Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Power Control
295
As explained earlier, 410 milliamps is the highest current load that can
be drawn from T2’s “intended primary” under ideal conditions. However,
there are losses in power transformers, and, in this case, the losses are
doubled. Therefore, it is necessary to derate T2’s primary maximum current
value by 10%. Consequently, the maximum current output from
this hypothetical isolation circuit will be only about 370 milliamps—a
little too small for most general-purpose isolation requirements.
If you decide to build this type of isolation circuit for your bench,
you’ll probably want to use transformers with significantly higher
volt-amp ratings. Unfortunately, it soon becomes apparent that power
transformers in the 400- to 500-VA range are expensive, but so are isolation
transformers. However, some “odd” value industrial transformers
with high VA ratings can be purchased very inexpensively from many
surplus electronic dealers. Just make sure that they are not ferroresonant
transformers, or power transformers intended for use with 400-
hertz AC power.
Curiosity Catcher
Figure 9-8 is an SCR latch circuit. When assembled as illustrated, depressing
the “normally open” momentary switch (SW1) provides a positive
gate pulse (relative to the cathode) and fires the SCR. Once conducting,
the gate has no more control over the SCR, and it continues to conduct,
powering the piezo buzzer, until the cathode/anode current flow is
interrupted by depressing the “normally closed” momentary switch
(SW2). Because the cathode/anode current flow drops below the holding
current of the SCR (it actually drops to zero), control is returned to the
gate, and the SCR will not conduct again until SW1 is depressed.
Figure 9-8
SCR latch circuit.
SW1
Momentary switch
NO
Piezo
buzzer
+
–
9-Volt
battery
1 k
NC
SW2
Momentary switch
SCR