Nuts & Volts - July 2004

Q&A

but I’m open to suggestions.

Doug Adams via Internet

A. Whatever the flip-flop, it’s easy to set the states of the outputs on power-up using the set and reset inputs. There are two types of JK flip-flops: set/reset and asynchronous reset. Let’s examine the set/reset 4027 first.

Its logic table (Table 2) shows that the outputs can be forced into any state through the set and reset pins, regardless of what exists on the clock (CLK), J, or K inputs. To create a stable state at power-up, one of these inputs has to lag behind the others.

An RC combination does just that, as shown in Figure 5. When power is applied, C1 charges through R1 until it reaches + 5 volts, which forces their respective outputs low and arms the flip-flop.

The logic table for an asynchronous reset 74HCT73 is shown in Table 3. As before, the capacitor pulls the reset pin low at power-up and forces the Q output low, then charges to + 5 volts for toggle operation.

Triac’s Sidekick: Sidac

Q. I recently came across the names of two electronic components that are unfamiliar to me: sidac and stabistor. Could you explain them? If you know of any more “weirdo” components, please let me know their names and functions.

John Agugliaro via Internet

A. You know what a diac is, right? Well, the sidac is the triac equivalent of a diac. Now in English: The sidac is a bidirectional negative resistance device. Let’s place a voltage across a sidac and see what happens.

At low voltages (typically under 70 volts), the sidac behaves like a switch in its off state. As the voltage across the sidac is increased, a point is reached where the switch turns on and conducts current heavily — and will continue to do so until the voltage is removed (Figure 6).

The difference between a diac and a sidac is polarity. A diac is a polarized device that acts like a diode — that is, it has one-way current flow, whereas a sidac can conduct current in both directions. It’s most commonly used to trigger triac gates, strobe xenon flash tubes and HID, ignitors for natural gas, and generators for high-energy pulses for flyback operations. A datasheet with applications www.littelfuse.com/data/Data_Sheets/E9Sidac.pdf) tells a more complete story.

The stabistor is a low voltage zener diode with one to four P-N junctions, where the forward voltage drop across the junctions provides superior dynamic impedance to low voltage zener diodes, which use avalanche electron tunneling. In English: It’s kind of like stacking a bunch of 1N4001 diodes in series to achieve a stable voltage reference in the