ELEC499 Design Project Report
ChargeGuard
8 April 2010
Group 10:
Brett Nelson
Colter McQuay
Geoff Costin
Michelle Stephenson
Supervisor: Dr. Amirali Baniasadi
Table of Contents
Abstract
1.0 Introduction
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1.1 Problem
1.2 Solution
2.0 Hardware
2.1 Overview
2.2 Power Supply
2.2.1 Design
2.2.2 Implementation
2.3 Relay
2.3.1 Design
2.3.2 Implementation
2.3.3 Improvement
2.4 TRIAC
2.4.1 Design
2.4.2 Implementation
2.5 Current Sensing
2.5.1 Design
2.5.2 Implementation
3.0 Microprocessor
3.1 Overview
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3.2 Firmware
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4.0 Future Expansion
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4.1 Multiple Outlets
4.2 Integrated in Adapters
5.0 Conclusion
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List of Tables and Figures
Figure 1: Sony Charge Adapter
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Figure 2: Canon Camera Charge Adapter
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Figure 3: Motorola Charge Adapter
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Figure 4: ChargeGuard Circuitry
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Figure 5: Power supply by Microchip
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Figure 6: Cosel Power Supply
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Figure 7: Relay
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Figure 8: TRIAC
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Figure 9: TRIAC Circuitry
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Figure 10: Hall Effect Sensor
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Figure 11: Current Transformer
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Figure 12: RMS to DC Circuit
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Figure 13: RMS to DC Converter
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Figure 14: Microprocessor
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Table 1: States and Descriptions
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Abstract
When charge adapters for devices like mobile phones, camera batteries, and mp3 players are plugged into the
wall, they waste power even without any devices plugged into them. ChargeGuard is an energy saving charging
station for rechargeable devices; the finished product could be realized as a multiple outlet device, similar to a
power bar. The final prototype for the project was a single-outlet version this product.
The circuitry prevents leakage current by having a microprocessor monitor the current being drawn through the
outlet. When there is only a small amount of current being drawn, the leakage current, the processor signals a
latching relay to open and prevent the current from leaking. When a device is plugged into the adapter to be
charged, the processor senses a larger current being drawn and closes the relay, allowing the device to charge
normally.
1.0 Introduction
1.1 Problem
Charge adapters are commonly used to charge mobile phones, digital camera batteries, and most
rechargeable devices. The problem addressed by this project is that current is leaked through these
charge adapters while they are not in use. Small leakage currents can accumulate to a significant amount
of wasted energy. The purpose of this project was to develop a product to minimize the energy losses
attributed to leakage current of devices. Some common charge adapters are seen in Figures 1-3.
Figure 15: Sony Charge Adapter
Adapter
Figure 16: Canon Camera Charge Adapter
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Figure 17: Motorola Charge
1.2 Solution
To prevent current from being leaked through charge adapters, an intelligent charging station was
designed and constructed. The device was designed to dynamically configure itself for any charge
adapter. Using this data, it is able to sense when a device is drawing current from the adapter, in which
case it allows the device to charge as it would normally. It can also sense when there is no device
plugged in and reacts to prevent current from being drawn. The circuit itself was required to have very
low power consumption, so as not to contribute to the wasted power.
2.0 Hardware
2.1 Overview
To prevent leakage current in idle charge adapters, the device required some specific hardware
components. A current sensing transformer was used to measure the amount of current being drawn
from the wall at any point in time. This waveform then needed to be conditioned so it could be read by
the microprocessor. An RMS-to-DC converter was used to condition the measured current to a DC signal
appropriate for the microprocessor. The microprocessor uses a TRIAC to close the circuit every second in
order to sample the current. This would theoretically allow the processor to sleep in between readings,
rather than constantly measuring the current. When the system senses that current above the leakage
threshold is being drawn, a latching relay is closed to allow the current to charge the device normally. A
detailed circuit diagram of ChargeGuard is shown in Figure 4.
Figure 18: ChargeGuard Circuitry
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2.2 Power Supply
The processor required 3.3 VDC, so a power supply was chosen which could convert 120 VAC to 3.3 VDC at
a maximum of 2 A. The goal was to design a power supply which could provide the necessary voltage
without the use of a transformer in order to minimize the overhead power required by the supply.
2.2.1 Design
The first power supply chosen for the project was designed by Microchip for use with a light
dimmer circuit. This power supply unit (PSU) was appealing because it involved no transformers
and no switching, and it was composed entirely of passive components. As a result, it would
have very low losses and would be fairly simple and inexpensive to construct. The circuit was
designed using a full-wave rectifier followed by a Zener regulator to generate 5 VDC. The next
stage was a pair of filters to clean the signal and smooth the voltage ripple. Since the circuit has
no electrical isolation between the AC and DC sides, the circuit is protected through the use of a
varistor which acts as a surge absorber. The supply was designed to convert 120VAC to 5VDC, so
an additional 3.3V regulator was used to achieve the desired final DC voltage. This design can be
seen here in Figure 5.
Figure 19: Power supply by Microchip
Upon construction, it was discovered that the current sourcing capabilities of the PSU (1.5mA)
were extremely low compared to what was required (>300mA). Attempts were made to modify
the design by changing the values of the RC current limiter in the circuit. The efforts yielded only
marginal improvements in current source capability (3.4mA). Although the Microchip PSU
offered appealing efficiency and low losses, it could not be used since it could not provide
enough current.
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2.2.2 Implementation
In the interest of time, a pre-fabricated power supply was used
which could convert 120VAC to 5VDC at 2A. The Cosel VAF1005
power supply, which was used, can be seen in Figure 6. This PSU
was then connected to the 3.3V regulator to achieve the final
desired voltage. This supply was found to be very stable and
reliable, with a more-than-adequate current capacity.
Unfortunately, it did exhibit undesirable losses with an efficiency
Figure 20: Cosel Power Supply
of about 74% and a current overhead of about 15mA. Despite the losses, this supply was still
chosen due to time constraints of the project, in addition to the fact that the prototype was
meant as proof-of-concept. Given more design time, a more efficient supply could have been
developed which would meet current sourcing needs. In future revisions of ChargeGuard, a
power supply similar to the one by Microchip would be designed.
2.3 Relay
A latching relay was to be used as part of the switch circuit since it would be ideal for the charging cycle
of the devices. This was because it would only need to be switched on at the beginning of the charge
cycle and switched off at the end. The latching capability would allow the processor to switch the relay
by applying a DC signal for an instant and then deasserting it; this would save power, and allow the
possibility for the processor to sleep.
2.3.1 Design
The Panasonic DK1A-3V-F relay was chosen because it is compact, inexpensive, and can handle
10A on the AC side; additionally, its coil is activated by a 3VDC signal at 66mA. It can been seen in
Figure 7. Since the digital outputs of the processor could not handle more than 25mA, the relay
was to be used in conjunction with a generic 2N3904 npn BJT in order to source the 66mA from
the power supply. The processor could control the relay indirectly by switching on the BJT
instead.
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2.3.2 Implementation
Upon receipt of the part, it was realized that the relay
required a reversal of polarity of the DC voltage applied to the
coil in order to alternate between switching it on and off. In
order to accommodate this, the single BJT circuit was changed
to an H-Bridge containing four BJTs. One pair could be
controlled by one digital output, and the other could be
controlled by a second output. Typically, a protection diode is
placed across the coil of a relay in order to protect transistors
and ICs from a brief high voltage that occurs when the coil is
Figure 21: Relay
switched off. With the implementation of the H-Bridge, a
protection diode could not be used since current could flow through the coil in either direction.
The relay and H-Bridge circuit was tested under the expected conditions without the use of
protection diodes, and it was found that no damage was incurred in any of the components.
Thus, this circuit was considered acceptable for use in ChargeGuard.
2.3.3 Improvement
The design could be improved by changing the single-coil relay to a two-coil model, which uses
one coil for switching on and one for switching off. This would allow for a reduction in
components since the H-Bridge circuit would no longer be necessary. Furthermore, a protection
diode could be placed across each coil since current flows through the coils in only one direction.
2.4 TRIAC
For the duration of time in which ChargeGuard is not charging a device, the line needs to be sampled
frequently to determine if a device has been connected. For this purpose, it was decided that using a
low-power TRIAC circuit would be best since it could be switched quickly and quietly using very little
current. Using a relay would have been undesirable since it would make a clicking noise every time it was
switched, and it would consume more power during switching. Additionally, TRIAC circuits have very
small switching times on the order of microseconds, whereas relays take milliseconds, effectively
minimizing the sampling time.
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2.4.1 Design
The TRIAC circuit was designed using an opto-coupler TRIAC driver and a power TRIAC, which
can handle 15A at 120VAC. The opto-coupler was used since it could be controlled using a small
DC signal to active the internal LED, which also provides isolation between the AC and DC sides.
The opto-coupler IC also contains a zero crossing circuit which serves to drive the power TRIAC.
With TRIAC switches, it is typical to also use a series RC snubber circuit in parallel with the power
TRIAC; this is done to protect against large voltage transients across the TRIAC, as well as to
compensate for forward biasing problems caused by inductive
loads. Several different snubber circuits were designed, but
some issues arose which could not be corrected. The snubber
tended to allow a small current to leak even when the switch
was off; although small, it was still significant enough to
invalidate the intent of ChargeGuard. Additionally, some
snubbers caused loss of controllability in the sense that the
TRIAC was always on and could not be deactivated by the DC
signal to the opto-coupler.
Figure 22: TRIAC
2.4.2 Implementation
Overall, it was decided that a
snubber would not be used with
the TRIAC for the prototype since
a suitable solution to the above
issues could not be found and the
TRIAC could still function without
one. Given sufficient time, an
acceptable snubber circuit could
be designed.
Figure 23: TRIAC Circuitry
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2.5 Current Sensing
Another important component of ChargeGuard is the current sensing circuitry. In order to determine
whether the relay should be opened or closed, the microprocessor must be able to read the current
being drawn through the outlet at any point in time. The current sensor needs to be able to measure low
levels of AC current and condition the measurements for acquisition by the analog to digital converter.
The required range of AC currents to be measured was between 0 and 200mA; furthermore, the analog
to digital converter in the microcontroller required a 0 to 3.3 V input signal.
2.5.1 Design
In order to measure the current, several options were considered. One method of current
measurement explored was to insert a low valued series resistance in the line and measure the
corresponding voltage drop across the resistor. The advantages of this method are three-fold:

there is no lower limit for current measurement,

the output voltage varies linearly with the current, and

it is very simple and inexpensive to implement.
This method, however, has disadvantages in that it continuously dissipates power proportional
to the square of the current and does not provide any form of isolation to the interface circuitry
or microcontroller. For this application, isolation and low power consumption are critical.
Although only small voltages appear over the sense resistor, the potential at the input of the
resister can be up to 120 V with respect to ground. This creates an indirect 120 V potential from
the analog input of the microcontroller to the ground, which would destroy the chip
Another option which was considered was the use of a Hall
effect sensor, which would measure the magnetic field
associated with a given current. For this specific application a
suitable Hall effect sensor could not be found because all
sensors either operated in a different frequency or current
range.
Figure 24: Hall Effect Sensor
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Electrical isolation can be effectively achieved through the use of a current transformer. A
transformer used for current measurement also steps down the current, reducing the wasted
energy by a factor proportional to the turns ratio. The stepped down current is fed through a
resistor and the resulting voltage (0 – 3.3V) is sampled by the microcontroller through a RMS-DC
converter.
2.5.2 Implementation
Overall, the most suitable current sensing solution was to use a
current transformer in conjunction with a RMS-to-DC converter. The
converter is required to take the AC signal and convert it to a DC
signal that the microprocessor can then sample. These components
were used primarily because of their low power
consumption and electrical isolation. Some
drawbacks of the design include the physical size
Figure 25: Current Transformer
of the transformer as well as the lack of measurability below a certain lower
limit. The use of a Hall Effect sensor in later models could potentially
Figure 27: RMS to
DC Converter
eliminate or mitigate these problems.
Figure 26: RMS to DC Circuit
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3.0 Microprocessor
3.1 Overview
The Nanocore12CS32TS was chosen as a suitable microprocessor for the project because of its low
power consumption, its ability to sleep, its small size, and its suitable number of analog and digital I/O
pins. The microprocessor uses the TRIAC to periodically activate the line in order to measure the current
draw; this is achieved through the use of the current
transformer and the RMS-to-DC converter. It switches the
TRIAC every second so that when a user plugs a device
into the charge adapter, charging begins seamlessly. This 1
second cycle was important because it would allow the
processor to sleep in between samplings. When the
processor is asleep, it only consumes 2µA of current,
whereas it consumes 15mA when it is awake and running.
Figure 28: Microprocessor
3.2 Firmware
The hardware discussed in previous sections, together with the microprocessor, merely comprises the
platform on which the firmware runs. A very specific algorithm had to be carefully designed in order to
correctly control this hardware.
There were several requirements that the firmware algorithm had to fulfill:

it must accurately determine the leakage current of the connected charge adapter

it must be able to dynamically reconfigure itself for a newly connected adapter

and it must correctly identify a sub- or super-threshold condition based on the leakage current
value and take the appropriate action.
It was decided that this could be best achieved by implementing a configuration mode for ChargeGuard.
During this mode, the algorithm gathers information about the currently-connected adapter as current is
observed to change over time. When sufficient data is collected, the algorithm can robustly and
accurately determine the value of the leakage current. Once configuration is complete, the algorithm
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alternates between charging and sampling states, depending on whether or not there is a device
connected to the adapter.
The firmware code was developed using a proprietary extension of BASIC called NQBASIC. The code
designed actually runs a finite state machine (FSM) that has the states represented in Table 1.
Table 2: States and Descriptions
State
CONFIGURE_NONE
Description
Relay
Triac
The default state when the processor
ON
OFF
is powered up. All of the min and
max current variables are set to zero
and all values are initialized.
CONFIGURE_FIRST_MIN
The state when the processor
ON
OFF
reaches its first minimum, virtually
happens after the first sample is
taken.
CONFIGURE_FIRST_MAX
The state when the processor finds
ON
OFF
its first maximum. The maximum
value has to be larger than the
maxCurrent variable value by a
certain relative threshold.
CONFIGURE_SECOND_MIN
The state when the sampled current
ON
OFF
is within the relative threshold of the
minCurrent variable.
CONFIGURE_SECOND_MAX
The state when the sampled current
ON
OFF
is within the relative threshold of the
maxCurrent variable.
CONFIGURATION COMPLETE
After the system is configured it will not enter configuration mode until it is either unplugged or it senses
basically zero current.
RUN_CHARGING
This state represents the charging
ON
OFF
state and will only change to the
leakage state when the current falls
within the relative threshold of the
minCurrent variable.
RUN_LEAKING
This state represents the leakage
OFF
Using to
state and will only change to the
sample
charging state when the current
current
sampled is above the relative
every 1s
threshold of the maxCurrent.
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4.0 Future Expansion
4.1 Multiple Outlets
The current prototype has one outlet for an adapter to be plugged into. The possible future expansion
for ChargeGuard could be to develop a device similar to a power bar, which could accommodate multiple
adapters. In this expansion some changes would need to be made to the existing circuitry. Each outlet
would need its own TRIAC, relay, current transformer, and rms-to-dc converter. Also, the power supply
would be required to handle a larger current and must be designed accordingly. If the number of inputs
and outputs on the processor becomes a factor, a serial to parallel converter could be used for the
outputs. This way a serial bit stream could be sent to the converter which would then send out the bits
in parallel to activate or deactivate the corresponding switches. A similar method could be used for the
inputs as well.
4.2 Integrated in Adapters
Another option for the development of ChargeGuard would be to create a very small circuit which could
be integrated inside the charge adapters themselves. Then the adapters could be plugged into any wall
socket and not be at risk for leaking current. In this situation the components required would be the
same as the prototype which was actually designed and built.
5.0 Conclusion
The intent of this project was to develop a device which would help to mitigate energy wasted by idle
charge adapters. This was achieved through the realization of the prototype, which was successful in
demonstrating this power-saving behaviour. Although the prototype did not possess multiple outlets and
the overhead power consumption of the device was quite substantial, the prototype was successful in
proving the original concept. Given more time further improvement could be made to the design in
several key areas. For instance, the efficiency of the power supply could be improved by utilizing only
passive components. In addition, lower-power TRIAC and relay circuits could be developed. With regard
to the firmware, a more effective timing scheme could be developed for the sampling algorithm, which
would also take advantage of the microprocessors sleep feature. Implementing such improvements
would bring the ChargeGuard prototype closer to a full commercial product.
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