cmsc838 week 02 | lecture 03 | feb 3, 2015 - umd...[source: scherz & monk, practical electronics...

Human Computer Interaction Laboratory

@jonfroehlich Assistant Professor Computer Science

CMSC838 Tangible Interactive Computing

Week 02 | Lecture 03 | Feb 3, 2015 Voltage, current, resistance

Multimeters

Ohm’s law

Design thinking

TODAY’S LEARNING GOALS

1. How to use a multimeter including how to check

resistance, voltage, current, and for contininuity

2. A conceptual understanding of voltage, current,

and resistance

3. Some basic circuit theory including Ohm’s Law

4. An introduction to design thinking and importance

of rapid prototyping (we might not get to this but

it’s super relevant to approaching projects in this

class and working together)

multimeters, circuit basics, & ohm’s law

volt∙age [vohl-tij] | measured in volts (V)

To get electrical current to flow from one point to another, a voltage

(electric potential) must exist between two points. A voltage across a

conductor gives rise to an electromotive force (EMF) that pushes free

electrons in a circuit

VOLTAGE, CURRENT, AND RESISTANCE

[source: Scherz & Monk, Practical Electronics for Inventors, 3rd Edition]






cur∙rent [kur-uhnt] | measured in amps (I)

Electric current is the total charge that passes through some cross-sectional

area A per unit time.








cur∙rent [kur-uhnt] | measured in amps (I)

Electric current is the total charge that passes through some cross-sectional

area A per unit time.

re∙sist∙ance [ri-zis-tuhns] | measured in ohms (Ω)

A material’s tendency to resist the flow of charge (current). In 1826, Georg

Ohm published experimental results regarding the resistance of various

materials using an empirical approach. He found a linear approach and

defined resistance as: R = V / I



WATER ANALOGY FOR DC CIRCUITS

[based on: https://learn.sparkfun.com/tutorials/voltage-current-resistance-and-ohms-law/all]

Pressure

Flow

Pipe width



Pressure ≈ Voltage

(measured

in volts)

Flow ≈ current

(measured

in amperes)

Pipe width ≈ resistance

(measured in

ohms)

ELECTRIC CURRENT Electrical current is the total charge that passes through some cross-sectional area A

per unit time. The unit of current is coulombs per second, but this unit is also called

ampere (A) or amp.

IΔQ is the amount of charge passing through an area in a time interval Δt

𝐼 = lim∆𝑡→0

∆𝑄

∆𝑡=

𝑑𝑄

𝑑𝑡

[based on: Scherz & Monk, Practical Electronics for Inventors, 3rd Edition]



Pressure ≈ Voltage

(measured

in volts)

Flow ≈ current

(measured

in amperes)

Pipe width ≈ resistance

(measured in

ohms)

Smaller pipe width ≈ greater resistance

Same

pressure ≈ same

voltage

Reduced flow ≈ reduced current



Note: Electricity always wants to flow from a higher voltage to a lower voltage!



What’s the relationship between current, resistance, and voltage? Enter: Ohm’s Law

5V 1KΩ

I

OHM’S LAW

𝑉 = 𝐼 ∗ 𝑅

Using Ohm’s Law, solve for I in the above circuit

Ohm’s Law states that the current through a conductor between two points is directly

proportional to the potential difference (voltage) across the two points.

𝑉𝑜𝑙𝑡𝑎𝑔𝑒 = 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 ∗ 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 = 𝑉𝑜𝑙𝑡𝑎𝑔𝑒

𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝐼 =

𝑉

𝑅

5V 1KΩ

I

OHM’S LAW

Using Ohm’s Law, solve for I in the above circuit Answer: I = V / R = 5 / 1,000 = 0.005 amps = 5 mA

𝑉 = 𝐼 ∗ 𝑅





𝑉

𝑅

5V 1KΩ

I

OHM’S LAW

The resistor is the ‘load’ in this circuit, which

dissipates power as heat. What happens if we

remove it?

𝑉 = 𝐼 ∗ 𝑅





𝑉

𝑅

5V

SHORT CIRCUIT

I

If you cannot a wire directly from the positive to the negative side of a power supply,

you’ll create a short circuit, which is a very bad thing!

[source: https://learn.sparkfun.com/tutorials/what-is-a-circuit]

With no load, R ≈ 0, which means

I = V / 0 (i.e., current tries to

become infinite!)

Well, OK, your power supply cannot

supply infinite current but it will max out and this could cause your wire to burn up,

damage your power supply, etc.

OPEN CIRCUIT

[source: https://learn.sparkfun.com/tutorials/what-is-a-circuit]

5V 1KΩ

What happens if we add a switch that is ‘open’ (that is, not pressed)

OPEN CIRCUIT

5V 1KΩ

What happens if we add a switch that is ‘open’ (i.e., is not pressed)

Answer: Nothing. A circuit, by definition, is movement that starts and finishes at the same place. Our electrons need to flow from +5 back to opposite side of our power source!

LET’S BUILD THIS SIMPLE CIRCUIT

9V 1KΩ

I

Breadboard View Schematic View

SOLVE FOR CURRENT USING OHM’S LAW

9V 1KΩ

I


Now, let’s solve for I in the above circuit

9V 1KΩ

I


Now, let’s solve for I in the above circuit Answer: I = V / R = 9 / 1,000 = 0.009 amps = 9 mA

SOLVE FOR CURRENT USING OHM’S LAW

9V 1KΩ

I


LET’S MEASURE THIS CIRCUIT EMPIRICALLY

USING OUR MULTIMETER!

MULTIMETERS ARE LIKE X-RAY GLASSES FOR CIRCUITS! THE MOST IMPORTANT DEBUGGING TOOL WHEN WORKING WITH ELECTRONICS

Multimeters Can Measure

Resistance

Voltage

Current

Continuity (short testing)

Some meters can even measure capacitance, transistors, check LEDs for burnout, temperature, and frequency

SOME HELPFUL MULTIMETER RESOURCES Adafruit’s Multimeter Tutorial https://learn.adafruit.com/multimeters/overview

Collin’s Lab: Multimeters (Video) http://youtu.be/rPGoMbVSUu8

TagentSoft’s Half-Hour Video Tutorial http://tangentsoft.net/elec/movies/tt06.html

Make Magazine’s How to Use a Multimeter

http://www.makezine.com/blog/archive/2007/01/multimeter_tuto.html

https://learn.adafruit.com/multimeters/overview

https://learn.adafruit.com/multimeters/overview

http://youtu.be/rPGoMbVSUu8

http://youtu.be/rPGoMbVSUu8

http://tangentsoft.net/elec/movies/tt06.html

http://tangentsoft.net/elec/movies/tt06.html



MEASURING RESISTANCE

[source: https://learn.adafruit.com/multimeters/resistance]

You can interpret the color codes on a resistor or use your multimeter!

This 10KΩ resistor is really 9.80K Ω

MEASURING RESISTANCE TIPS!


You can only test resistance when the device you're testing is not

powered. Resistance testing works by poking a little voltage into the circuit and

seeing how much current flows, its perfectly safe for any component but if its

powered there is already voltage in the circuit, and you will get incorrect readings

You can only test a resistor before it has been soldered/inserted into a

circuit. If you measure it in the circuit you will also be measuring everything

connected to it. In some instances this is OK but I would say that in the vast

majority it is not. If you try, you will get incorrect readings and that's worse than

no reading at all.

Resistance is non-directional, you can switch probes and the reading will be

the same.

If you have a ranging meter (as most inexpensive ones are), you'll need to

keep track of what range you are in. Otherwise, you will get strange readings,

like OL or similar, or you may think you're in KΩ when really you're in MΩ. This is

a big problem for beginners so be careful!

MEASURING MY 1KΩ RESISTOR

9V 1KΩ

I


MEASURING MY 1KΩ RESISTOR You hook up your multimeter in parallel with the resistor. Note: you must turn off the

power supply to measure the resistance of an object.

9V .984KΩ

I


MEASURING MY 1KΩ RESISTOR My 1KΩ resistor is actually 0.984KΩ

MEASURE A POTENTIOMETER! Grab and measure the varying resistance of your potentiometer


9V .984KΩ

I


MEASURING MY 1KΩ RESISTOR My 1KΩ resistor is actually 0.984KΩ

How about my power supply?

MEASURING VOLTAGE

[source: https://learn.adafruit.com/multimeters/resistance; http://www.sciencebuddies.org/science-fair-projects/multimeters-tutorial.shtml]

Switch your multimeter to measure DC voltage. Just like when you measure the

resistance of an object, to measure voltage, hook up the multimeter in parallel.

SWITCH MULTIMETER TO MEASURE VOLTS

[source: Platt, Make: Electronics, 1st Edition]

MEASURING VOLTAGE


This 1.5V battery reads 1.588V. Why? The 1.5V written on the battery is a nominal voltage—or the “average” you may expect from the battery. In reality, an alkaline battery starts out higher, then

slowly drifts down to 1.3V, then finally to 1.0V and even lower.

MEASURING VOLTAGE TIPS!

[source: https://learn.adafruit.com/multimeters/voltage]

You can only test voltage when the circuit is powered. If there is no voltage

coming in (power supply) then there will be no voltage in the circuit to test! It

must be plugged in (even if it doesn't seem to be working)

Voltage is always measured between two points. By definition, voltage is the

difference between two points. There is no way to measure voltage with only one

probe, it is like trying to check continuity with only one probe. You must have

two probes in the circuit. If you are told to test at a point or read the voltage at

this or that location what it really means is that you should put the negative

(reference, ground, black) probe at ground (which you must determine by a

schematic or somewhere else in the instructions) and the positive (red) probe at

the point you would like to measure.

If you're getting odd readings, use a reference voltage (even a 9V battery

is a reasonable one) to check your voltage readings. Old meter batteries and

wonky meters are the bane of your existence but they will eventually strike! Good

places to take reference voltages are regulated wall plugs such as those for cell

phones. Two meters might also be good :)

MEASURING VOLTAGE TIPS (CONTINUED)!

[source: https://learn.adafruit.com/multimeters/voltage]

Voltage is directional. If you measure a battery with the red/positive probe on

the black/negative contact and the black probe on the positive contact you will

read a negative voltage. If you are reading a negative voltage in your ciruit and

you're nearly positive (ha!) that this cannot be, then make sure you are putting

the black probe on the reference voltage (usually ground)

DC voltage and AC voltage are very different. Make sure you are testing the

right kind of voltage. This may require pressing a mode button or changing the

dial. Unless otherwise indicated, assume DC voltages

Multimeters have different input impedences that affect readings of high

impedence circuits. For example, measuring a sensor that has 1Mohm

impedence with a 1Mohm impedence meter will give you only half the correct

reading

9V 1KΩ

I


MEASURING MY POWER SUPPLY You hook up your multimeter in parallel with object you are measuring.

9.26V 1KΩ

I


MEASURING MY POWER SUPPLY You hook up your multimeter in parallel with object you are measuring.

MEASURING VOLTAGE DROP ACROSS LED AND POTENTIOMETER You hook up your multimeter in parallel with object you are measuring.


MEASURING CURRENT To measure current, switch your multimeter to DCA, switch the cables to measure

amperage, and connect the cables in series with your circuit.

MEASURING VOLTAGE VS. CURRENT


MEASURE WATER FLOW USING A TURBINE IT MUST BE INLINE WITH THE FLOW OF WATER!

MEASURING CURRENT


When measuring current, think of the multimeter as a little turbine sensor for electrons. It needs to be

inline (in series) in order to measure flow!

MEASURING CURRENT: TWO EXAMPLES

[source: https://learn.adafruit.com/multimeters/resistance; http://www.sciencebuddies.org/science-fair-projects/multimeters-tutorial.shtml; Platt, Make: Electronics, 1st Edition]

SWITCH MULTIMETER TO MEASURE AMPS


SWITCH MULTIMETER TO MEASURE AMPS


Some meters require that you move your leads to a different socket. If you are unsure

about how much current is in your circuit, it’s best to start with the high current socket

first (10A in this case) to begin your measurements. If you observe low current in your

circuit, then you can safely switch over to mA.

LET’S MAKE AND MEASURE THIS (SLIGHTLY MORE) COMPLICATED CIRCUIT I want you to hook this up and then play with the multimeter to measure voltage and current.

WHAT’S CONSIDERED A LOT OF CURRENT? What’s considered a lot or a little amount of current? It’s a good idea to have some idea

of typical current draws when working with electronics.


100W lightbulb Draws about 1A

Microwave Draws 8-13A

Laptop Draws ~2-3A

Toaster Draws about 7-10A

SmartPhone Draws ~200mA loading a webpage

Low-Power Microchip A few μA or pA

Lightening Strike Around 1000A

Dangerous Current Levels 100ma – 1A is sufficient to induce cardiac/respiratory arrest

Typical LED Draws 20mA

DANGEROUS CURRENT LEVELS This is dependent on the current type (AC or DC) and frequency. A person can feel

~1mA of AC at 60Hz while 5mA of DC. This class deals exclusively in DC.

[source: Scherz & Monk, Practical Electronics for Inventors, 3rd Edition; https://www.sparkfun.com/news/1385; http://en.wikipedia.org/wiki/Electric_shock]

Current can be dangerous—high amperage is what can

kill you or cause tissue damage.

When you come into contact with a live wire or

energized object, the amount of current that passes

through your body to ground depends on the voltage

level and your internal resistance.

The NIOSH states "Under dry conditions, the resistance

offered by the human body may be as high as 100,000

Ohms. Wet or broken skin may drop the body's

resistance to 1,000 Ohms," adding that "high-voltage

electrical energy quickly breaks down human skin,

reducing the human body's resistance to 500 Ohms."

DC Current Probable effect on human body

1 - 5 mA Tingling sensation

5 - 10 mA Pain

10 - 20 mA Involuntary muscle contractions

20 - 100 mA

Paralysis, heart stoppage

LICKING A 9V BATTERY

[source: http://youtu.be/mhSW_5iuy5k

]

To reinforce our learning, let’s come back to

the water analogy for DC circuits

Assume same size holes, which hole would see the

greatest flow rate?


lower pressure

higher pressure

[based on: Scherz & Monk, Practical Electronics for Inventors, 3rd Edition; http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html]


greatest flow rate?

Answer: the flow rates increases with higher pressure

lower pressure

higher pressure


water flow rate



greatest flow rate?

Answer: the flow rates increases with higher pressure

lower pressure

higher pressure How does this relate to voltage and current in a

DC circuit?


water flow rate


lower pressure lower voltage

higher pressure higher voltage


water flow rate current

Water pressure is equivalent to voltage

(a voltage placed across a conductor

causes free electrons to move)


Volume flow rate is equivalent to

charge flow rate (current)

𝑉𝑜𝑙𝑢𝑚𝑒 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 =𝑐𝑚3

𝑠𝑒𝑐

𝐶ℎ𝑎𝑟𝑔𝑒 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒

= 𝑐𝑜𝑢𝑙𝑜𝑚𝑏𝑠

𝑠𝑒𝑐𝑜𝑛𝑑

= 𝑎𝑚𝑝𝑒𝑟𝑒𝑠

= 𝑐𝑢𝑟𝑟𝑒𝑛𝑡






Can you draw a roughly equivalent

figure using multiple batteries

and LEDs?






How does current relate to voltage and resistance given Ohm’s Law?






How does current relate to voltage and resistance given Ohm’s Law? Answer: I = V / R (but V is actually ΔV—that is, the change in V). This equation I = ΔV / R is similar to water flow in a cylindrical pipe

[based on: Scherz & Monk, Practical Electronics for Inventors, 3rd Edition; http://en.wikipedia.org/wiki/Hagen%E2%80%93Poiseuille_equation; http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html]

WATER ANALOGY FOR DC CIRCUITS Electricity was originally understood to be a kind of fluid and Ohm’s Law for a circuit

and Poiseuille’s Law for fluid in a cylindrical pipe.

F = flow rate in cm3 / sec





I = charge flow rate in coloumbs/sec

What’s the equation here for current given

the above?





I = charge flow rate in coloumbs/sec

WATER ANALOGY FOR DC CIRCUITS There are lots of water analogies for DC circuits including these helpful diagrams from

Charles Platt in Make: Electronics


WATER ANALOGY FOR DC CIRCUITS This one is from Scherz & Monk, Practical Electronics for Inventors, 3rd Edition


5V 1K

Ω

I

A NOTE ABOUT CURRENT FLOW

Conventional flow notation: the notation we will use in this class is the universal standard (but wrong), here electric charge (current) is shown to move from the positive to negative side of a power supply.

5V 1K

Ω

I

Actual electron flow: electrons move from the negative side of the battery to the positive side. This is the reality! However, convention keeps us from changing and all formulas used in electronics pretend that the current I is made up of positive charge carriers.


You don’t really need to worry about this in class as we’ll always be using conventional flow

notation but it’s worth knowing that electrons flow in the opposite direction!

BENJAMIN FRANKLIN TO BLAME!


Benjamin Franklin (often considered the father of electronics) was doing pioneering work in

electronics. He had the convention of assigning positive charge signs to the mysterious things that

seem to be moving and doing work. Later, Thomson found that moving charges (which he called

electrons) were moving in the opposite direction of conventional current I used in equations.

cmsc838 week 02 | lecture 03 | feb 3, 2015 - umd...[source: scherz & monk, practical electronics...

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