phy222 lab 3 what are all these knobs? - syracuse...

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PHY222 – Lab 3 What are all these knobs? Power supplies, digital multimeters and all their settings

February 2, 2017

Print Your Name

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Print Your Partners' Names

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You will return this handout to the instructor at the end of the lab period.

Table of Contents 0. Introduction: What are all these knobs? 1 1. Activity #1: Power supply knobs and voltmeter (DMM) settings 5 2. Activity #2: Making connections 6 3. Activity #3: Another mode to use the power supply 7 4. Activity #4: Using an ammeter 9 5. When you are finished ... 12

0. Introduction: What are all these knobs?

Abstract Concepts that are part of the lab activities

0.1 Current

0.1.1 Current is the amount of charge per second, measured in Coulombs/s, flowing out

of a power source, past a point on a wire, or through something like a light bulb, motor,

radio, etc.

0.1.2 Current is usually represented by the letter I in equations.

0.1.3 The unit of current is the ampere, defined to be one coulomb per second.

0.1.4 Many times, you will measure current in milliamperes (mA). A current reading of

10 mA is the same as 0.010 A.

0.2 Ohm’s Law

0.2.1 Ohm’s Law is V = IR.

V is the difference in electric potential (in volts) between two points in a circuit.

I is the current flowing along the path between those two points.

0.2.2 The meaning of Ohm’s Law is that voltage V is proportional to current I.

R is the proportionality constant between the voltage V and the current I. R is

called the resistance.

Instructions

Before the lab, read all sections of the

Introduction to What are all these

knobs, and skim through the rest of

the lab packet.

What are all these knobs?

© Sampere

0.2.3 The unit of resistance is the Ohm, represented by a Greek uppercase omega: .

0.2.4 Ohm’s Law, the proportionality between voltage and current, is true for many

things that conduct current but not for everything. Light bulbs are an example of

something that conducts current but does not obey Ohm’s Law. If you apply different

voltages to a light bulb and measure the light bulb currents, you get different values of

the ratio V/I. This makes it impossible to assign a fixed resistance R to a light bulb.

Things which do have a fixed resistance always yield the same V/I ratio no matter what

voltage you apply to it. Then it is possible to say that V/I = R is the resistance, because

the ratio is always the same.

0.3 The graphic symbols for things without and with resistance

Wires, or things that have little or no resistance are represented by straight lines. The idea

behind resistance is that it resists the flow of electrical current, so resistance is represented by a

jagged line (which should make you think of a difficult path).

Wire, or something with little or no resistance:

Something with significant resistance:

0.4 Batteries and power supplies

You are familiar with batteries. Your cell phone no longer works when the battery is fully

drained. The battery is the source of voltage which causes the electrical currents in your phone.

Batteries are available in specific voltages. Many batteries, such as A, AA and the small coin cell

batteries are 1.5 V. Other common types are 3V and 9V.

We will use power supplies in place of batteries in many of our experiments,. We can

adjust certain output parameters, making these extremely useful for conducting experiments and

powering apparatus. You will investigate these parameters during this lab session.

0.5 Ammeters

0.5.1 An ammeter is an instrument that measures the rate at which electric charge flows

through a wire in amperes, which are the same as coulombs per second. An ammeter also

tells the direction the current flows by the sign of the reading or the direction of the

needle swing (see 0.6.4).

0.5.2 The only way the ammeters used in our labs can know how much current flows

through a wire is if the wire's current actually flows through the ammeter. If you had a

single wire and wanted to know the current flowing through it, you would have to cut the

wire and connect the two cut ends of the wire to the ammeter so that the current passes

through the ammeter.

0.5.3 Measurements in electric circuits do not require you to cut wires, but you do have

to arrange the circuit connections so that the current you want to measure is diverted to

flow through your ammeter. In a circuit diagram, the connection looks as shown in

Figure 1.

A

current-carrying wire ammeter

Figure 1 An ammeter connected to measure the electric current flowing through a wire


© Sampere

0.5.4 Of the two places on the ammeter where you connect a wire (to measure the

current flowing through it), one will be named COM (short for Common. If the current

flows out of the ammeter's ground terminal after flowing in the other terminal, the

ammeter identifies the current as positive. If the current flows into the ground terminal

and out of the other terminal, the ammeter identifies the current as negative. That is how

you can tell which direction the current is flowing. (If the current is positive, then

electrons are moving in the opposite direction.)

0.6 Voltmeters

0.6.1 In a circuit with resistors and

batteries, an electron in the wire sees some

places as higher in energy than others – and

harder to get to – while an electron will see

other places as lower than others – and

getting to those low places is easy, like

rolling down a hill. However, there is no

way a human can look at a circuit and

immediately see which points in the circuit

look high and which points in the circuit

look low from an electron's perspective.

Those high and low places (to an electron)

are there even if the circuit is completely flat

on the table.

0.6.2 One thing is usually easy for a

human to see. To electrons, batteries and

power supplies look like escalators, moving

electrons uphill, from the low places to the

high places. You can look at a battery and

see which terminal is + and which is –. To

an electron, the + terminal is low (because

negative electrons are attracted to positive)

and the – terminal is high. Since batteries

pull electrons in at the + terminal (the low

end) and push them out the – terminal (the

high end), batteries act like electron escalators.

0.6.3 Elsewhere in circuits, which places are

high and which places are low is not

immediately obvious to human eyes, and this is why humans use voltmeters. To use a

voltmeter, you touch its two terminals to two different points in a circuit. The voltmeter

compares the two points, determines which is high and which is low, and tells you what

the height difference is in volts, that being the measure of height that the electrons

respond to. (Height in volts has to do with electric forces and has nothing to do with

height in meters from which things fall due to the gravity force. Different forces have

different measures of height, but it is the same idea in both cases.)

Figure 2 The digital multimeter you will

use in your lab activities.


© Sampere

0.6.4 Using a voltmeter to measure the voltage difference between different parts of a

circuit requires you to touch the two voltmeter terminals to the two different parts of the

circuit. Figure shows how a voltage measurement looks in a circuit diagram.

resistor

V voltmeter

Figure 3 Using a voltmeter to determine the voltage ("height") difference between two ends of a resistor

0.6.5 The voltmeter reads out the difference in height between the two points it touches

in volts. If the voltage is positive, the ground terminal of the voltmeter touches the circuit

point that is – from an electron's point of view – higher than the other point. If the

voltage is negative, the ground terminal touches the high point. This is confusing

because it means a circuit point with a positive voltage is lower than the comparison

point. The confusion is due to the fact that everyone talks about electricity as if it were a

flow of positive particles moving from + to –, in spite of the fact that it really is a flow of

negative particles moving from – to +.

0.6.6 To make this less confusing, pretend – along with everyone else – that

electricity is a flow of positive particles. Then a positive voltage reading means the

ground terminal of the voltmeter is touching the lower point and the other terminal

is touching the higher point, from the positive particle's point of view. Conversely, a

negative voltage means it is the ground terminal that is touching the high point.

0.7 DC and AC

0.7.1 Electric current that always flows in one direction is called DC, for direct current.

Current that keeps changing directions is called AC, for alternating current. Ammeters

and voltmeters can measure current and voltage for both kinds of current, but you have to

tell the meter which kind of current it is measuring before you do the measurement.

0.7.2 On the big rotary switch that determines what the meter will measure, you will

see either A–, A~, V–, V~ or A , A~

, V , V~

or , . The symbols with wiggle

lines indicate AC, and the symbols with straight lines indicate DC. Thus, either A~ or A~

might be used to indicate the AC ammeter function, and similarly either V– or V might

be used to indicate DC voltmeter function.

0.7.3 In this lab, all currents are DC, so you will never use the meters to measure AC

current or voltage.

0.7.4 The instrument shown in Figure 2 is a digital multimeter or DMM. These can

function as voltmeters, ammeters, and can also measure resistance.

V A


© Sampere

1. Activity #1: Power supply knobs and voltmeter (DMM) settings

Equipment: GPS-3030DD power supply

Tenma DMM

Assorted banana plug wires

1.1 Using the power supply in constant current mode.

1.1.1 Push the HI LO switch in and out several times while watching the display on the

A meter. This meter displays the current that the power supply is providing, and is

measured in amps (A). Leave the switch in the depressed LO setting.

1.1.2 Turn the COARSE VOLTAGE knob all the way up and down.

1.1.3 Plug a wire

into the DMM

terminal labeled

and the other end

into the (+) terminal

of the power supply.

Set the voltmeter

scale to 20 V DC or

30 V DC,

whichever the

voltmeter has, and

turn the meter on.

Q 1 What happens when

you adjust the COARSE

VOLTAGE knob?

1.1.4 Turn the

COARSE CURRENT to about the 12 o’clock

postion.Turn the

VOLTAGE knob

again while

watching the display.

Q 2 What happens now?

1.2 Leaving the current knob at the 12 o’clock position, adjust the COARSE VOLTAGE knob to

about the same position.

Power button

HI/LO Current switch

Black (-) and red (+)

output terminals

Current and Voltage FINE

and COARSE control knobs.

Figure 4 The control knobs of the power supply used in these lab activities.

VΩ


© Sampere

Q 3 What is the voltage value?

Q 4 What do you think this means?

1.3 Vary the FINE VOLTAGE knob and observe the V readout.

Q 5 What is the difference between the COARSE and FINE controls?

1.4 Using these knobs, set the power supply to display 14.0 V, 14.1 V, … 14.5 V. Then have

another lab partner repeat that starting at 20 V, and the third lab partner start at 25 V.

Q 6 Describe how you used these controls to adjust the voltage desired.

1.5 Turn all four control knobs fully counter-clockwise (CCW).

2. Activity #2: Making connections

Abstract An experimental and qualitative activity

2.1 Insert one end of a banana plug wire into the red (+) terminal of the power supply and the

other end of the same wire into the negative (-) terminal.

2.2 Plug a wire into the DMM terminal labeled , and the other end into the (+)

terminal of the power supply. You will need to insert this into the banana plug that is already in

the power supply. This is called stacking the connection.

2.3 Plug a 2nd wire into the COM terminal of the DMM. The other end of this wire plugs into the

(-) terminal of the power supply.

2.4 Set your DMM scale to read 200 m .

2.5 Turn the COARSE VOLTAGE fully CW.

Q 7 What are the readings on each scale?

VΩ

V


© Sampere

2.5.1 This is because the current is set to 0A, telling the power supply to maintain a

current of 0A. In order to maintain the value, the voltage MUST remain at 0V.

2.6 Using the COARSE and FINE CURRENT knobs, set the current to .100A.

Q 8 What is the reading on the V readout of the power supply?

Q 9 What is the reading on the readout of the multimeter?

Q 10 Why do you think they differ?

2.6.1 Change the current to 0.5 A

Q 11 Compare the voltage readouts. How do they differ?

2.6.2 Turn the current all the way up.

Q 12 What is the maximum current allowed by the power supply in this LO setting?

Q 13 How do the voltage readings compare now?

3. Activity #3: Another mode to use the power supply

3.1 Do you see how the CURRENT knob(s) were able to control the voltage output by the power

supply? However, once the power supply needed to increase the voltage beyond the limit that

you set, you were restricted from further increasing the current, and neither the current, nor the

voltage increased.

3.2 Let’s explore another way we can use this power supply. Start by examining the large 14 Ω

resistor. It is made of a sufficiently thin and long metal wire. Notice how the ends of the wire are

connected to the terminal at each side of the long tube.


© Sampere

3.3 Begin by turning all power supply knobs fully counter-clockwise. Connect the circuit as

shown in Figure 5 . First, connect the loop that includes only the power supply and the resistor.

Then connect the voltmeter.

voltmeter V

resistor

Figure 5 The figure on the left shows a schematic of the experimental setup. The photo on the right shows the

initial loop – the power supply and resistor. The voltmeter has not yet been added.

3.4 Have your TA check and approve your circuit before continuing. TA initials

3.5 Turn both VOLTAGE knobs to about the 12 o’clock position, then using both CURRENT

controls, set the current to .250 A. Notice that the red CC light is illuminated. Set the DMM to

the 2 setting.

Q 14 What is the voltage reading on the power supply?

Q 15 What is the voltage reading on the multimeter?

3.6 The voltage has exceeded the maximum range of the multimeter. Change the scale to the

smallest setting that will show a reading.

Q 16 What setting did you need to use?

3.7 Change the multimeter setting to the 200 V scale.

Power supply

+ -

V


© Sampere

Q 17 What is the measured value now?

Q 18 Which setting should you use and why?

3.8 Change the DMM setting back to the 20V scale. Using the CURRENT controls, adjust the

current to read .500 and 1.00 amps while observing the voltage.

Q 19 Explain why changing the current knob affects the voltage reading?

3.9 Now increase the current to 1.500 A

Q 20 What happened when you attempted this? Did any lights change on the power supply? If

so, what changed?

3.10 Gradually, increase the VOLTAGE knobs to reach the desired current of 1.500 A.

Q 21 What are the voltage readings on the power supply and DMM now?

Q 22 What do you need to do to get a voltage reading on the DMM now?

4. Activity #4: Using an ammeter

4.1 Before dismantling your circuit, turn both CURRENT knobs fully CCW to make certain that

current will NOT flow. Then turn both VOLTAGE knobs fully CW. Gradually increase the

current to 0.150 A (150 mA). Leave the current knobs in this setting and adjust both voltage

knobs fully CCW to turn off the output of the power supply.

4.2 Wire the circuit as shown on the following page in Figure 6.


© Sampere

4.3 Before continuing on, make sure your TA approves your circuit and initials in the space

provided. TA

4.4 Set the DMM to the 2m scale. In this setting, your DMM will only measure a current up to 2

mA.

4.5 Slowly, turn the FINE VOLTAGE up 0.1 V at a time and watch the reading on the DMM.

You should see this value increase. If not, please call your TA to assist. Eventually, you will see

a reading of 1. mA on your DMM. Yes, that blank space typed on this line is intentional. This

reading indicates that you exceeded the range of the DMM. You need change to the next higher

range, or 20 mA.

4.6 NOTE: You must NEVER exceed 200 mA when using the mA terminal on this DMM or

you will blow a fuse and render the ammeter non-functional. This will cost you and your

lab partners 5 points on lab reports. Remember, we just left the power supply capable of

outputting only 150 mA, so for now, you are safe!

4.7 Gradually, increase the voltage until you reach 150 mA.

Power supply

Ammeter

Figure 6 The circuit for Activity #4. An ammeter is connected in series with a resistor and power supply.

µA mA

COM


© Sampere

Q 23 What is the voltage reading on the power supply when you reach 150 mA?

Q 24 What happens to the current reading if you try to further increase the VOLTAGE?

Q 25 What happens to the red CC and green CV lights as you adjust the voltage knob to go

below 150 mA and return to 150 mA?

Q 26 Does further increasing the voltage allow you to exceed 150 mA?

Q 27 An experiment requires that you not exceed a voltage of 6 V while being able to vary the

current and voltage. Explain how you would set up your power supply to ensure that you do

not blow up the circuit by exceeding 6 V.

Q 28 An experiment requires that you not exceed a current of 75 mA while being able to vary

the current and voltage. Explain how you would set up your power supply to ensure that you

do not blow up the circuit by exceeding 75 mA.


© Sampere

5. When you are finished ...

5.1 Make sure

5.1.1 All questions are answered and this report is turned in.

5.1.2 Your wires are all disconnected

5.1.3 The power supply is switched off and all controls are fully CCW.

5.1.4 Your DMM is switched off.

phy222 lab 3 what are all these knobs? - syracuse...

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