the preamp disconnected I get the proper voltages from the power supply. When I connect the preamp, the +15V rail drops to +6V. The outputs from the inverter are 6V and 160V, respectively. (R. C., via email).

The reason why the both the 15V and HT rails are dropping under load (ie, when the preamp is connected) is because the 15V outputs of the KC-5347 power supply are not designed to supply the current drain required for the valve preamp. They’re designed to provide only a few tens of milliamps whereas the valve preamp needs about 300mA.

In fact, the 12AX7 valve heater alone needs around 150mA, while the DC-DC converter used to provide the HT requires almost the same amount.

So you’ll need to power the valve preamp from a 12V power supply capable of providing the required current. When you do this, you should find that the voltages don’t drop significantly when the preamp is connected.

TV choke magnet mystery

Could you please tell me what purpose a permanent magnet fitted to the top of a choke/transformer serves? I refer to part of the HV/EHT circuitry of an old TV (picture supplied) and I have also often found quite small chokes with strongly magnetic ferrite cores when salvaging bits and pieces from CRT monitors and TVs. I figure it modifies the components’ inductive behaviour but can’t get my head around exactly how.(B. J., via email).

We passed this query to our Serviceman writer and here is his reply: as a guess, this looks like a transductor used in North South Correction circuits for old delta CRT sets like Sanyo (circa 1976). Rotating the magnet attempts to straighten the horizontal lines at the top (and bottom) of the screen. Vertical and Horizontal pulses are mixed together via this transductor to produce a butterfly waveform which is injected into the deflection yoke. This is always a compromise with other adjust pots nearby.

This particular board looks like it is from an EMI HMV/Healing C211 chassis.

Query About UHF Prescaler Capacitors What wonderful projects are possible with modern components. A few years ago, a project such as the UHF prescaler described in the October 2006 edition of SILICON CHIP would have hardly been possible for the average constructor, or at least not with the confidence that it would work without any hassles or a bench equipped with equipment worth as much as one’s home. I shall be building one for use with 23cm and 13cm ATV Tx design and construction. The reason for this letter is to query the value of the two 10nF capacitors for the input and output of IC1. The reactance of the two capacitors is only 0.31Ω at 50MHz. 100pF with a lower self-resonant frequency would still have a reactance of only 31Ω and even 10pF would operate quite effectively with its 310Ω of reactance. At microwave frequencies of around 3GHz, for 10pF the reactance would be insignificant but their reduced inductance and self-resonant frequency would be a decided advantage as well as reducing the possibility of lower frequency instability. I have noted the excellent supply decoupling technique employed. Perhaps the 10nF values were typographic errors? I do not intend this letter to be taken as a criticism for I am genuinely interested in the design philosophy, having designed pro and non-pro projects over many years. (V. B., via email). The value of the coupling capacitors on either side of IC1 may seem a little high but we found that the sensitivity of the prescaler dropped significantly at 50MHz if they were reduced much below 10nF, at least with the X7R dielectric capacitors being used. However, if you are not too concerned about the sensitivity at 50MHz, you could use lower values for these capacitors. We would suggest either 1nF or 470pF.

Notes & Errata LED Tachometer Pt.1, October 2006: the display reading in both Fig.1 and Fig.2 should be 1200 rather than 3200. The text describing the operation on page 27 is correct. LED Tachometer Pt.2, November 2006: the overlay diagram for the DC Relay Switch board (Fig.9) shows D1 with reversed orientation. The cathode (striped end) of D1 should be to the right. Mini Theremin Mk.2, July & August 2006: equalising coil L1 needs to be wound so that its self-capacitance is as low as possible. In practice, this means that the windings should be jumble-wound by hand without regard to neatness. Do not wind each layer with each turn placed adjacent to the next as would be done by a coil-winding machine. Battery Zapper, May 2006: if readers intend to use this project with 24V batteries, the following changes are recommended: (1) Increase the two 100Ω 5W resistors to 180Ω parts and replace both ZD1 and ZD4 with two 6.8V 1W zener diodes in series. (2) Improve ventilation by drilling holes in the sides and top of the case and fit a heatsink to diode D3. (3) Insert four 0.5Ω 5W resistors in parallel between the drains of Mosfets Q3 to Q6 and the battery positive to prevent oscillation when doing the "Condition" check. DC Relay Switch, November 2006: the overlay diagram (Fig.2) shows D1 with reversed orientation. The cathode (striped end) of D1 should be to the right. In addition, the parts list should include: 1 1N4148 diode (D3) 4 M3 x 12mm countersunk Nylon screws 4 3mm Nylon washers 4 M3 nuts SMS Controller, October & November 2004: in certain circumstances, user commands such as EN and DIS may operate on the wrong input or output port. A firmware update (v1.2) is available from the website to correct this problem. Note that as this problem is only evident with certain combinations of long strings, there is no requirement to perform this update if your controller is operating satisfactorily.

Warning! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable.