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Low Voltage Rails for Plastic Power Amplifier I am about to start the “Plastic Power Amplifier” featured in the April 1996 issue of SILICON CHIP and would be obliged if you would let me know the following: (1) Would it be OK to use trannies I have, which are 300VA toroi­dals, giving a 40 volt rail, without compromising the circuit? (2) If OK, I guess the setting of the quiescent would remain the same? (3) Approximately what output would I have – 100 watts? (4) Do you use only off-the-shelf devices and parts; eg, matched pairs, o/p devices? Many thanks for your previous articles. I hope the Plastic Power amplifier is as good as my SILICON the same thing would happen if a car thief was to leave the wires connected after trying unsuccessfully to steal my car. I would then return to the carpark and find nothing but a pile of molten metal! I am confid­ent that the circuit was constructed and installed correctly. It operates fine over any period of time when the key is in acces­sory position. However, when the ignition is on, +12V is present at the coil connection and the circuit overheats. Could this be a design fault? If so, how could it be recti­fied? (T. V., North Adelaide, SA). • We think that your unit may be malfunctioning. When transis­tor Q1 is turned on, the current through the coil should be no more than about four or five amps, as determined by the coil resistance and its associated ballast, if it has one. Hence, the transistor should only be dissipating about 6 or 7W, when it is turned on. However, the transistor is only turned on for about 0.7 seconds in 2.9 seconds (0.7s on, 2.2s off) or 24% of the time. Therefore, even if the circuit is powered up continuously, the power transistor should only dissipate less than 2W. This will make it hot but is not likely to be enough to melt the plastic case or your car! It should be possible to check for correct operation of the Immobiliser with it out of the car. Connect a 1kΩ resistor bet­ween the collector of Q1 and the +12V supply and then apply 94  Silicon Chip CHIP amplifier using boards SC111287 (Decem­ber 1987) which more than compares with a Tandberg amplifier I had (to me anyway). (H. M., Balga, WA). • You can run the amplifier with 40V rails; the quiescent current setting would be the same. However, the power output would be markedly reduced, to around 50W into an 8Ω load or 100W into 4Ω loads. We have not bothered to use matched output devices in our circuits because they are generally not readily available. Howev­er, the use of matched pairs can produce a slight reduction in the harmonic distortion of an amplifier. power. Use your multimeter to measure the voltage between collec­tor and emitter of Q1. It should be +12V (or close to it) for 2.2 seconds, then close to 1V for 0.7 seconds, and so on. If it does not follow this sequence, check the operation of Q2 and the 555. Zener diode tester has incorrect transformer I purchased a zener diode test kit from Dick Smith Elec­tronics and have noted a couple of changes to the original cir­cuit as published in the March 1996 of your magazine: (1) T1 has prewound secondary winding with 136 turns and not 40 as per circuit. The primary winding was 18 turns and not 20 as per circuit and this was for the constructor to wind. The transformer in your article was stated to be a 2:1 step up transformer, thus the windings on the supplied transformer made it, by my calcula­tions, to be a 7.5:1 step-up transformer. (2) ZD1 was a 75V 5W diode not 56V 3W as in the March 1996 circuit. I decided to construct the circuit as supplied and on com­pletion the output voltage measured 470V instead of 112V. I checked and double checked with no change to my test results, so I decided to rewind the transformer with 36 turns to the second­ary and 18 turns to the primary, as I felt this would make it a 2:1 step up, and also be more in line with the original circuit. The output voltage now read 141V. As ZD1 is higher than original specs, the higher voltage output of 141V is probably acceptable. On testing known value zener diodes, the circuit appears to be measuring correctly. Could you please advise me of the implications of changing the ratio of turns on windings as I have done with my circuit. (G. M., Seven Hills, NSW). • We are aware that Dick Smith Electronics has been supplying a different transformer. However, the zener diode should still be 56V as specified, in order to be certain that the various versions of MTP3055 which may be used will not break down. The turns ratio for the transformer does not need to be precisely 2:1 since the circuit has current feedback from the Source of Q1 and this controls the overall level of power deliv­ered to the zener diode under test. However, the turns ratio should still be in the region of 2:1 for correct operation, given that zener diode ZD1 is 56V. Accordingly, with the 136 turn secondary, the primary winding should be somewhere between 60 and 70 turns. Alternatively, as you have found, the transformer can be wound with lesser turns on the primary and secondary and with a ratio of 2:1. We have advised Dick Smith Electronics to this effect. Making Clifford work in daylight I built your Mini Electronic Cricket (Clifford) as de­scribed in the December 1994 issue of SILICON CHIP. I would like to know how to reverse the LDR so it comes on in the light. (I. B., Strathpine, SA). • It should be possible to make the circuit operate in ambient light by swapping the positions of the LDR and the associated 47kΩ resistor. Notes & Errata Fluorescent Lamp Starter, August 1996: the circuit diagram on page 16 shows D1-D4 as 1N4004 diodes. They should be 1N4007 1000V types, as specified in the parts list. 2-Amp SLA Battery Charger, July 1996: the wiring diagram on page 57 has reversed polarity signs on the output cable crocodile clips. The cable coming from the lefthand side of the SC PC board should be positive.