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PWM Line-Voltage Interface Brightens Your Outlook
William Grill, Riverhead Systems
Several of William Grill's designs have included pulse-width modulator (PWM) circuits that control
LEDs. But what about the control of line-powered lamps and fixtures? You can find several
commercial PWM controllers, but build one yourself, save money and learn how to implement a
microcontroller-based design. This circuit is no flash in the pan.
This circuit accepts a PWM signal from an external source, processes the signal, and uses the
result to turn on an optically coupled triac (triode for ac) in sync with 60-Hz 120V ac line power.
The circuit provides both an educational and an affordable project for those Blue Light Special
gadget freaks and model-railroad enthusiasts who want to add a bit more control in illumination
projects.
CAUTION
The circuit derives its power directly from a 120V AC line connection without a step-down power
supply or any electrical isolation, so exposed components are electrically "hot" and could pose a
shock hazard. Neither William nor Design News accepts any responsibility for your safety or the
safety of others who build or use this circuit. We cannot be responsible for how you build,
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fabricate, use, or package this circuit to get it into a form you can use. Work on a nonconductive
surface. Keep the unpackaged circuit away from children and others. During testing, connect test
instruments or probes to the UNPOWERED circuit and then with one hand in your pocket, plug in,
or turn on, the circuit and read your measurements. Package the final circuit in a non-conductive
package. If you must use a metal enclosure, ensure that you have carefully grounded the metal
case to the ground terminal (green wire) on your line-power plug.
Circuit Description
This circuit uses an inexpensive microcontroller (MCU) to read the a filtered PWM signal that
comes via an isolated input. The MCU then uses that signal to synchronize the PWM input with
the phase of the 110V AC line power. The synchronization ensures careful control of a load
device. See the schematic diagram below. For a larger schematic diagram, go to:
www.gfreak.com/GF159/GF159_Schematic.jpg.
One optical coupler (U1) isolates the external PWM-Control inputs from the rest of the circuit.
This PWM signal could come from another circuit, a microcontroller's PWM output, or a similar
source. A second optical coupler (U2) includes an optically driven diac (diode for AC) that
connects to the triac to control the line-powered load device.
An inexpensive Microchip Technology 12F675 MCU (IC1), provides the data-processing
functions. The lamp-related hardware uses a triac (TRIAC) and an optically coupled MOC3010
diac (U2) of the types commonly used in lamp-dimmer and AC motor-control circuits. The
remainder of the circuit hardware isolates the PWM Control inputs and provides the circuit with
power directly from a 110V AC outlet. (Jameco Electronics sells the MOC3010 as part number:
26278. It is a non-stock part at Allied Electronics.)
PIC12F675 Pin #
Pin Name
Connection
1
Vcc
To 5V from 78L05
2
GP5
Not Connected
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PIC12F675 Pin #
Pin Name
Connection
3
GP4
Jumper to Ground, connects to 5V through a 10Kohm resistor
4
GP3
Input from 120V AC through 1-Mohm resistor
5
GP2
Output to MOC3010
6
Vref
Not Connected
7
AN0
Input from MCP6002
8
GND
Ground
The GP4 input at pin 3 on the MCU lets users "invert" the sense of the PWM input. If you leave
the jumper in place so it grounds (logic 0) the GP4 input, a 0-percent PWM input at the PWM
Control points results in no power to the load through the triac. A 100-percent PWM input causes
the circuit to put 100-percent power through the load.
Depending on your application, you might choose to reverse these conditions so a 0-percent
PWM input at the PWM Control inputs causes the circuit to apply 100% power to the load and a
100% PWM input causes the load to receive 0-percent power. In this case, remove the jumper
between the MCU's GP4 input (pin 3) and "ground" (the 120V AC white wire) so the MCU senses
a logic 1 at this input, provided via the 10 Kohm resistor that connects to the +5V supply.
When you apply power to this circuit, the MCU causes a short delay. Then the control program
uses the MCU's internal clock and a counter to measure the period of the AC-line signal present
on the MCU's GP3 input at pin 4. This signal comes directly from the "hot" side of the 110V AC
signal through a 1-Mohm resistor. The 5.1-V zener diode (D1) prevents the GP3 input from going
above 5.1 volts, which could damage the MCU.
The PWM control signal reaches the circuit through a CNY17-1 optical coupler (U1). The output
of this coupler passes through the MCP6002 operational-amplifier circuit and to the resistor and
capacitor on the op-amp's output, which create a low-pass filter. Next, the MCU reads the filtered
voltage at pin 7 (AN0) and converts it to a 10-bit digital value. The analog-to-digital converter
(ADC) measures this input signal once per AC line-voltage cycle.
You should provide a continuous PWM signal for the PWM Control input with a voltage that
swings between 0 and 2 to 5 volts and with a current of at least 8 mA. Apply this signal to the
PWM Control + input and ground the PWM Control - line. The CNY17-1 device can handle a
maximum reverse voltage of 6V and a maximum forward current of 60 mA. For complete
specifications and a data sheet for the Vishay Semiconductors CNY17 family, visit: <A HREF =
"http://www.vishay.com/docs/81863/81863.pdf">www.vishay.com/docs/81863/81863.pdf</A>.
The 60-Hz line frequency and the effect of the PWM period and of the op-amp-based filter, limit
the controller’s output pulsed-phase response. I recommend you use a PWM frequency of 900 Hz
or greater at the PWM Control input. PWM frequencies above 900 Hz reduce the filter-related
ripple on the voltage applied to the ADC at the MCU's AN0 input.
The MCU code uses the 10-bit PWM Control value and the measured AC-line period to define the
corresponding delayed output control pulse. In the circuit shown in the schematic diagram, and
with the MCU running the supplied program, a small PWM Control signal would cause the MCU
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to place pulses very near the end of each half cycle. (See the oscilloscope images below.) The
maximum PWM Control signal would produce pulses at the very start of each half cycle. Thus,
the MCU synchronizes its triac-control pulses with the AC-line cycle times.
The MCU's GP2 output (pin 5) generates the pulses that control the MOC3010 diac (U2) that
turns on the triac and delivers power to the load device. William coded these pulses for a width of
about 50 μsec. A pulse causes the triac to turn on. A triac device turns off on its own when the
voltage across it reaches, or transitions through, zero volts.
The MCU translates the filtered PWM Control signal into a linear phase across each half cycle of
the AC-line. Internal timing loops, used in the controller code, each have a resolution cycle time
of 50 μsec. The code uses only eight bits of the digitized 10-bit PWM Control value, so the code
resolves each 180-degree half cycle to within about 1.4 degrees. In the prototype, William found
that minor differences in the code's loop-execution times contributed to some additional jitter,
resulting in a total error of up to 2.8 degrees. This 2.8 degrees is the phase variation you might
observe during each half cycle (180 degrees), given a constant PWM Control input. (The minor
differences come from sources of errors in the translated phase position and include the multiple
branches associated with the code execution. Depending on the dynamic variables processed in
the code, the position of the output control pulse ‘product’ may appear to be offset and thus ‘jitter’
in time. The controller’s internal timing is synchronized to the 60Hz.)
Power Supply
Because this circuit obtains power directly from the 110V AC line, you must pay careful attention
to component values and ratings. The 0.47-μF 250V capacitor (C1) acts like a "ballast" that
supplies current to the circuit. The larger the capacitance, the more current available. The two
1N4003 diodes, shown near C1 in the schematic diagram, provide a positive AC voltage to the
22-μF 50V capacitor (C2) that helps filter the AC signal that goes into the 78L05 voltage regulator
(U3). This regulator produces steady 5V DC power for the MCU and the devices that connect to
it. (For 120V AC applications, you must use a 0.47-μF capacitor and diodes rated for AT LEAST
200 volts. Diode ratings include a peak-inverse-volts, or peak reverse-voltage, specification, and
the two 1N4003 have a PRV or PIV of 200V.)
For the capacitor at C2, do not use a 22 μF capacitor with less than a 50V working-voltage rating.
A higher voltage rating will work, though. The 5.1 Kohm load resistor (R1) and the LED in series
with it ensure the circuit draws enough current to load the 0.47 μF ballast capacitor and maintain
a voltage less than 25V across the 22 μF capacitor (C2).
The 0.47 μF value of the ballast capacitor was chosen so the 78L05 voltage regulator could
provide about 6 to 8 mA for the circuit. This design could use a 0.27 μF ballast capacitor for a
load of between 4 and 6 mA. Mismatching the capacitance value to the load might cause a
voltage higher than the 50-V specified for the 22 μF capacitor (C2).
In most circuits you want a power source that provides an isolated low voltage. You can
substitute a plug-in power cube that supplies DC power to the input of the 78L05 voltage
regulator. (The input voltage to the 78L05 regulator should be between about 8 and 20 V DC.)
You still must connect the MCU to the 120V AC line with the 1-Mohm resistor and with the 5.1V
zener diode (D1) so the MCU can detect the line-voltage timing.
Remember, you have NO ELECTRICAL ISOLATION between the controller, other components
and the AC line. In this project, the CNY17-1 optical isolator (U1) electrically isolates the external
PWM Control input from the line voltage. Consider everything else electrically ‘hot,' and thus a
potential shock hazard.
One Design News Gadget Freak described an MCU-based model-railroad light set that uses
PWM control. I used the model-railroad circuit to demonstrate this PWM phase controller. I have
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taken the beacon output from that circuit and uses its modulated PWM output, as shown in the
short video for this Gadget freak design.
Waveforms from the Circuit
In the scope photos below, the upper trace shows the voltage across the triac, so the flat
"plateaus," or 0-volt portions of the trace indicate the triac is on and conducting current. Thus, the
voltage across it comes close to zero volts.
The bottom trace represents either the phase-control signal from the MCU or the PWM Control
signal from the user's external PWM device. Each photo identifies the lower-traces source. The
video for this Gadget Freak case includes dynamic scope images.
Scope screen 1. With 0-percent PWM, shown in the lower trace, the triac has only a minimal on
time--she short 0-volt plateaus.
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Scope screen 2. This image shows the voltage across the triac (upper trace) and the MCUgenerated control pulse (lower trace) delivered to the diac switch (U2 in the schematic diagram).
The upper trace represents about a 20-percent pulse-width modulation at the circuit's PWM
Control input.
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Scope screen 3. This image shows the same result across the triac for ~20-percent PWM. Note
the triac turns on in each half cycle. The lower trace shows the PWM input signal measured at the
PWM Control input to the circuit.
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Scope screen 4. At a PWM Control of ~70 percent (lower trace), the triac stays on (upper trace)
for a longer time.
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Scope screen 5. A change of the PWM Control input signal to ~50 percent (lower trace) turns on
the triac (upper trace) for approximately half of each AC line cycle.
The information used to time the placement of the triac-on pulse in each half cycle of the 60-Hz
line power depends on the filtered voltage applied to the MCU's AN0 input the reference voltage
set for the MCU's ADC. In this circuit, William used a programmable option to select the MCU's
supply voltage (the 5V output from the 78L05 regulator) as the ADC's reference.
For a complete data sheet for the PIC12F675 MCU, visit: <A HREF =
"http://ww1.microchip.com/downloads/en/DeviceDoc/41190F.pdf">ww1.microchip.com/download
s/en/DeviceDoc/41190F.pdf</A>.
You can use this PWM circuit with a fader application, found in EDN magazine's 12 November
2009 issue. Other applications include an illumination ring (EDN, 9 July 2009) and a
programmable table-driven PWM sequence controller (EDN, 20 July 2006). Find EDN's archives
at www.edn.com/archives.
Build Instructions
You can use a small piece of perf-board (holes on 0.1x0.1-inch centers) to mount the
components and make the connections. This PWM circuit's triac can carry several amperes, so
use appropriate gauge wire that matches the current drawn by your load (motor, lamp, etc.). The
larger the wire the better. (Remember, smaller wire-gauge numbers indicate larger wire cross
sections.) William did not include a fuse on the output, so you might include one when you
package your implementation of the circuit. Place the fuse in the circuit between the 120VAC
connection and your line-power plug.
For questions about compatibility, code or circuit modifications, contact William Grill at:
contact@riverheadsystems.com.
Bill of Materials
Amt.
Description
Allied Part No.
1
CNY17-1 Optical Isolator
431-0064
1
MOC3010 Optical Isolator
See Text
3
47-ohm, 1/8W Resistor
296-4733
2
2.2 Kohm, 1/8W Resistor
296-4725
1
5.1 Kohm, 1/4W Resistor
296-6517
1
10-Kohm, 1/8W Resistor
296-4721
1
1-Mohm, 1/8W Resistor
296-6647
1
Green LED
405-0106
1
0.47 μF, 250V Capacitor
862-0602
1
4.7 μF, 50V Capacitor
852-7070
2
10 μF, 10V Capacitor
541-0585
1
22 μF, 50V Capacitor
852-7074
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2
1N4003 Diode
411-0003
1
L6004L5 Triac
846-0051
1
12F675 PIC Microcontroller
383-0292
1
MCP6002 Op Amp
383-0227
1
1N5231 Zener Diode
431-0365
1
78L05 Voltage Regulator
288-0630
1
2-Pole Terminal Block
502-0421
2
2-Pole Terminal Block
502-0492
2
8-Pin DIP Socket
374-4541
1
6-Pin DIP Socket
374-4540
Programming the MCU
William used a Microchip PICSTART Plus programmer but other PIC MCU programmers will do
the job, too. The figure below shows the configuration screen provided by Microchip's free
MPLAB integrated-development environment software that will work with the assembly-language
code included below the figure.
Details of the controller configuration settings:
Code Listing
; FROM SCRATCH AND derived from PWDIM16Z_GREATEST.ASM
11/9/09
; PW TO DIMMER CONTROLLER this is now a pwm to analog to dimmer
controller
; makes the app a bit more flexible and eliminates 2nd controller
;
; F675 bit A/D
;**********************************************************************
***
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GPIO
equ 5; 6
STATUS
equ 3
PC
equ 2
trisr
equ 0x85
; reserved equ 7
dwell
equ 0x30
index1
equ 0x31
;minimum
equ 0x0A ; min # keyservice loops : sequence 2*23/60 min
;keyimage
equ 0x32 ; masked key image
;savephase equ 0x33 ; max # keyservice loops : sequence 10*23/60 max
temp1
equ 0x34 ;
temp2
equ 0x35
;
cnt
equ 0x36
datareg equ 0x37
;
;keyloop
equ 0x20 ; keyservice/debounce loop servie counter
temp
equ 0x21
bits1
equ 0x22
dwelltimr equ 0x23 ; maintains active count of dwell for each tabled
value
newcount
equ 0x24
mresult
equ 0x25 ; product of result sub service
pointer
equ 0x26 ; pointer of the tabled value
cyclecnt
equ 0x33 ; saved value of phasecount representing number of
timing loops in poitive half cycle
phasecount equ 0x28 ; working counter of timing loops in poitive half
cycle
bits
equ 0x2A
phase
equ 0x2B ; working counter of number of loops to assert
output
newphase
equ 0x2C ; saved value recovered from the tabled profile
resindex
equ 0x2D ; maintained indexed count of the dwell per the key
inputs
number
equ 0x2E
timer
equ 0x2F
ad0
equ 0x1F
ad1
equ 0x9F
addata
equ 0x1E
;
#define b0
STATUS,5
#define output GPIO,2
#define input
GPIO,3
; bits
#define outflag
bits,0
#define debounc
bits,1 ; state flag for key service
#define firstpass bits,2
#define pflag
bits,3
#define shutdwn
bits,4
#define up
bits,5
#define down bits,6
;
#define sflag
bits1,0
;#define pretest
bits1,1
;
#define carry
STATUS,0
#define zero
STATUS,2
GF159 William Grill: 110V AC PWM
;
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*********** START ********************
goto s1 ; reset vector
nop
nop
; intpr vector
nop
nop
s1:
clrf
GPIO
movlw 0x11 ; 0001 0001
bsf
b0
movwf ad1
movlw 0x3B ; 11 1011
movwf trisr
bcf
b0
movlw 1
; enable
movwf ad0
; ********************************************************
topp:
goto
realstart
keyserv:
goto
xkeyserv
result:
goto
dresult
delay10:
nop
nop
nop
nop
nop
nop
retlw
0
; ********************************************************
realstart:
clrf
bits
clrf
bits1
clrf
temp
movlw
0xFF
movwf
temp1
movwf
temp2
clrf
cyclecnt
real3:
decfsz temp1 ; wait ~2 sec 254*254*16
goto
real3a
decfsz temp2
goto
real3a
goto
real3b
real3a:
decfsz temp
goto
real3a
movlw .4
movwf temp
goto
real3
; ********************************************************
real3b:
;
recall the full scaled phasevalue would be interpreted as 0 degr
bsf
firstpass
topb:
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btfsc input
; wait for - phase
goto
topb
topq:
btfss input
; wait for + phase
goto
topq
; ************************44 instructions ***********************
topa:
; all loops are 50 instructions
btfsc firstpass
; test state flag for special process complete
goto
topa41
; 6 ; non calibration + phase
movfw phasecount
movwf phase
; phase is working counter to desired phase
btfsc zero
; qual for maximmum value
goto
topa11
topa3:
call
delay10
call
delay10
call
delay10
call
delay10
decfsz phase
goto
topa2
goto
topa1
topa2:
btfss input ; test/qual input hasn't transistioned
goto
topa5
nop
nop
goto
topa3
;
topa11:
bsf
output
call
delay10
call
delay10
call
delay10
call
delay10
call
delay10
topa1:
bsf
output
topa4:
;
coast to negative transistion
btfss input
; wait for - phase
goto
topa5
nop
nop
nop
nop
nop
nop
nop
nop
bcf
output
; makes a 10 us pulse
goto
topa4
topa41: ; 5
nop
nop
nop
nop
call
delay10
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call
delay10
call
delay10
call
delay10
incf
cyclecnt
bcf
firstpass
btfss input
; wait for - phase
goto
top10a1
nop
goto
topa41
; ******************************************************
topa5: ; 3 in negative phase
bcf
output
; just in case
movfw phasecount
movwf phase
btfsc zero
; test for maximuum
goto
topa10
; normal - cycle
btfsc phase,7 ; test if phase is >127 loops
goto
topa8
goto
topa6
; 10 loop to pulse
topa7:
nop
nop
nop
nop
nop
nop
nop
nop
nop
topa6: ; 12
nop
nop
nop
nop
nop
call
delay10
call
delay10
call
delay10
nop
decfsz phase
goto
topa7
topa10:
bsf
output
call
delay10
call
delay10
call
delay10
call
delay10
call
delay10
top10a1:
call
keyserv
movfw newphase
; save value 'newphase'
movwf phasecount ; set up for next cycle
goto
topq
topa8: ; 9
call
keyserv
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movfw newphase
movwf phasecount
; assumes call plus routing equals 150 instructions
decf
phase
decf
phase
decf
phase
; 150
topa9:
call
delay10
call
delay10
call
delay10
call
delay10
nop
nop
nop
nop
nop
decfsz phase
goto
topa9
btfsc input
goto
topa
bsf
output
nop
nop
nop
nop
nop
nop
nop
nop
nop
bcf
output
goto
topq
; ****36************
xkeyserv:
; THIS INTENDED TO PROVIDE SERVICES FOR BUTTON BASED DIMMER
bsf
ad0,1 ; start conversion
nop
f1:
bcf
output
nop
movwf
temp
call
delay10 ;
call
delay10
;
;
movfw
addata
;
movwf
temp
;
rrf
temp pre- buffer offset adjust
;
rrf
temp
;
rrf
temp,w
;
andlw
0x1F
;
addwf
addata
;
bcf
carry
rrf
addata
;
movlw
2
;
subwf
addata
GF159 William Grill: 110V AC PWM
btfss
incf
carry
addata
movlw
movwf
btfsc
decf
0x7F
temp
addata,7
addata
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;
;
movfw
addata
btfsc
GPIO,4 ; test/adjust for inversion at opticoupler 0%PWM->no
power out
subwf
temp,w ; 7F-addata
movwf
temp
; **********************equalized 114 instructions************
dresult:
clrf
mresult
movlw
7
movwf
index1
movfw
cyclecnt
movwf
number
rlf
temp
a:
bcf
carry
rrf
number
rlf
temp
btfsc carry
goto
ba
nop
nop
nop
nop
c: decfsz index1
goto
a
call
delay10
c2:
movfw mresult
movwf newphase
retlw 0 ; product in mresult and newphase
ba:
movfw number
addwf mresult
goto
c
stop:
end