Fullscreen HTML5Med Res (??MB)HTML4

Using Polyswitches for loudspeaker protection I wanted to build the 50W audio amplifier published on the March 1994 issue and I figured the 2-way bookshelf speakers published in January 1993 issue would be an excellent match. I would like to protect the speakers using Polyswitch protectors but the thing is, how would I connect the Polyswitches on the crossovers and what sort of Polyswitches do I need? I also want to know what kind of Polyswitches would suit a “Nippon America” 10-inch 100W RMS sub­ woofer with an impedance of 4Ω? I would like to run it with the 50W amplifier. One last question: can Polyswitches be connected in parallel to double their maximum current rating? (M. C., Yarraville, Vic). • We featured Polyswitches in the 50W and 100W amplifier designs published in the December 1988 issue of SILICON CHIP. Suitable devices are stocked by Jaycar Electronics. You can insert them individually in series with the tweeter and woofer or just have one to protect each system. For the latter approach, we would suggest the Cat. RN-3415 from Jaycar. These could also be used to protect your 10inch speakers. You cannot connect Polyswitches in parallel as they will not share the current equally. Another vote for a signal strength meter In reference to the letter from B. P., of Port Macquarie, in the April 1994 issue, I too would be blessed if you would design a signal strength meter and present it as a project. As he states, commercial units are expensive and no doubt this is due to their flexibility and accuracy; a design based around a tuner/front end from a VCR should be just as good. I have included with my letter the circuit of a tuner/front end which is made by Sanyo and are easy to come by secondhand – most of their Beta machines are fitted with them. The beauty of this unit is that it only requires +12V for the supply and 0-12V to tune across the entire band. Band­ switching is achieved by taking one of three lines high as required. This makes it an ideal choice for a portable unit as it can then be Notes & Errata Microprocessor-Controlled Nicad Battery Charger, September 1993: a number of errors have come to light in the circuit on page 17. Pin 6 of IC2 should be labelled pin 1 in Fig.1 and the 330Ω resistor associated with Q3 should be in series with the emitter resistor. VRI and the 30kΩ resistor are reversed com­pared with the PC board layout on page 20. The lower of the two series resistors to earth from pin 9 of IC1 should be 2.2kΩ. Finally, the 100µF capacitor at the output of the 5V regulator is shown with incorrect polarity on the wiring diagram on page 20 and the 12V relay has been omitted from the parts list. 4-Bay Bow Tie UHF Antenna, July 1994: readers in eastern states who are having difficulty purchasing 4.74mm dia, 0.91mm wall thickness aluminium tube for this project should note that it can be purchased from the Alcan Aluminium Centre, Lidcombe, NSW (Phone 02 647 9900) or ordered from them through their local Alcan Centre. Readers in other states may also do this but they will have to pay freight from Sydney to their location. The reflectors and dipoles may be made from thicker tubing or rod; eg, 6.35mm diameter with suitable adjustment being made to the dipole mounting clip dimensions. The reflector, dipoles and the connector harness pieces could also be made powered by eight 1.5V cells or a 12V sealed lead acid bat­tery. In designing the meter, the following should be taken into account: (1) It needs to be able to detect and display signal strength from 1µV up to 2-3V (via a switched attenuator); (2) It should have a large, easy to read display, either a moving-coil meter or a digital readout (if not both); (3) It should be easy to calibrate without the use of expensive test equipment and have optional outputs at the rear for IF (to use the meter as a substitute tuner) and Video/Audio to feed to a monitor for picture quality evaluation. Judging by the comments I have had from others, a project like this would be extremely popular, as most from 4.74mm dia aluminium rod. For the harness pieces this material will need heating and hammering on an anvil in the areas where holes have to be drilled. The connector pieces could also be made from 3.2mm aluminium rod which is readily available from CIG welding cen­tres. Discrete Dual Supply Voltage Regulator; April 1994: the PC board pattern and the overlay pattern featured on page 31 has an error in that pin 3 of IC1b is not grounded. The board can be corrected by connecting a short link across to the adjacent GND track. Fast Charger for Nicad Batteries, May 1994: this circuit has caused confusion to many constructors because of its method of dV sensing to end the fast charging mode; it will not work unless it is actually charging cells. If you attempt to test the circuit without a nicad battery load, the output voltage will rise until pin 7 reaches +4.25V whereupon the circuit will switch off. For further background on this circuit, refer to the arti­cle on using the TEA1100 IC on page 6 of this issue. Dual Diversity FM Tuner, August 1994: the varicap diode D5 is shown on the circuit the wrong way around. It is shown correctly on the wiring diagram of this month’s issue, on page 68. commercial units start around $1500. There have been other designs presented before but in general they were uncalibrated or involved lengthy construction. (S. W., Hamilton, NZ). • The problem with designing such an instrument is not so much the tuner front end but that of calibration. Without calibration, such instruments are fairly useless. Tuners do not respond equal­ly over their band and therefore it is necessary to produce a calibration curve for each individual instrument if the results are to be accurate. This really does make it difficult for us to produce a useful product unless we can come up with a simple calibration procedure. At the moment, we don’t SC have a solution. August 1994  93