April 10, 1997

A dime for your thoughts

I read with interest Bill Travis’ article, "For want of circuit-protection devices…" (EDN, Feb 3, 1997, pg 46). I had a few comments about overcurrent-protection devices, especially as they relate to the picture above. First, at my organization we prefer to use pennies instead of dimes. A penny carries almost the same current as a dime, and a penny is significantly cheaper (nearly a 10-to-1 cost ratio!). Of course, if you require higher current capacity, you can resort to nickels, quarters, or even half-dollars.

Second, you can further reduce costs by using Canadian money, which will show your company’s good intentions by making your product truly international. It is also good manners to help neighbors to the north by removing their currency from circulation. However, if you are seeking CE marking, you may be required to use appropriate sizes of European money.

Finally, some engineers here feel that circuit protection is better if you place the coin "heads up." I don’t feel this way, especially considering reciprocity principles. Additionally, in ac circuits, half the time the current flows from heads to tails, and the other half of the time the current flow is reversed. In this case, what would be the correct orientation?

William Reams
via the Internet

Design Ideas patent

The Design Idea "Capacitor charger improves high-voltage supply" (EDN, Dec 19, 1996, pg 76), by Kurk Mathews and Jesus Rosales, is an excellent method of charging an energy-storage capacitor. I hold US Patent 5,485,361 on that method. The patent includes a detailed description of the circuit’s operation, with design equations that yield a design that charges a specified capacitance to a specified voltage within a specified time, with the smallest possible peak currents in the power components and the least possible power dissipation in the components’ parasitic series resistances. My colleague, Dr Richard Redl of ELFI SA, Switzerland, and I will publish an expanded technical description in the September 1997 issue of IEEE Transactions on Power Electronics.

Please note that it is illegal to make, use, or sell equipment embodying that charging method without having a license under that patent. I will be glad to issue licenses under the patent to interested parties and will send copies of the patent on request. Please enclose a self-addressed #10 envelope with postage for 2 oz.

Nathan O Sokal
President, Design Automation Inc
73507.247@compuserve.com

Invent this

I enjoyed Clive Maxfield’s article on the use of 1’s complement in an ALU. It took me back a few years to a college course I took in which we designed and built a simple microprocessor using 22V10s. I thought I was really smart for coming up with the inverter block, but then I seem to recall inventing the ring buffer, too.

I was intrigued, though, that you included a signed-byte comparator as a part of a basic ALU, because a number of chips don’t include one. (The good ol’ 8051 comes to mind.) For unsigned bytes, a SUB instruction works just as well, and, with some coding, you can compare signed bytes, too.

JT Klopcic
via the Internet

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