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working voltages because the “tracking” carbon has a negative resistance coefficient and therefore, only becomes visible at higher voltages, and hence higher dissipation, when the tracking resistance drops substantially. (V. E., Highett, Vic). • Australian Insulation standards for household appliances are available from Standards Australia – phone (02) 9746 4700 or fax (02) 9746 8450. An insulation resistance of around 20MΩ for a washing machine is fairly typical. The low resistance is unlikely to be due to any mains filter but is due to the insulation of the heating element (if it has one) or the insulation of the various timer switching contacts, solenoids and the motors. In fact, washing machines with heating elements often cause nuisance tripping with core-balance safety switches and it is a good idea to have them protected separately from other household circuits. Smoking inductor in power supply We are having difficulties with the 40V/8A power supply meeting the specifications outlined in your January/February 1992 issues. The unit is smoking the inductor L1 which is the 17-745-22 powdered iron core wound with 10 turns of 1.2mm enamelled copper wire. This is heating up to the point of it being a fire hazard if left to draw more than about 4.5A for more than a few minutes. (P. S., Perth, WA). • It does seem strange that this problem is appearing after four and a half years. However, the Neosid 17-745-22 core is not ideal for the job. The 17745-23 is better. Also reducing the turns to four or five rather than six will reduce core satura­tion. Similarly, using a few 0.8mm wires in parallel rather than 1 x 1.2mm wire for winding should reduce resistive losses. Alter­ natively, use an ETD49 transformer core as per the Nicad Charger in October 1995. Use the 0.5mm gap and 10 turns of 4 x 0.8mm wire as shown on page 59 of this issue. The secondary winding is not required. Notes & Errata Woofer Stopper MkII; February 1996: depending on which type of piezo loudspeakers are used, they can pro- Fence controller needs more output I would like to make some comments about two of your pro­jects I constructed. The first is the Electric Fence Controller described in the July 1995 issue. It was constructed correctly and all components, including the ignition coil work OK. Unlike electric fences on different properties that I have been to, the output of this device has very little bite. I made changes to replace the timing resistors so as to give a 0.1 second pulse every second and increase the wattage of the 6.8Ω resistor. I also built the Engine Immobiliser described in Decem­ber 1995 – a great idea. However, the 75V zener diodes shorted after testing and driving my car. This may be an isolated incident but to make sure I placed a 560Ω 0.5W resistor in series with the new 75V zeners and placed a 0.22µF 630VW capacitor across the collector and emitter of Q1. There has been no trouble since. (D. C., Narangba, Qld). • We should point out that Notes duce audible clicking at the rate the signal bursts to a high and low level. This can be cured by adding a 47µF 16VW electrolytic capacitor between the base and emitter of transistor Q3. The positive side of the capacitor connects to the base. The capacitor effectively slows down the rate that the burst signal rises and falls to eliminate any audible noise in the speaker. We should also point out that if the tweeter drive level control (VR2) is set too high, it can cause the same symptom. Minivox Voice Operated Relay; September 1994: diode D1 is shown with the incorrect polarity on the overlay diagram on page 33. LPATS: Striking A Blow Against Lightning; November 1996: the text on page 8 and in Fig.1 on page 6 refers to parabolas as the paths of possible lightning strikes. The term used should have been “hyperbola”. Engine Immobiliser; December 1995: there have been reports of the zener diodes in this circuit failing. In line & Errata were published on the Electric Fence Controller in the December 1995 issue. These noted that Australian Standard AS/NZS 3129: 1993 now specifies a maximum output voltage of 10kV instead of 5kV. In order to increase the output voltage by the required amount, the 6.8Ω 1W resistor should be changed to 1.2Ω 0.5W. While the coil on-time for the electric fence may need some minor adjustment for different coils we are inclined to the view that if the coil does not give a good output it is probably defective. We have seen one kit version of our electric fence controller where the circuit was working correctly but the HT output was non-existent. It turned out that the coil was a dud. As far as adding a resistor in series with the zeners for the engine immobiliser, this cannot be recommended as it will prevent the zeners from protecting the MJ10012 transistor. Howev­ er, in line with our circuit practice for high energy ignition systems, the specified zener diodes should be rated at 5W instead of 1W. with our circuit practice for high energy ignition systems, the specified zener diodes should be rated at 5W instead of 1W. Video Transmitter/Receiver; October 1996: it has been pointed out that some video camera modules have a DC output instead of AC. If these are used with the Video Transmitter it will not work. The cure is to connect a 100µF non-polarised electrolytic capacitor in series with the input socket. This can be wired directly between the RCA input socket and the input on the PC board. Fuel Injector Monitor; August 1995: we have recently seen a fuel injector monitor in which only eight of the LEDs would light instead of the full 16. The problem is that differing switching thresholds on the 4053 (IC2) can cause faulty switching of the LM3914 dot/ bar modes. If this occurs, the cure is to replace zener diode ZD1 with a 1µF electrolytic capacitor, with its negative lead SC connected to pin 3 of IC5. December 1996  99