solar power supply - eecs at uc berkeleyboser/courses/40/assignments/lab... · uc berkeley, eecs...

UC Berkeley, EECS Department B. E. BoserEECS 40 Lab LAB2: Solar Power Supply UID:

Enter the names and SIDs for you and your lab partner into the boxes below.

Name 1 SID 1Name 2 SID 2

Device Characteristics

In this lab we familiarize ourselves with the characteristic of important electronic devices. We start with a reviewof the IV (current/voltage) characteristics for resistors and then explore the IV characteristics of the laboratorysupply.

Electronic devices can be characterized by the relationship between voltage and current at their terminals.Figure 1 shows the IV characteristic of two resistors. What are their values?

RA =1 pt.

0

RB =1 pt.

1

Devices whose IV characteristics occupy the first or third quadrant (passive sign convention) are passive, i.e.they dissipate power when voltage is applied or current is flowing. IV characteristics in the second or fourthquadrant indicate an active device. Rephrase this rule for active sign convention!

1 pt.

4

−10 −5 0 5 10

−10

−5

0

5

10

Voltage [V]

Cur

rent

[mA

]

RARB

Figure 1 IV (Current/Voltage) characteristic for two resistorsRA and RB. Passive sign convention.

Figure 2 Resistors with value 680 Ω and 5 % tolerance (colors blue-grey-brown-gold).

1 September 27, 2011 LAB2 v1015 http://ucbfeedback.com

Figure 2 shows a picture of resistors like those we use in the lab. The colored rings encode the value of theresistor. You can find that code e.g. on Wikipedia under “electronic color code” or “resistor”. Use the Ohmmeter(DMM set to measure resistance) if you are not sure about the value of a resistor. You will find that only certainresistor values are available. Most resistor types are only made with values 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68,82 and decimal multiples or fractions thereof. In many situations the closest value can be substituted (e.g. 4.7 kΩfor 5 kΩ). Alternatively it is possible to combine two or more resistors in series and/or parallel to synthesize adesired value (e.g. two 10 kΩ resistors connected in parallel produce exactly 5 kΩ). When you design a circuit youneed to round calculated component values to available ones and decide if the rounding error is acceptable or amore precise value must be synthesized from several parts.

Figure 3 Color coded resistors (colors orange-orange-red-gold).

What is the value of the resistors shown in Figure 3? Include the unit in your answer, e.g. mV, MW, kOhm anduse proper capitalization.

1 pt.

2

Laboratory Supply

The laboratory supply is an interesting device. You may think of it as a big battery with adjustable voltage. Whileit certainly meets this need, it works equally well as a current source.

Program the supply for Vout = V and Iout = mA. What are the voltage V and current I if a resistor ofvalue R is connected to the supply?

Resistor R Calculated Measured10 Ω V =

1 pt.

3

1 pt.

5

I =1 pt.

4

1 pt.

6

100 Ω V =1 pt.

5

1 pt.

7

I =1 pt.

6

1 pt.

8

1 kΩ V =1 pt.

7

1 pt.

9

I =1 pt.

8

1 pt.

10

10 kΩ V =1 pt.

9

1 pt.

11

I =1 pt.

10

1 pt.

12

Draw the IV characteristic of the supply (use active sign convention):


0 1 2 3 4 5 6 7 8 9 100

10

20

30

Voltage [V]

Cur

rent

[mA

]

Although most of the time we use the laboratory supply as a voltage supply, the current source feature has animportant use also: By programming the current of the supply to a value that is 20 to 50 % higher than the nominalcurrent draw of the circuit we are testing, we can protect our device from frying in case of a wrong connection.

Suppose you have designed a new audio amplifier and are now ready to test it in the laboratory. The circuitrequires Vdd = V to operate and based on your calculations, the maximum current draw of the circuit isImax = mA. To what output voltage Vs and maximum current Is do you program the laboratory supply?

Vs =1 pt.

11

Is =1 pt.

12

One last question: is the laboratory supply an active or passive device? The answer is not as trivial as it looksat first. In our experiments, the supply delivered power to the resistor and hence clearly was an active device. Butwhat happens if we connect a negative voltage to the output of the supply? Now the voltage changed sign but thecurrent keeps flowing in the same direction as before, hence the supply must be absorbing power: it now operatesas a passive device!

This is not as odd as it appears first. Consider a rechargeable battery: it clearly can supply power to an attacheddevice and hence operate as an active device. During recharging, however, the battery absorbs power from thecharger and hence is a passive device.

If you are still not convinced, program the second output of the laboratory supply to a negative voltage and thecurrent to a few ten milliampere, then connect the supplies together and measure the voltage and current flowing.Which one acts as a passive, and which one as an active supply? Incidentally you can perform this experimentonly with good, expensive laboratory supplies. Less expensive ones will not operate as passive devices.

Solar Cell

With our newly gained insight into electronic components, we are ready to tackle a serious problem. What aboutsolving the world’s energy problems? We’ve got to set your goals right!

We will use an Osram BPW34 photodiode, visible in the right panel of Figure 4 as a tiny dark square. Downloadthe datasheet from the website for more information.

Even though the device is very small, we can get a measurable amount of power from it even with indoorillumination. Since the power is proportional to the active area of the device, we can easily calculate the powerfrom a scaled up solar panel by multiplying our measured results by the ratio of the areas.

Since our lab has no windows, we’ll use a desk lamp (incandescent or Halogen, fluorescent does not work forthis experiment) to “simulate” the sun. Vary the distance of the solar cell from the light source from 10 cm to 50 cmin 10 cm steps and measure the short circuit current Is (solar cell leads shorted) and open circuit voltage Vo (solarcell leads open). Record your results in the plots below:


Figure 4 Solar cell test setup.

0 5 10 15 20 25 30 35 40 45 50 55 600

20

40

60

80

100

Distance [cm]

Shor

tCir

cuit

Cur

rent

I s[µ

A]

0 5 10 15 20 25 30 35 40 45 50 55 600

100

200

300

400

Distance [cm]

Ope

nC

ircu

itVo

ltag

eV

o[m

V]

Also record the maximum open circuit voltage Vo,max and short circuit current Is,max that you can get from thecell while keeping a minimum distance of 10 cm from the lamp, and the minima Vo,min and short circuit currentIs,min when covering the cell with your hand.


Vo,max =1 pt.

13

Vo,min =1 pt.

14

Is,max =1 pt.

15

Is,min =1 pt.

16

Vo,max/Vo,min =1 pt.

17

Is,max/Is,min =1 pt.

18

Which ratio is larger, that of the open circuit voltages or short circuit currents? The answer is rooted in thephysics of solar cells: incident photons cause electrons to move. Consequently, the short circuit current is propor-tional to the incident light. The voltage on the other hand has to do with material properties of the solar cell anddoes not change significantly.

Show the working measurement setups for Vo,max and Is,max to the GSI.

Keep these results handy (make a copy before handing in the lab report); you will need them for future labs!

Now all we need to do is scale up the size of the solar cell and the world’s energy problems are solved. Whatis the power P delivered by the solar cell for short circuit or open circuit condition?

P =1 pt.

13

Yes, you can answer this question without making a single measurement!How much power does a solar cell deliver? Well, on a sunny day and with the sun directly above us, the power

delivered by the sun is approximately 1 kW/m2. In 2008, the average annual electricity consumption for a U.S.residential utility customer was 11 MWh. Assuming the sun shines year round 12 hours per day, what is the areaA required to supply this energy with solar cells? Specify the units of your result as m2.

A =1 pt.

14

Surprising, eh? Admittedly, there are few flaws in this analysis. The sun does not shine twelve hours each day,not even in sunny California. And it is not directly above us, either. Solar cells are not 100 % efficient. The bestones are around 50 %, 20 % is more typical. But of course we do get a sufficient amount of energy from the sun,the problem is collecting and storing it.

Let’s figure out how much power we can harvest from our tiny solar cell. Place it approximately 20 cm fromthe light source and pay attention not to move it during your experiments. Measure the voltage Vx across the celland current Ix flowing by connecting different load resistors RL to the cell. Draw the circuit diagram with the solarcell, load resistor, voltmeter and ammeter in the space below. Use circuit symbols, not pictures, for the elements.

1 pt.

20

Record your measurements on the plots below (active sign convention for the solar cell). You will need moremeasurements around the power maximum to get an accurate measurement of its value. Do not move the bread-board or lamp during the measurement.


0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

100

200

300

400

Conductance 1/RL [mS]

Volt

age

Vx

[mV

]

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

50

100

150

200


Cur

rent

I x[µ

A]

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

50

100

150

200


Pow

erP x

=V

xI x

[µW

]


0 50 100 150 200 250 300 350 400 450 5000

100

200

300

400

Current Ix [µA]

Volt

age

Vx

[mV

]

Summarize the condition for maximum power in the table below.

Vx2 pts.

21

Ix2 pts.

22

Px2 pts.

23

RL2 pts.

24

Our solar cell measures approximately 3 mm2. How many cells (N) would be required to produce 1 W of powerand what active area A would they occupy?

N =1 pt.

25

A =1 pt.

26

Show your experimental setup in the maximum power condition to the lab GSI.

Extracting energy from solar cells is not trivial. First, we usually do not load the cells with resistors, but ratherwith lights, computers, cell phone chargers, and whatever other electrical loads we might have. Imagine we wouldhave to design all these devices with appropriate voltage and current requirements so that they would efficientlyextract power from the solar cell. For example, we would need light bulbs for all sorts of different voltage ratings.A second problem is that the optimal RL is not constant, but depends on the amount of light falling on the solarcell. You can easily confirm this by repeating the above experiment for a different distance of the cell from the lightsource. Our electronic devices would not only have to meet the specific IV characteristics of our solar cell, but alsobe able to adapt it to the changing optimal RL.

Fortunately clever electrical engineers like you figured out electronic circuits that accomplish these adaptationsautomatically. Complicated as they may be, they work so well that the average customer of a solar panel is noteven aware of the marvels happening inside.

What Have We Learned?

In this lab we familiarized ourselves with two electronic components, resistors, and (tiny) solar cells. We havealso learned how to characterize the components using the DMM. In subsequent labs we will be using thesemeasurement results to design electronic circuits based on the solar cell. So be sure to keep them handy (make acopy of this report before turning it in).

Parts List

• Solar cell

• Resistors (1/4 W, miscellaneous values)


Password:


solar power supply - eecs at uc berkeleyboser/courses/40/assignments/lab... · uc berkeley, eecs...

Documents