but I’m open to suggestions.
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.
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
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
Table 3. 74HCT73 Logic
range of 0.7 to 3. 7 volts by forward biasing them. The
forward current is critical to the actual voltage, as shown in