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

Power Control
291
There are several important points to understand about the operation
of this circuit. The diac will reach its breakover voltage and trigger the
triac at the same relative point during each half-cycle of the AC waveform.
This relative point will depend on the charge rate of C1, which is
controlled by the setting of P1. In effect, the setting of P1 controls the
average power delivered to the load. P1 can control the majority of the
AC half-cycle because C1 also introduces a voltage-lagging phase shift.
Without the phase shift, control would be lost after the peak of the AC
power cycle was reached. Throughout the entire power control range of
this circuit, the power wasted by the triac is negligible, compared to the
power delivered to the load.
PL1 is a standard 120-volt AC three-prong plug. If you build this project
in an aluminum project box, the ground prong (round prong) should be
connected to the aluminum box (the chassis in this case). For safety reasons,
the 120-volt AC hot lead should also be fuse-protected. P1 is mounted
to the front panel of the project box for easy access.
I used a flat, rectangular aluminum project box large enough to set
the soldering iron holder on. I also connected the soldering iron internally
to a phenolic solder strip with a strain relief to protect the cord.
This, of course, is a matter of opinion. You might want to wire the circuit
to a standard 120-volt AC socket for use with a variety of soldering irons.
The triac, diac, and C1 can be assembled on a small universal perfboard
or wired to a phenolic solder strip.
The 15- to 25-watt lamp is a standard 120-volt AC incandescent lightbulb
of any style or design you like (it might also be any wattage you
desire, up to 60 watts). It is mounted on the outside front panel of the
project box and serves several useful indicator functions. First, it indicates
that the power is on and that the circuit is functioning. Second,
with a little practice, the brightness of the bulb is a good indicator of
about how much power you are applying to the soldering iron. For
example, if you’re using this circuit with a 60-watt soldering iron, and
you adjust P1 until the bulb is about half as bright as normal, you’re
supplying about 30 watts of power to the tip. Third, the lightbulb makes
a good reminder to turn off the soldering iron when you’re finished
working. (I can’t count how many times I have come into my shop and
found the soldering iron still turned on from the previous day.)
The best value for C1 will probably be about 0.1 F. After building
the circuit so that it can be tested using the lightbulb as the load, try a
few different values for C1, and choose the one giving the smoothest
operation throughout the entire power range. C1 must be a nonpolarized
capacitor rated for at least 200 volts.