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Speed control for fish tank pump I have a friend living on a property with a home electrici­ty supply. He has had the system in operation for about 17 years. It was built before commercial systems were available and is decidedly home-made. Amongst other voltages available in his system, he has a 32V DC supply from 16 lead-acid accumulators and from this he runs an electric air pump to supply air to his tropical fish. Due to the number of fish and the number of tanks he has he could not find a commercial pump to suit, so he has a home-made pump run by an aircraft electric motor which he runs from half the 32V bat­tery supply; ie, at 16V. He then alternates for video signals because its bandwidth is inadequate for this application. Instead, you will need a wideband op amp or discrete amplifier which can handle a bandwidth of at least 5MHz. The LM833 will easily handle the audio side of things and just one circuit will be able to drive four outputs, each with its own 100Ω series resistor and 0.33µF coupling capacitor. Fixing hum from DC plugpacks Have you any articles from previous editions of SILICON CHIP on how to deal with the problem of hum from DC plugpacks? So far, I have tried shielding (the earth) and connecting a 10µF capacitor across the output. No luck. (T. F., Malanda, Qld). •  Hum in DC plugpacks can be a very difficult problem to solve, depending on the application. The most effective approach is to use a 3-terminal regulator circuit to follow the plug­pack and this will certainly bring the ripple down to a very low value. We published a suitable circuit and a small PC board in an article entitled “The Eliminator” in the May 1992 issue of SILI­CON CHIP. The PC board can be installed in a small plastic utili­ty case which can then be glued to the back of your DC plugpack. the half bank manual­ly until both halves need charging, as near as he can estimate. This pump runs 24 hours a day and 365 days each year and has done for a few years now. It only stops when the rubber diaphragm tears and he switches to an identical standby pump until repairs are made. Now the point of this letter is would the motor speed con­ trol described in the June 1997 issue work on the 32V supply (probably 38V when charging) and be able to supply 15V to the motor? It should be well able to supply the current because he thinks it only draws a few amperes but the higher voltage is the concern. I suppose it would require the voltage divider supplying A1+ and the voltage divider supplying A1- to be altered. I see that In some situations though, the hum problem may be caused by a lack of earthing in the circuit being powered. This is impossi­ble to solve unless the DC plugpack concerned has an Earth pin on the plug section and a corresponding earth output connection. Such plugpacks are very rare. NTSC to PAL video conversion I have a faint recollection of seeing a project which will convert NTSC signals to PAL signals. I have a VHS PAL hifi stereo VCR. I would like to play VHS-NTSC format tapes in it and the BUK456-60 can stand 60V from drain to source but does this simplistic view cover all the difficulties? (R. B., Seymour, Vic). •  We can see no real problems with your application. The only thing that could worry the Mosfets is noise spikes from the charger. These are hardly likely to exceed 60V but if you are concerned, an iron cored choke and an electrolytic capacitor on the input to the speed control would remove the threat. There is no need to alter any other components, although you should check the temperature of the voltage regulator tab (REG1), as it may possibly need a small heatsink at this higher voltage. If the motor only draws a couple of amperes then you will only need one Mosfet. watch on my PAL TV. Do you have a project that will do this? If so, which edition of your magazine was it in and what was the esti­mated cost? (D. H., Melbourne, Vic). •  We described an NTSC-to-PAL converter in the April 1997 issue of SILICON CHIP. However it is of no use with your VHS VCR. If you want to play NTSC tapes, the only way to do it is with an NTSC VCR. You could then use our NTSC-to-PAL converter to watch the resulting video signal on a PAL TV. A better ap­proach would be to purchase a dual-standard VCR which could play both sorts of tapes SC and feed them to a PAL TV. Notes & Errata Remote Controlled Gates For Your Home, August 1997: the relay wiring to both motors M1 and M2 on the circuit diagram (page 69, August 1997) is incorrect. The diagram at right shows the corrected relay wiring for motor M2. Motor M1, which is driven by relays RLY1 and RLY2, should be wired in exactly the same fashion as shown here. The parts layout diagram shown on page 70 of the August 1997 issue is correct. September 1997  93