AN521

AN521
AN521
Interfacing to AC Power Lines
Author:
Doug Cox
Microchip Technology Inc.
FIGURE 1:
PIC16C5X SERIES INPUT
PROTECTION CIRCUIT ON I/O
PINS
VDD
INTRODUCTION
This application note describes a simple method for
measuring parameters from the AC power line.
Parameters such as zero crossing, frequency, and
relative phase can be measured. This method is useful
for measurements on 50, 60, and 400 Hz power
systems with voltages up to several hundred volts. The
method requires only one external component, a
resistor, and is more reliable than previously published
methods using capacitors or bulky, expensive
transformers.
APPLICATIONS
P
I/O pin
N
VSS
Input
buffer
Simplified structure of an I/O pin.
This measurement method can be used in any
application where power line parameters are used for
system measurements or control. Typical applications
are; switch timing (what part of the power cycle should
the system be activated), power factor correction,
power measurement, and power line monitor. An additional application is to generate timing or clock functions using the relatively stable power line frequency.
This method is also useful for calibrating oscillator frequency for accurate timing measurements when an
inaccurate reference such as an RC oscillator is used
to clock the PIC16C5X.
The least expensive method to limit current is to use a
high value resistor. A block diagram is shown in
Figure 2. The power line voltage is current limited by the
resistor and then clamped by the input protection
diodes internal to the PIC16C5X. A typical input waveform is shown in Figure 5. A 115 VAC, 60 cycle sine
wave will traverse from 0 to 2V in 32 µs, therefore a typical threshold of 2V on a PIC16C5X I/O port will permit
zero crossing detection accuracy of about 30 µs. If the
typical capacitance on an I/O pin is 5 pF, then R should
be (t = RC) 6 MΩ or less for best zero crossing accuracy. A 5 MΩ resistor with 115 VAC applied to it will limit
current to 32 µA, a value which is well within the safety
margin of the PIC16C5X.
THEORY OF OPERATION
FIGURE 2:
This application takes advantage of the input static
protection circuitry that exists on all I/O pins of a CMOS
PIC16C5X. These protection circuits are designed to
short the inputs to the power supplies when a large
overvoltage is applied, thus protecting the chip from
static electricity spikes. On PIC16C5X microcontrollers,
this protection circuit is two large P-N diodes on each
input (Figure 1). These diodes will short any voltage
higher than VDD to the VDD supply and any voltage less
than VSS to the VSS supply. They can take several milliamps of current without any damage to the chip. High
voltages can be applied directly to the chip inputs as
long as they are current limited.
 1997 Microchip Technology Inc.
110 VAC
High
LIMITING CURRENT USING AN
EXTERNAL RESISTOR
R = 5 MΩ
RA0
PIC16C5X
IPEAK = 162V/5 MΩ = 32 µA
DS00521C-page 1
AN521
The user needs to be aware that the circuit required to
connect the T0CKI input to an AC power line is slightly
different than the other I/O pins. Each of the I/O pins
has two diodes for input protection whereas the T0CKI
pin has only one protection diode connected to Vss
(Figure 3). Therefore, it is necessary to connect a diode
externally between the T0CKI pin and VDD in order to
clamp the voltage on the T0CKI pin to VDD + 0.6V
(approximately). See Figure 4. It is also recommended
that resistor R be at least 2 MΩ.
FIGURE 3:
A short is the most unlikely failure mode for a resistor,
and resistors are more reliable than transformers or
capacitors, which are the alternate components for
measuring line parameters. This reliability can be
enhanced even further by using two resistors in series.
Both would have to fail short to cause catastrophic
failure, a very unlikely event.
The second possible failure mode is that excessive
injection of current into the PIC16C5X might cause the
protection diode to open. This would allow the input to
go to power line peak voltage (162V) and short the
input transistor gate oxide, causing device failure. The
maximum continuous injection current into an I/O pin is
specified at ± 500 µA.
INPUT STRUCTURE OF MCLR
AND T0CKI PINS
T0CKI &
MCLR
pins
N
Input
buffer
FIGURE 4:
CONNECTING AN AC POWER
LINE TO THE T0CKI PIN
VDD
Simplified structure of T0CKI and MCLR pins.
Grounded gate NMOS device provides ESD and
overvoltage protection.
T0CKI
R
RELIABILITY
PIC16C5X
Reliability of production devices that are directly connected to AC power is always a concern. Two failure
modes are possible. First, the series resistor of Figure 1
might fail short, destroying the microcontroller.
FIGURE 5:
Connecting to TMR0 input. R ≥ 2 MΩ
INPUT WAVEFORM
-25.0000 msec
Ch. 1
=
Timebase =
Ch. 1 Parameters
Rise Time =
Fall Time
=
P-P Volts
=
0.00000 msec
1.000 volts/div
5.00 msec/div
184.001 µsec
174.005 µsec
6.187 volts
Freq.
+ Width
Preshoot
RMS Volts
=
=
=
=
59.9797 Hz
8.26099 msec
0.000 volts
3.894 volts
25.0000 msec
Offset
Delay
Period
- Width
Overshoot
Duty cycle
=
=
=
=
=
=
3.000 volts
0.00000 sec
16.6723 msec
8.41132 msec
0.000 volts
49.54 %
Waveform at part pin (RA0) •R = 100k; Line: 60 Hz, 110V
DS00521C-page 2
 1997 Microchip Technology Inc.
Note the following details of the code protection feature on PICmicro® MCUs.
•
•
•
•
•
•
The PICmicro family meets the specifications contained in the Microchip Data Sheet.
Microchip believes that its family of PICmicro microcontrollers is one of the most secure products of its kind on the market today,
when used in the intended manner and under normal conditions.
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the PICmicro microcontroller in a manner outside the operating specifications contained in the data sheet.
The person doing so may be engaged in theft of intellectual property.
Microchip is willing to work with the customer who is concerned about the integrity of their code.
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable”.
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of
our product.
If you have any further questions about this matter, please contact the local sales office nearest to you.
Information contained in this publication regarding device
applications and the like is intended through suggestion only
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
No representation or warranty is given and no liability is
assumed by Microchip Technology Incorporated with respect
to the accuracy or use of such information, or infringement of
patents or other intellectual property rights arising from such
use or otherwise. Use of Microchip’s products as critical components in life support systems is not authorized except with
express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property
rights.
Trademarks
The Microchip name and logo, the Microchip logo, FilterLab,
KEELOQ, microID, MPLAB, PIC, PICmicro, PICMASTER,
PICSTART, PRO MATE, SEEVAL and The Embedded Control
Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries.
dsPIC, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB,
In-Circuit Serial Programming, ICSP, ICEPIC, microPort,
Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM,
MXDEV, PICC, PICDEM, PICDEM.net, rfPIC, Select Mode
and Total Endurance are trademarks of Microchip Technology
Incorporated in the U.S.A.
Serialized Quick Turn Programming (SQTP) is a service mark
of Microchip Technology Incorporated in the U.S.A.
All other trademarks mentioned herein are property of their
respective companies.
© 2002, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
Microchip received QS-9000 quality system
certification for its worldwide headquarters,
design and wafer fabrication facilities in
Chandler and Tempe, Arizona in July 1999. The
Company’s quality system processes and
procedures are QS-9000 compliant for its
PICmicro® 8-bit MCUs, KEELOQ® code hopping
devices, Serial EEPROMs and microperipheral
products. In addition, Microchip’s quality
system for the design and manufacture of
development systems is ISO 9001 certified.
 2002 Microchip Technology Inc.
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