ENGR 40M Solar-powered cell phone charger Due Prelab: Oct 3-5, 24 hours before lab; Lab: beginning of lab, Oct 4-6

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Objectives

In this project, we’ll build a solar-powered cell phone/iPod/tablet charger. A solar panel charges a battery, which provides power to charge a cell phone. But a cell phone requires 5 V to charge, and the battery only provides about 3.7 V, so we’ll need a small converter circuit to get the 5 V output. We’ll use this to explore the concepts of voltage, current, and power. By completing this lab, you will: • Be able to explain how a solar cell extracts energy from sunlight • Become comfortable using a multimeter to measure voltage and current • Practice calculating voltage, current, power, and efficiency from measurements • Learn to solder • Build something useful!

2 2.1

Parts and equipment Lithium-polymer batteries

We will use an off-the-shelf rechargeable lithium-polymer (LiPo) battery to store energy from the solar panel. The battery chemistry produces a nominal potential of 3.7 V, but it may be as high as 4.2 V when fully charged. When the voltage drops below 3.7 V, the battery is nearly dead and you should recharge it. LiPo batteries rather flammable - you have have heard about battery fires in laptops or electric cars. Because of this, nearly all LiPo batteries have built-in circuit protection to prevent overcharging. Even so, you should ensure that the charging current is less than 1 A, and check periodically that the battery isn’t getting too hot. The protection circuit prevents accidents by disconnecting one of the terminals when it detects too much current, or when the battery voltage is too high or too low. However, you still need to be careful not to puncture the battery, which is why we’ll give you two pieces of plastic to cover the battery and protect it. Not using the plastic is not safe (and will cause a large loss of points).

2.2

DC-DC voltage converter

We won’t talk much about the voltage converter now, except to say that it takes an input between 2.5 and 5 V and provides a 5 V output on the USB port. At the end of the class, we’ll come back to investigate exactly how the voltage converter works. It is important to remember that the voltage converter doesn’t contain any energy sources (which is sometimes called a passive circuit), so the power it outputs at 5 V must always be less than the power it consumes at the input voltage. The goal of any converter is to be energy efficient; that is, to transfer the highest possible percentage of the input power to the output. In this lab we will measure how efficient the converter really is.

ENGR 40M Solar-powered cell phone charger

2.3

Spring 2016

Soldering

Soldering is the process of joining metal pieces - wire, electrical components, and circuit boards - together with molten metal solder (lead or tin). When the solder cools, the resulting connection is permanent, and provides strong electrical conductivity. Go read the soldering page from Sparkfun, and watch both videos: https://learn.sparkfun.com/ tutorials/how-to-solder---through-hole-soldering/all One important thing to remember is that solder is primarily for electrical connections, and doesn’t have great mechanical strength. If there will be any mechanical force applied to the joint, it is a good idea to have a parallel mechanical path for the stress. Observe in the pictures below how mechanical support is built into the plastic housings, so that there is no stress on the electrical connection.

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Prelab

Each lab assignment will have a prelab section, which you need to complete before you come to lab. Some of the prelabs will be done as in-class exercises. P1: The first part of this week’s prelab is to check that you have all the pieces in your lab kit using the inventory sheet. Report any missing parts to a TA when you come to lab. Also, bring your phone and charging cable when you come to lab; you’ll need them to test the charger.

P2: Given what you know about the solar panel, diodes, battery, and power converter, draw a diagram showing how you can connect them to build the solar charger. The battery must charge when the solar panel is exposed to the sun, and not discharge when it’s in the dark. The voltage converter should always be able to draw power, regardless of whether the device is in the sun or not.

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ENGR 40M Solar-powered cell phone charger

3.1

Spring 2016

Characterizing your multimeter

In order to not affect the circuit, an ideal voltage meter should take no current, and an ideal current meter should have no voltage across it. Needless to say, our multimeters are not ideal, but in most situations they’re close enough that we don’t need to worry. However, we will be trying to precisely measure the power efficiency of our solar charger, and these non-idealities, especially when measuring current, will change our results. So before you start your lab we will measure some of these effects. It’s much easier to analyze ideal components, so we often try to model non-ideal components with a group of ideal components. In this case, we can model our non-ideal voltmeter as an ideal voltmeter in parallel with a resistor:

Rm

We use this model because an ideal voltmeter should take no current, but our non-ideal meter does take a little. The resistor models the fact that some current flows through our meter when a voltage is applied across the terminals. P3: Set up one DMM to measure voltage, and set it on the 20V scale. Now take a second DMM (your partner’s) and set it to measure resistance. Connect the two meters together and adjust the resistance scale to get a good reading. If the display of the resistance DMM has a single ’1’, that means that the resistance is larger than its max range, and you should use a higher resistance range. What value do you measure for Rm ? Change the scale of the voltage meter to 2V, and 0.2V. Does the resistance change?

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ENGR 40M Solar-powered cell phone charger

Spring 2016

P4: While you are measuring the value of Rm , the voltage meter also measures a voltage. What voltage does it measure? You get bonus points if from this information you can guess how your DMM measures resistance.

P5: Now we need to model the non-ideality of the current measurement. An ideal current meter has zero resistance, so the voltage across it is always zero. Your meter will have a voltage drop when it measures current. If you are going to use a resistor to model this voltage drop, does this resistance go in series with the current meter, or in parallel to it?

P6: Now put one DMM in current mode to measure 20mA current, and set the other to measure resistance. Adjust the resistance scale to get a good reading. What resistance do you measure?

P7: Now repeat this resistance measurement for the different current scales. The resistance will change. Again bonus will be given if you can guess why the resistance is changing and what the meter is trying to keep constant?

3.2

Characterizing your solar cell in light

In the lab you will solder wires to your solar panel, but for now, you should use two of your clip leads to connect to the panel. Clip one on to the silver colored positive connection on the back of the panel, and the other on the negative connection. 4

ENGR 40M Solar-powered cell phone charger

Spring 2016

P8: Go outside when the sun is shining, and use your multimeter to measure the short-circuit current and the open-circuit voltage. Remember to orient the solar cell to catch as much light as possible (cast the largest shadow possible). The sun is very bright. Working inside, or on a cloudy day will give you much smaller results.

P9: From these measurements, can you estimate an upper bound on the amount of power the solar panel can provide?

If you look carefully at your solar cell (bright light is best) you will see it is not one monolithic piece, but rather a large number of strips, each about 10 mm wide. So far we have characterized the solar panel when the same light falls on all the different solar cells diodes that make up the panel. Now we’re going see what happens when some cells are bloked. P10: A silicon diode has a forward voltage of about 0.6 V. Given the panel voltage you measured, how many diodes must it contain?

P11: Measure the short circuit current if you block the sun from one of these strips using a finger or a piece of dark tape. For bonus points, explain your results using the model of the solar cell as a stack of current sources in parallel with diodes.

P12: Extra credit: Find a way to measure the solar panel at points other than open-circuit voltage and short-circuit current. Submit a plot of your results.

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ENGR 40M Solar-powered cell phone charger

3.3

Spring 2016

Characterizing the solar cell in the dark

So far we have looked at the current through the solar cell when light is shining on the cell. However, if we are going to connect the solar cell to the battery to charge it, we also need to consider what will happen if the battery remains connected and the sun is no longer shining. When the sun is not shining on the solar panel, it behaves like any other stack of diodes. If the panel is connected directly to the battery, current can flow from the battery through the solar panel. Leakage current

Solar panel

Battery

P13: Measure this leakage current by measuring the current through the cell when it is connected directly to your battery.

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ENGR 40M Solar-powered cell phone charger

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Spring 2016

Lab procedure

4.1

Characterizing the converter

You will need: • 1k and 100 Ω resistors • 2 27 Ω power resistors • Converter board (small red circuit board with a USB port) • LiPo battery • One of your USB-A to USB-mini cables Solder wires for ground and Vin onto your converter board. It is convention in electronics to use a black wire for the ground connection, and a red wire (or other light color) for Vin . Please follow this convention, so that others can quickly understand your circuit.

L1: Connect the power converter to your battery. How much current does the converter board draw when nothing is connected to the USB port?

Cut the USB cable in half, and strip back the outer jacket on the USB-A (big connector) side. Strip about 1/4” of the 5 V and ground wires (red and black respectively), and tin the ends by melting a small bit of solder onto the wire. Connect the converter board, power resistor, and battery as shown below. Converter

USB 5V

IN+

Load resistor

IN-

USB ground

You’re going to perform the same procedure for four different load resistors: 1. Measure the current going from the battery into the converter board. You may need to use the 10A scale (and the associated jack) on your meter. 2. While measuring the current, use a second meter to measure the voltage across the converter board inputs1 . 1 We

have to do this because the meter’s non-ideality affects the voltage supplied to the board. If you measure them independenly, you can introduce significant error into your results.

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ENGR 40M Solar-powered cell phone charger

Spring 2016

3. Calculate how much power the battery is supplying to the converter board. 4. Measure the resistance of your resistors, because they have a ±5% tolerance and could be significantly different their nominal value. Measure the voltage across the load resistor. From these measurements compute how much power is dissipated in the resistor. Repeat the measurements above for the remaining resistors. Use two 27 Ω resistors in parallel to get 13.5 Ω. L2: Power converter measurements: Battery Voltage

Current

Resistor Power

Voltage

Current

Power

Efficiency

1kΩ 100Ω 27Ω 13.5Ω Efficiency is defined to be the ratio of output power divided by input power. The website for the converter board says the efficiency is 96%. L3: Based on your measurements, draw a graph of the efficiency versus output current.

L4: What is the converter’s maximum efficiency? Does this meet the advertised efficiency?

Grab your phone charging cable, and connect your phone to the converter board. Check that your phone begins charging when you plug it in.

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ENGR 40M Solar-powered cell phone charger

Spring 2016

L5: Determine how much current your phone is drawing. Show your work or briefly explain what you measured.

4.2

Assembly

Now it’s time to assemble the charger. It’s up to you to figure out how to wire it up, so talk with a TA if you aren’t sure about the schematic you drew for the prelab. You will need: • LiPo battery • Converter board • Solar panel • Diode • Plastic sheet, and double-sided tape 1. Solder the charger together according to the circuit you designed in the prelab. 2. Use double-sided tape to stick the components to the solar panel. Make sure to insulate any exposed contacts that might cause a short circuit. 3. When you’re done, test your charger by plugging in your phone. The package we’ve provided provides decent protection for the circuit, but it’s not particularly pretty. We encourage you to experiment and build a better one that is more aesthetically pleasing or more solid. Just make sure that you can still get the converter board out for a later lab - don’t glue everything together just yet!

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Analysis

Our solar cells are rated to produce 1W in direct sunlight. But to produce this much power, the panel voltage must be about 7V:

Power = V·I

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ENGR 40M Solar-powered cell phone charger

Spring 2016

However, we are charging the battery by connecting it directly to the solar panel, which forces the panel voltage to be 3.7V. A1: In this setup, how much power is being delivered to the battery?

A2: Where does the rest of the power go? The photons are still hitting the solar panel and producing 1W of electrical power. Use your knowledge of how to compute the power flow of each device to figure out where this power is lost. Does it flow out of the solar cell into the rest of the circuit?

A3: Bonus: As you just found, the solar panel to battery efficiency isn’t great. Suggest one or more ways you could improve it.

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ENGR 40M Solar-powered cell phone charger

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Spring 2016

Reflection

Individually, answer the questions below. Two to four sentences for each question is sufficient. • What did you learn from this project, and how confident are you about what you have learned? We want the truth here, and not just a summary of the topics from this week. If you didn’t learn anything because it was all review or because you’re completely confused, say so. • What was the most valuable thing you learned? • What skills or concepts are you least sure about? • What is something you wish you’d known before you started working on this lab?

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Build Quality rubric

Build quality will be graded in a scale from “+” to “-”. We expect that most projects will be in the check to check-plus range, with plus reserved for rare and exemplary work. Plus • All solder joints are exemplary—clean and shiny with neither too much nor too little solder • Wires are color coded and exactly the right length • Wires are stripped back exactly as much as necessary • Connections between components are minimized and well-planned • Package is unique and elegant Check plus • All solder joints are clean • Wires are color coded and about the right length • Package is robust, and all components are firmly held together Check • One or two solder joints could be improved • Wires are color coded, but some wires are longer than necessary • One connection between wires could have been avoided with better planning • Package holds everything in place, but some parts may be loose Check minus • Many solder joints are dirty or use too much solder • Many wires are too long, or some are much too long • Wires are color coded using a non-standard scheme (e.g., red for ground) • Multiple unnecessary connections between wires Minus One or more of the following major problems: • Circuit doesn’t function • Packaging doesn’t protect the battery • Wires are prone to short-circuiting

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