Back-to-back capacitors explained

I am building the Model Train Controller from the April 1997 issue and I came across the Notes & Errata for this project in the August 1999 issue. In that note it said that the 4700μF capacitor connected between switch S1 and the -12V rail needs to be replaced with "two back-to-back 4700μF 25VW capacitors connected between switch S1 and the 0V rail". What I want to know is what does "back-to-back" mean? Are the capacitors connected in parallel, series or what? (G. M., Caringbah, NSW).

Hmm. Perhaps we should have explained that better. The background to the change was explained in an answer to a letter on the train controller on page 91 of the August 1999 issue. In the original circuit (April 1997), the 4700μF inertia capacitor connected to S1 and the -12V rail could never be reverse-biased but it did cause an initial lurch in the train because it was discharged.

One solution to that problem is to connect the negative side of the capacitor to the 0V rail. That fixes the initial lurch but it does means that the 4700μF capacitor will be reverse-biased when the throttle potentiometer is wound down for reverse operation of the loco. Clearly, this cannot be allowed to happen and the simple solution would be to use a non-polarised electrolytic 4700μF capacitor. Non-polarised electrolytics can be operated with a positive or negative bias voltage or with none at all. The problem is that the non-polarised electros are not readily available in large values.

So we specified the next best thing which is two capacitors "back-to-back". In effect, you connect the two capacitors in series but with their negative leads connected together (ie, back-to-back). One positive capacitor electrode goes to S1 and the other positive capacitor electrode goes to 0V.

Electric fence output voltage uncontrollable I have built the Electric Fence kit from April 1999 and when I try to set the capacitor charging voltage via VR1 (to 340V) I find that it shoots all over the place (starting from about 56V, up to 1000+ and off the scale of my meter) with very small changes in the pot position. I’d be grateful if someone could suggest what might be wrong. (J. N., via email).  Check the components around VR1. Check that you have 4.7V across ZD1. Also check that you do not have high resistance in the wiper of VR1. Our tip is that the bottom leg of VR1 is open circuit.

Data logger for pH readings

I have made the pH meter for swimming pools described in the April 1988 issue and it all checked out OK. I would now like to connect the pH meter to an ADC so the readings could be timed and stored using a computer. I have searched the web for a circuit which could read microamps using an ADC but with no success. (J. R., via email).

Have a look at the Mini-log, an 8-bit data logger published in the July 1996 issue. It has an ADC and is based on the Basic Stamp II.

Capacitors for SLA battery charger

I refer to the July 1996 Silicon Chip article "Charge SLA Batteries Away From Mains". The circuit requires two 0.68μF 250VDC polyester caps. I’m having trouble obtaining same. However, I can get suitable electrolytics. Can I use these, paying attention to polarity?

The instructions suggest that the circuit is suitable for 12V 6.5Ah and greater capacity SLA batteries. Is it suitable to fast charge a 12V 2.4Ah Nicad battery pack? (J. P., Wirrabara, SA).

 The 0.68μF 250VDC capacitor is difficult to obtain but you can use a 1μF 250VDC instead. These are available from Jaycar or Altronics.

This charger is only suitable for SLA batteries. It is not suitable for Nicads.

Questions on the fast charger

I recently built the "Fast Battery Charger" as described in the February & March 1998 issues. When I tested the charger according to the test procedure, I could not get the 200mV across the two 0.1Ω resistors. The most I could get was about 87mV. I also measured the DC current; 1.6A DC into a 12V 1.8A cordless drill battery.

A few minutes after it started to charge the battery the charger started to buzz and continues until the time out period ends and then the buzz changes slightly but does not stop.

I also tried charging a 6V battery and the same thing happens (the buzz is louder). All the other voltages I was supposed to check seem to be OK.

Could you tell me how to stop the buzzing and what might be wrong with the charge current?

There is one other problem I would like you to explain and that is the number of turns on the inductor. It specifies 10 turns bifilar wound. Is that 10 turns for each winding for a total of 20 turns or 5 turns for each winding for a total of 10 turns? When I wound on the total of 20 turns I could only wind 19 coils with the 20th coil above the 19th coil.

Also, why do you need the spacer between the E cores and how precise does it have to be? (E. L., Midvale, WA).

The actual current which charges the battery is both switching at high speed and also following a pulsating DC waveform shape at the mains frequency. This makes it difficult to measure with a standard digital voltmeter.

Measurement of the current can only be made with a true RMS meter or by calculating the current via the waveform on an oscilloscope. The value of current read from a standard multimeter will be erroneous. Perhaps the best way to tell if the current is correct is to check if the heatsink gets quite hot during charge and if a battery charges in the expected time.

The squeal in the transformer is normal. It can be quietened by potting the windings in epoxy. The number of turns on the transformer should be two lots of 20 turns. In other words, wind the two lengths of wire together for 20 turns. The windings will go to at least two layers.

The gap between the cores sets the inductance of the transformer and its saturation characteristic. Therefore it should be the 1mm as specified, within ± 0.15mm.

Headlight pinouts and tacho interfacing

I am looking for the pinout details for a car headlamp, preferably with Low/High beam. The reason is that my car’s headlights are fine on high beam (practically cook a rabbit at 10 yards) but low beam is awful. It seems that the power is earthing out via the high beam filaments.

Also, on the Speed Alert published in November & December 1999, is it possible to use a Hall Effect device instead of the coil sensor? I don’t fancy having to wind the coil. Can you please help? (W. S., via email).

Have a look at the choice of input arrangements used in the Tacho published in the April 2000 issue. You could modify the input op amp along the same lines.

We don’t have info on car headlight pinouts but generally one side of both filaments connects to a common terminal.

Problems with 3A train controller As a member of the "Logan District Model Railway Club Inc.", I have been asked to write to you to seek some advice concerning the 3A Train Controller featured in the February 1993 issue. We realise that this is a fairly old circuit but still hope that you may be able to help us with our problem. The circuit has all the features we need for our large "HO" club layout which has six "plug in" positions to connect our controllers. We wanted to build some new controllers for the club. After looking around at numerous circuit diagrams, by sheer coincidence our secretary, Darren Lee, and myself both came up with the same circuit. All the club members agreed that this circuit appeared to represent all the features we wanted. Following this, Darren built one and so did I. This is where the problems started! He built the circuit exactly as shown in the article. However, I modified my unit by replacing the two trimpots with linear potentiometers mounted on the box sides. This gives the operator the ability to adjust the inertia and braking to suit his own requirements. Unfortunately, both units failed to operate. The test supply is from an old Triang Controller claiming to be 15VAC but on testing is 17VAC. The unit was connected to this supply but not connected to the track. Our knowledge of electronics then limits us from checking much further. I know that on the output side of the bridge rectifier the voltage is DC and the output is 1.414 times the input. This makes the feed to the circuit approximately 24VDC. This checks out as correct. Both our units have been assembled on Veroboard. These have been carefully checked numerous times to ensure that no error exists in the circuit. Our questions are as follows: (1). Could the input VAC be too high, thus damaging some of the components? (2). A silly thought, but was an error discovered after the article was published and rectified in a later issue of the magazine? (No insult intended!) (3). Nobody can tell us the handling capacity of the Veroboard strips; ie, can one strip carry 3A? (4). Is it acceptable to change the trimpots as mentioned above? (M. B., Logan City, Qld). The most likely reason for the malfunction in your train controller circuits is mistakes in the Veroboard layouts. From bitter experience we know how easy it is to make mistakes. Your best approach is to obtain the PC board as it is much easier to assemble. You can purchase it from RCS Radio Pty Ltd, 41 Arlewis Street, Chester Hill, NSW 2162; phone (02) 9738 0330. The board is type number 02102931 and is $12.65 plus $3.30 for postage and packing. You can change the trimpots to pots, as you have done.

Coil failure in ignition system

I have built the High Energy Ignition system as described in the June 1998 issue and the unit has worked well. But the other day my ignition coil failed and I think this caused the output transistor to fail as well.

I checked for continuity from the transistor’s heatsink to case and there was a short (it was still connected up to my car, with power off when I checked it). I can find no reference to this transistor in either the Dick Smith Electronics or Jaycar catalog.

Is there a higher-rated substitute I can put in? By the way, is there a simple go/nogo test for the output transistor? I think it was a Darlington type. (M. K., via email).

The coil probably failed because the transistor’s collector became short circuited to the case. This would have meant that the full battery voltage was connected across the coil which would burn it out fairly quickly.

This short from the transistor to case would suggest that it is the insulating washer between the transistor and case or the bush which has failed. Check that there are no sharp edges around the mounting hole for the transistor as this will give a starting point for any arc-over between the transistor and case.

Use either a new silicone washer or two mica washers.

You can check the output transistor (it is a Darlington type, by the way) by using your multimeter to measure the resistance between base and emitter, between base and collector and collector to emitter. The 1999/2000 Dick Smith Electronics catalog shows how it is done on page 236.

Voltage regulator for motorbike

I own some older motorcycles that have permanent magnet, single phase alternators. These have very simple shunt regulators that "waste" surplus power. I’ve heard that a better way to regulate is with a DC-DC converter. I didn’t get more detailed info but am wondering if something along the lines of your 2A SLA battery charger described in July 1996 could be used? (J. P., via email).

Normal alternator regulators work by switching the field current on an off but clearly this is not possible with a permanent magnet alternator and so a shunt regulator is used. A DC-DC converter is not the solution but a switchmode series regulator could be. We doubt whether the 2A SLA battery charger would be suitable as its current and voltage ratings are unlikely to be anywhere near adequate.

None of our existing circuits could be easily adapted. However, even if they could, it would not make any more charging current available from your alternator and the amount of power that is "wasted" is probably quite small relative to the power developed by the engine.

So we doubt that there is any real advantage to be gained by using a switchmode charging circuit with your existing alternator.

Feedback on the Ultra-LD amplifier

I’ve just finished assembling the 100W Ultra-LD stereo amplifier (March & May 2000). I must say that I’m pretty impressed with the sound – the clarity and stereo imaging especially.

I have some feedback on the construction process. The Jaycar kits were well done. The power and audio wiring was difficult using the 1mm PC pins. I ended up using PC-mount spade pins with Utilux connectors on the wires. I used shielded cable that was quite stiff and it would have eventually twisted the pins out of their pads when moving things around during assembly.

I noticed that the bias setting was a very sensitive procedure. Once set at 4.4V, it would drift around aimlessly between 4.3V and 4.6V. If you blew on the TO-220 heatsinks in the middle of the PC board, the bias would shoot up to 4.65V. It’s just a bit too sensitive for my liking.

I don’t know how it will affect the sound but it’s something you may want to look into.

I used the 35V-0-35V plus 55V-0-55V transformers from Harbuch Electronics (who offered great service by the way). However, hooking up the 12V fans presents a bit of a problem in this configuration. I have the speaker protector kit (described in August 2000) on order from Jaycar which will allow me to hook it up but in the meantime, I have to be careful how long I run the amplifier without overheating it. (M. D., via email).

Thanks for the feedback. The bias on any class AB amplifier does tend to wander about so it’s not a worry. Also you can run the amplifier without a fan as long as you don’t consistently drive it to high power. However, we really don’t understand why you have not used the fan hook-up described in the May issue.

Basic Stamp2 Xout function

I have been doing some programming on the Basic Stamp2 module and am interested in using the Xout function. This provides remote control functions via the reticulated mains power lines within a building. Special 110V interface modules are available in USA for this purpose.

Is this facility available for the Australian 240VAC power system? Are there approved "control via mains" systems in use here and if so, are the interface units available for purchase? (K. M., via email).

We referred your question to Microzed Computes, the Australian agents for The Basic Stamp2. Their answer is as follows:

Our understanding is that X10 is being discouraged in Australia by power supply authorities because of developments in power line accessing of meter readings, using a protocol that would collide with X10.

A more reliable option is CE BUS from Clipsal. This uses a separate 2-wire, low tension bidirectional bus. CE Bus has more features and should interface with the Stamp.

Notes & Errata Opto-Electronic Ignition, October 2000: the circuit featured in Circuit Notebook on page 58 shows a 470Ω resistor connected to the collector of Q2 via a .01μF capacitor. This resistor should be 470kΩ.

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