The Case of the Precision Analog Eight-Hour Timer

By Wayne Pickette

Problem solved in 1998 for the NEC Ultralight computer battery charger called the “Klip charger” The design was 1/3 the size and 1/4 the cost of all the competition.

The secret:
1n914A diode vs. 1n4001 as known since 1973. The difference between the two diodes is the reverse blocking leakage.

I was operating International Research in Palatine IL at the time. I had my desk from early years shipped from California to Palatine. It had some 1968-9 era parts in the center drawer.

When I began to design the circuit, I went to Radio Shack and purchased a 555 timer and a cmos logic gate and a programmable 3-pin voltage regulator.
When I put the circuit together on my breadboard, I used diodes from my center drawer. The initial time test result was 7.64 hours, a heat gun and a circuit cooler could not shake this reading. I tuned the time to exactly eight hours then I did the temperature test again. Not a single perceptible variance was detected.
I went to again to Radio Shack with a parts list, to purchase parts for a production prototype. When I assembled the production prototype, it timed for 1/2 hour!

I began to exchange parts between the two breadboards when I remembered from back in 1968 the 1n914A had been used to isolate vacuum tube signals from solid state circuitry. Getting out my meter I compared the two diodes…….I understood. I called supplier there were not a single 1n914A in stock anywhere! After a few phone calls I found a company in Connecticut that could make the parts for me at the cost of $.92 apiece. It raised the price of the charger $1.19. I do not have pictures it was so long ago but could, I think recreate the circuit, and I did have a copy of the $10,000 check NEC wrote me for the design! I got them grandfathered in UL approval by driving the circuit from an UL approved wall DC transformer supply.

Born in Illinois then raised in California from age six months, Wayne Pickette heralds from California, where he began to study electronics at the age of thirteen. At the age of seventeen he purchased his own computer.