we apply to our universal motor.
The DC power component of the controller is supplied
by a transformerless capacitive power supply. With yet
another peek at Photo 2, all of the transformerless
capacitive power supply components are located below the
470 µF electrolytic capacitor (C3), which is to the right of
the TRIAC heatsink. You should be able to easily pick out
the transformerless capacitive power supply’s five
components, which include resistor R5 (100Ω) , capacitor
C2 (type X2 1 µF), zener diode D1 ( 5.6V, 1.5 watt), general-purpose rectifier D2 (1N4007), and electrolytic capacitor
C3. The transformerless capacitive power supply you see in
Schematic 2 will work for us (keep a stable voltage
across C3) as long as the current we draw from the
transformerless capacitive power supply is equal to or less
than the current that is feeding the power supply.
The amount of output current the power supply
provides depends on the value of R5, C2, D1, and the
magnitude and frequency of the input AC voltage.
(C3) is the output charge collection point that stores
and supplies voltage gleaned from the rest of the
transformerless capacitive power supply components. Here’s
the formula that will reveal the amount of current taken
from the 120 VAC mains:
• IN = Current available at
power supply output
• VRMS = AC mains input voltage
• VZ = Voltage drop across zener diode
• f = Frequency of AC mains input voltage
• C1 = Value of universal motor controller capacitor C2
• R1 = Value of universal motor controller resistor R5
I had a great deal of fun playing with this formula. So,
to make it just as much fun for you to manipulate the
parameters, I whipped up an Excel page (included in the
download package available at www.servomagazine.
com) that calculates IN using your input values. Screenshot
1 is a spread of values using a tolerance of 20% for the
blocking capacitor and 1% for the blocking resistor. The
other parameters are hand-picked assumptions with the
exception of the VZ value (zener voltage drop), which I
extrapolated from the 1SMA5913BT3 datasheet.
I overdesigned my universal motor controller
Screenshot 1. I used a 1.0 µF X2-type capacitor and a
100 two-watt SMT power resistor in this transformerless
capacitive power supply to provide ample current for driving
LEDs and optocouplers. I also overdesigned the PIC18F2620
into this circuit as to not want for I/O if I needed it later. You
can design in just as much PIC as you want and just as much
DC current as you need for your personal universal motor
transformerless capacitive power supply as I didn’t end up
using as many current-hungry LEDs and optocouplers as I
had envisioned. As you can see from Screenshot 1, I have
about 30 mA of available current. The formula for
computing the available current is generous. The actual
amount of available current will be somewhere around
80% of the computed value, or 24 mA.
The PIC18F2620 is also overdesigned as I have plenty
of PIC I/O left over after servicing the controller’s AC and
logic interfaces. The main reasons for choosing the
PIC18F2620 for this application were its built-in oscillator,
ample I/O, and EUSART. A smaller PIC footprint will also
work with our controller AC and DC components. Just
make sure your PIC has enough I/O lines to do the job.
It’s pretty obvious in Photo 2 as to where the PIC,
optocouplers and all the supporting microcontroller
components are located.
It is important to note that our transformerless
capacitive power supply has its DC positive component
referenced to the 120 VAC mains Line, which is also
connected to the triac’s MT1 terminal. The reasons for the
zener diode’s cathode connection to the 120 VAC mains
Line are twofold. First, we want to trigger our BTA16-
600CW using a low-going pulse. The reason for selecting a
low-going trigger pulse is dictated by the quadrants in
which we desire to trigger the BTA16.
A careful study of Figure 1 shows us that a typical triac
can be triggered by a positive or negative going pulse.
Since we’re triggering the controller’s BTA16 with a low-going pulse, we can only trigger it in quadrants II and III.
For inductive loads, triggering in quadrants II and III is
desirable as the triggering energy is not excessive here.
Simple diac-based lamp dimmer circuits — which are
fashioned similarly to Schematic 1C — usually trigger in
auadrants I and III. Transpose the diac trigger diagram in
Figure 2 over Figure 1 to see why. quadrant I is also a
desirable triggering quadrant as it does not require
excessive triggering current. Note that quadrants I and III
are complementary as far as triggering and AC cycle conduction are concerned. For most applications, quadrant IV
is to be avoided as it takes the most triggering energy. Note
also that both the positive and negative portions of the AC
cycle can be processed with a low-going triac trigger pulse.
The BTA16-600CW requires a minimum of 35 mA and
a maximum of 60 mA of gate current flow to trigger. The
PIC18F2620 can sink 25 mA per I/O pin. I’ve paralleled 3
I/O pins to provide a total of 75 mA of sink current
capability. The 47.5Ω 1% resistors (R1, R2, and R3) are
present to level the I/O pin load across the three current
sinks and limit the maximum amount of gate current drive.
In our universal motor controller application, I’ve selected
a minimum logical high level of three volts. The BTA16-
600CW datasheet tells me I need to be ready to sink a maximum of 60 mA of current through the triac gate. Using
Ohm’s Law, that equates to:
R = 3 volts / 0.060 Amperes
44 SERVO 11.2008