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## ENERGY SCAVENGER Project #37

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Introduction

- Intended Function
- Salvage energy from transmission lines and make it usable energy source
- Goals
- Design power converter that produces 5V DV output at a minimum of 70% efficiency with an AC voltage source 1.0-3.0V and output of up to 100mW.
- Benefits
- An alternate energy source, cheaper and readily available
- Can be used anywhere with a transmission line
- Environmental Friendly – Saves waste of batteries

The Idea

- Fluorescent tubes under transmission line

Transmission Line

- Tested underneath ~200kV lines
- Power lines are ~25 years old which provides greater leakage and better coupling

Current Inducing Coils

Sensor Coil

Long Loop Coil

- N: 220
- Area: 0.13225 m2
- 25 Gauge Copper Wire
- Foam Base

- N: 1
- Length: 200 ft: ~70m
- Area: ~85 m2
- Speaker Wire
- 2, 5 Foot Aluminum Poles

Final Coupling Design: Long Loop Method

- Power now on mW scale!
- Maximum power at ~500 Ω
- 4.01 V Open Circuit Voltage
- Power is will increase with an increase in length

Rectifiers

- Full-wave Rectifier
- Half-wave Rectifier
- Voltage Doubling Rectifier

Full-wave Rectifier

- Rectifier Output:

994.217mV ~ 988.108mV

- Boost Converter Output:

774.509mV ~ 769.279mV

Half-wave Rectifier

- No need to boost input voltage
- Transmitting voltage under 5V
- Capacitor usage will steady out the output for usage.
- Lower Cost (Less diode usage and transformer cheaper then the center tap)

Half – wave Rectifier

- Frequency set to 60Hz
- Input Voltage: Vpp = 1.66V
- Output Resistance: 4.9 Ohm
- - Capacitor: 330uF

Half – wave Rectifier

- The ripple value ( 25 mV / 563.40 mV) = 0.04

Boost Converter

- 0.5V DC input, 5V DC output
- 70% Efficiency over entire converter
- MOSFET switch (with snubber)
- Vout ripple < 5%
- Line regulation of 10%
- Load regulation of 5%

Design Considerations

- Inductor Choice
- L >> Lcrit
- Lcrit = VL·D(1/f)/(2imax), given 100mW at 5V that is 20mA so Lcrit = 0.5·0.9(1/30k)/(40mA) = 375uH
- Ripple control
- Output ripple under 5%
- C = 436nF
- Power loss
- MOSFET, low on state resistance (MTP36N06V, 40mΩ on state resistance)
- Diode, low voltage drop (MBR130, 0.35Vf drop but does not fit well in breadboard)

Simple Tolerance Analysis

- 100mW output with 70% efficient converter then starting power = 100/0.7 = 143mW
- Assuming drop over rectifier is about 20%, we have 114.4mW to work with
- Transistor drop D1I2R + Diode drop D2(I*Vdiode) + Wire drop (negligible)
- = 0.9*20mA*40mΩ*20mA + 0.1*20mA*0.35 = 14.4uW + 0.7mW
- = 0.714mW
- The boost has 0.714/114.4 = 0.62% drop but given fast switching and other possibilities such as bad isolation from other circuitry such as the feedback and PWM, this drop could easily increase

Control

- Square wave gate control
- Adjustable frequency & duty cycle
- High output voltage for efficient gate driving

Feedback

- Output feedback control
- Output between 1.8V~2.5V to Comp (pin1) of UC3843

Feedback (2)

- Internal oscillator and comparator which drive an SR latch to drive final PWM output

Feedback (3)

- Cannot ground LM741 negative end therefore requires use of -12V source
- Circuit output = k(Vref – Vout)
- For k >> 1, Vout ≈ Vref
- Drawback are, k is not infinite therefore output is not exact, also k CANNOT be too large or else any disturbance will drive the duty cycle to extremes of 0 and 1 therefore we choose a mediocre value of k = 5

Separate Testing

- MOSFET operation
- Finding relation between COMP voltage and output duty
- Boost output ripple
- Capacitance vs ripple
- Efficiency
- Transistor / diode choice
- Transient response

MOSFET Operation

Vgate vs VdsON

Output Ripple

Largest Ripple at 1.5k (higher power levels) with 438mV ripple which is 0.438/5 = 8.76% ripple, a bit more than our specified 5% using a 1uF capacitor, this was easily lowered under 5% with 3 of these capacitors

Efficiency and Regulation

Note: The transient response test could not be done because the function generator source always became deformed when applied to circuit

Test Results

- MTP3606 MOSFET was operational with a 12V gate driving voltage from PWM, no excessive gate drives needed
- Ripple was higher than expected with 1uF capacitor but was easily reduced by increasing capacitance
- Unstable efficiency with highest of 65.69%, about 5% lower than our goal
- Transient responses are unknown, but was also less important than other issues since we are running on low power and we have snubbers on both the MOSFET and amplifier
- A worst case scenario of delay of UC3843 is 300ns, given an extensive input wire length of 1m with a typical wire inductance of 1000nH/m, then if current reduces during turn-off at rate 20mA/300ns, with 1000nH inductor then VL = -0.067V which is trivial regarding almost any Power MOSFET voltage blocking tolerance

Modulation

- When arranged with rectifier, the feedback circuit would malfunction
- However using a simple voltage divider (potentiometer) for the PWM COMP input as in initial design schematic instead of the feedback, the circuit could perform rationally
- Never got to connection with power meter but I would speculate another feedback error

Final Results

- Unnoticeable ripple, with minimal switching spikes (0.5Vin)
- Clean square wave gate drive with operational feedback

Final Results (2)

- 0.6Vin shows feedback characteristics compared to last figure

Final Results (3)

- Maximum efficiency of 65.69%, lowest 22.13%, average around 40%
- Largest ripple (560Ω 45mW load) at 0.5% with 330uF capacitor, 3.7% with 22uF, 8.5% ripple with 1uF capacitor
- Max line regulation 4.54% at largest load (45mW)
- Max load regulation 3.16% at lowest input voltage (0.4V)

Problems & Solutions

- Goal of 70% efficiency was the only part that was not reached, but it was also the most important aspect of the boost converter since boost converters are supposedly higher efficiency compared to others
- Did not get circuit on PCB
- Excessive ESRin an already low power circuit
- Could not use some more efficient parts such as low power components, PMEG1020 diode (0.25V drop) and IRF6609 MOSFET (2mΩ on resistance)
- Note: MTP36N06V has 40mΩ resistance, 1N4150 diode has (0.65 – 0.85V drop) which is huge considering that is about 15% of our goal

Problems &Solutions (2)

- Feedback malfunction when integrated with rest of circuit
- Possible gain change when circuit impedance is changed since we are taking amplifier input from the negative end (amplifier feedback end)
- A possible solution may be to use a zener diode to reference the amplifier positive end to minimize differences or to lower the resistance value controlling the k = gain parameter so feedback is not as sensitive (another compromise) to change as to jump between 0 and 1 duty instantly

Energy Meter Overview

- PIC16F877A used to sense, calculate, and display energy consumed
- Energy value displayed on four 7-segment digits (still standard in the industry)
- Much cheaper than LCD displays
- BCD to 7-segment driver was built with only fundamental and cheap parts
- 2 channels of the A/D 10-bit converter in PIC was used
- ADC is used to sense voltage and current through load with respect to +5V reference voltage
- 20 Ω 0.1% precision resistor used in series with load to sense current
- 4 MHz oscillator was used
- Code was written in C

Original Design Modifications

- Instrumentation amplifier was not used (1NA2126)
- No scaling of voltage and current was needed
- Separate 8-bit AD7574 ADC was not needed
- ADC on PIC16F877A used instead
- Provides sufficient sampling
- Allows more input/output ports to be available
- 20 Ω 0.1% precision resistor used instead of 1 Ω 1.0%
- Allows for more accurate current readings
- Power factor calculation was not needed
- Since DC voltage and current are measured instead of AC

Starting Out With the PIC

- Use PIC16F8877A to make a LED flash
- Move on to drive one digit of 7-segment display without transistors
- Drive all four digits of 7-segment display
- Test ADC for voltage and current values
- Coding for energy calculation
- Debugging and energy scaling problems

General purpose 2N3906 pnp transistors are used to drive individual digits (connected to common anode)

- Prevents overloading of microcontroller pins
- Amplifies current drive capabilities
- Ground logic signal is the output of the PIC pin to turn a digit on
- 5V logic signal to turn digit off

7-Segment Coding Functions

- Function needed to isolate the digits of the energy sum variable
- Modulo operation is used
- Function needed to control the digit driver
- Quickly change from digit to digit to eliminate flickering
- Human eye can notice around 60 Hz or 16.67 ms between flashes
- Function needed to convert binary digit to appropriate signals to control segments of display

Voltage and Current sampling

- AN0 = Analog voltage input
- AN1 = Analog current input
- Input is actually a voltage which is scaled in the code to represent the current
- AN3 = 5 V reference for ADC

ADC Continued

- Resolution
- Nominal analog change required to change digital output by 1 bit
- R = 2-nVREF = 2-10bits(5 V) = 4.88 mV
- Voltage measurement verified by using the 7-segment display to show voltage value
- At 5 V input, 7-segment shows 1 0 2 4
- Display decreases as voltage decreases
- Current measurement verified similarly

Energy Calculation

Long energy_sum = 0; //initialization

While(1) //Continually (main loop)

{ …

energy_sum = energy_running_sum(energy_sum, instant_v, instant_i);

…

}

//function

energy_running_sum( long energy, long voltage_mV, long current_mA)

{

energy = energy +(voltage_mV*current_mA)/7200

}

Energy Measurements

Tested Results of Energy Meter

Possible Causes of Errors

PIC16F877A calculates in 16 bits

Max Voltage (5V) = 1024 (ADC is 10 bits)

Scale voltage to represent mV

1024(4.883) = 5000 mV (5 V is designed DC output)

Cannot have current greater than 13 mA (65 mW output)

(5000 mV) (14 mA) = 70 mW

Max increment is 216 = ~65,000 µW

OVERFLOW! 70,000 – 65,000 = 5,000 µW will be added instead!

Only voltage can be sensed to represent current

Scaling the energy value to be added with respect to time

More times we divide, the less accurate it will be

Decimal values are cutoff

Ethical Issues and Solution

- Stealing power from transmission line
- If this device is used by great number of people, it may be a great loss to the power company.
- Bill users (text messaging system and sync usage data)
- Instead of commercializing the device to charge small electronics, it may be used to form a whole new power grid providing power to homes

Recommendations

- Running electricity to a home remote from the grid is prohibitively expensive. Every few hundred feet can cost thousands of dollars.

Questions?

Thank You

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