Letters (EN, Jan. 1993)

Write to Letters, Electronics Now, 500-B Bi County Blvd., Farmingdale, NY 11735, USA.

NEW ADDRESS

The address and telephone number for TSW Electronics (the supplier for the "PC Controlled Test Bench" series that started in June 1992) have changed.

They can now be reached at 10751 Southwest 30th Place, Davie, FL 33328. (305) 452-1190.

DISKETTE DIFFERENCES

In the September issue of Radio-Electronics, there was an article in Q &A concerning the difference between the 3 1/2-inch double-density (DD) and high-density (HD) diskettes. You anticipated a host of comments from industry experts. Here's one.

For the past 22 years, I have been designing flexible diskettes and their ancillary test equipment. I have designed 8-, 5 1/4-, and 3 1/2-inch products for many major manufacturers. For three-and-a-half years, I worked for Sony Corporation to develop an industry standard that was adopted by the American National Standards Institute (ANSI), the European Computer Manufacturers Association (ECMA), and the International Standards Organization (ISO). During this time, I served as the chairman of the ANSI subcommittee that standardized the current PC format.

There are many good reasons why you should not punch the HD hole in DD diskettes, although there are some similarities between the two products. They share the same case (except for the extra hole and the silkscreened "HD "), and the magnetic media looks virtually the same. When the DD product was designed, there was no intent to use it for higher densities without some major changes. To double the density and be able to store 1.44 megabytes it is necessary that the coating thickness be reduced to about half that of the DD product.

The coercivity of the oxide was increased slightly to accommodate the higher recording frequency. It was also found out that we had to decrease the diameter of the hub to allow reliable recording at the innermost tracks. Those are not trivial changes, as some advertising would have you believe. Those changes were accompanied by many process changes to improve reliability. Those changes resulted in a requirement that the record current for HD be higher than for a DD product.

If you record a DD product using the HD record frequency and current, the recording will have greater peak shift and bit jitter. If you record and play back on one drive (and never plan to interchange data with another drive), and if you always keep your drive in top shape with frequent maintenance, you might not have any problem with punching holes in DD diskettes and recording them at HD densities. However, if you are among the vast majority of users who neglect their drives until they fail (as I do), you will most certainly have data failure.

WILLIAM B. PROCTOR; Cupertino, CA

RECYCLED TRANSFORMER

The "250-Watt Power Inverter" (Electronics Now, October 1992) looks like an ideal place for a transformer from a dead microwave oven. Those transformers are really rugged, readily available (when the oven dies, it is usually the magnetron or the digital control board failure that seals its fate), and easily converted for the low-voltage, high-current use.

Just remove the high-voltage and three-volt filament windings (a wood chisel carefully applied does it in about ten minutes with practice). Punch out the two magnetic shunts between the primary and the secondary. All of the transformers I have wound take one turn per volt, which makes it easy to calculate the turns ratio. I just size the secondary wire to match the load requirement.

The transformer should provide about the same power output as the oven from which it came. For the power inverter, just use two 12-turn windings with 12 or 10 AWG wire and you are all set-and you save $52.78. Keep up the good work.

LLOYD HARTENBERGER, Chester, IL

DIGITAL TACH FOR DIESEL CAR

In the November issue of Radio-Electronics /Electronics Now in Q&A

Mr. J. Hewitt of Florida, NY was looking for a way to hook up a digital tachometer to his diesel car.

He stated that he had put a disk with holes around the circumference of the alternator and was using an optical pickup.

It appears to me that he was overlooking the forest just to see a tree.

The alternator is an excellent signal source to drive a tachometer. However, he will need to go inside the alternator and attach a diode to the junction where the stators feed the rectifier diodes, run the other lead of the diode out of the alternator through some kind of insulated feed-through to his tachometer circuit. (Hook up the diode to give the desired polarity of the signal to drive his circuit.) The number of pulses per second will depend on the crankshaft speed, ratio of the crankshaft to alternator-pulley diameters, and the number of poles in the alternator.

If there's access to the teeth on the flywheel, this is another excellent source of pulses. It requires that a magnetic pickup be placed in close proximity to the flywheel teeth to detect each tooth as it passes the pickup.

-DUANE L. MITCHELL, Trivoli, IL R-E

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