## Basic DC Electronics

August 25, 2015 Session

Fred L. DeRoos

WA0GMH

Direct versus Alternating Current

We have two type of current:

– One is called Direct Current (DC)

– The other is called Alternating Current (AC)

DC is what you get from a battery or a power supply

for your radio. The current flows in one direction.

Static electricity and lightening are also DC.

AC is what you get from the wall outlet in your home.

The current first flows one way, then reverses and

flows the other way.

Graph of DC

0 volts

Graph of AC

0 volts

One Cycle

What is Current?

Electric current is the flow of electrons (or any charge carrier)

The more electrons that flow, the higher the current.

Current is measured in amperes, amps (A), named for the French

physicist named Andre Marie Ampere.

In electronics we usually use milliamps (mA) and microamps (uA).

There are 1,000 mA in one amp.

There are 1,000 uA in one mA.

So there are 1,000,000 uA in 1 amp.

What Makes the Electrons Flow?

In order for the electrons to flow, there has to be a

potential difference between where the electron is

and where it is going to flow.

Potential difference is measured in volts (V), named for the

Italian physicist Alessandro Volta.

Most measurements are in volts, but we also use millivolts (mV)

and microvolts (uV).

Sometimes you will also see kilovolts (1,000 volts).

Potential Difference

Potential difference is simple the difference in voltage between

two points in a circuit.

So if one point is at 10 volts and the other is at 0 volts, the

potential difference is +10 volts.

If one point is at 20 volts and another at 10 volts, the potential

difference is still +10 volts.

We often measure potential difference referenced to ground.

What is Resistance

Whenever electrons flow through a wire, or any conductor,

the composition of the wire resists the flow of the

electrons. This is called resistance.

Resistance is measured in Ohms, abbreviated as Omega (?).

The ohm is named for Georg Ohm, a German physicist and

mathematician.

We also speak of resistance in thousands or millions of ohms

(k and M or Megaohms).

Resistance

There are three types of materials, with respect to resistance

– Insulators – have very high resistivity

– Semiconductors – resistivity can be varied

– Conductors – have relatively low resistivity

The resistance varies with the length, area, temperature and

resistivity of the material.

For a wire, the longer it is, the higher the resistance

The larger the area (bigger wire), the lower the resistance

Resistance

Resistance = (l x ?)/A

l is the length of the conductor

? is the resistivity of the conductor

A is the area of the conductor

? varies with temperature

Some materials increase resistivity with increased temperature

(positive temperature coefficient) others decrease their resistivity

(negative temperature coefficient).

Relative Resistivities

Silver (lowest) (1) Germanium (107)

Copper (1.06) Sea Water

Gold (1.53) Drinking Water

Aluminum (1.77) Deionized Water (1×1013)

Nickel Sulfur

Iron (6.29) Air (1024)

Tin Quartz

Lead (13.8) TeflonTM (highest) (1031)

Stainless Steel

Nichrome (100)

( ) = relative resistivity compared to silver

Ohm’s Law Relates volts, amps and ohms

E = I x R

I = E/R

R = E/I

E is measured in volts

I is measured in amps

R is measured in ohms

Remember, we need a potential difference to make amps flow.

So if we put a voltage across a resistance, a current will flow.

Some use V for volts

Math Review

A = BC is an equation that means that some number A

is equal to the product of B and C.

A = BC can also be written as A = B x C

To find C, we can divide both sides of the equation

by B.

A/C = BC/C The two C’s cancel out, so

A/C = B

Example Using Ohm’s Law To Calculate Current

Potential

Difference

Resistance to the

flow of electrons

Ohm’s Law

I

I

0.003 A = 3 mA

Resistors

Resistors are marked with a color code so we can identify them.

Various Resistors

Types of Resistors

Resistors use to be made of compressed carbon. These

were called carbon composition resistors.

-Inexpensive to make, but tend to change value with age

and can be noisy.

-Nearly always go up in value with age. Sometimes way up!

Newer resistors are either carbon film or metal film. The metal

film resistors are very stable and are quiet.

There are also metal oxide resistors. These are usually the higher

power resistors. Can withstand high temperatures.

There are also wire wound resistors. Usually high power. Not used

for RF due to inductance. Act like coils of wire.

Resistors

In addition to resistance, resistors are also specified as to the wattage they can dissipate and their tolerance.

Typical wattages are ¼ watt, ½ watt, 1 watt, 2 watts

5 watts and 10 watts.

Power resistors can have wattages of several hundred

watts.

Best to operate a resistor at no more than ½ of its power rating.

Most resistors are ±5 % of the expected resistance value.

Some are ±1 %. Older ones may be ± 10 or 20 %.

Inside a Resistor

Power Dissipation in a Resistor

E = I x R

I = E/R = 9v/100 ohms = 0.09 A

P = E x I

So P = 9 v x 0.09 A = **0.81 watts **

What Can We Do With Ohm’s Law?

If we are given any two of the values we can calculate

the third value.

If we know the voltage and resistance, we can calculate the current.

If we know the current and resistance, we can calculate the voltage.

If we know the voltage and the current, we can calculate the

resistance.

Resistors in Series

Rtot = R1 + R2 + – – – + Rn

So if R1 is 1,000 ohms and R2 is 2,000 ohms

Rtot is 1,000 + 2,000 = 3,000 ohms

If the voltage across the resistors is 3 volts,

the current through the resistors will be 0.001 A

So the voltage across the resistors will be 1 V and 2V

Resistors in Series

E = I x R

1 k

2 k

I = E/R = 3/3000 = 0.001 A

O volts

3 volts

E = 0.001 x 1000

E = 1 volt

E = 0.001 x 2000

E = 2 volt

1 volt + 2 volts = 3 volts

Math Review

1/Rtot = 1/R1 + 1/R2

1/Rtot = (R1 + R2)/R1R2

Rtot = R1R2/(R1 + R2)

This is the equation we use to calculate

the resistance of two resistors in parallel.

If the two resistors have the same value,

the parallel resistance is ½ of the resistance.

Two 1,000 ohm resistors in parallel equals 500 ohms.

Two 10 k resistors in parallel equals 5 k ohms.

Resistors in Parallel

R1 = 1k ohms, R2 = 2 k ohms and R3 = 3 k ohms

1/Rtot = 1/1000 + 1/2000 + 1/3000

1/Rtot = 0.001 + 0.0005 + 0.0003 = 0.0018

Rtot = 555 ohms

Resistors in Parallel

If all resistors are equal, 1/Rtot is equal to 1/R1 + 1/R2 + 1/R3

1/Rtot = 3/R, so Rtot = R/3

With two equal resistors in parallel, the total is R/2

With ten equal resistors in parallel, the total is R/10

Why Use Series and Parallel Resistors?

Sometimes we need a resistance value we don’t have.

We can combine resistors to “make” the value we need

Sometimes we need to make a resistor of higher wattage.

Power is spread over several resistors.

Let’s say you need a 2 k resistor that is within 1 %. By putting two 1 k resistors in series you would have the equivalent of a 2 k resistor. You could select the 1 k resistors so that the sum is 2 k. Some will be above 1k and some will be below 1 k. For

example one 1 k resistor might measure 950 and another 1050

Ohms. Placed in series you would have 2 k

Simplifying Resistor Networks

Simplifying Resistor Networks

Rtot = R1 + R3equiv + R2

Series Circuit

The power source, the switch and the three bulbs are

connected in series. Opening the switch disconnects the

power from the three bulbs.

If one bulb burns out, none of the bulbs will light.

40 volts

40 volts

40 volts

Parallel Circuit

The battery and the switch are connected in series with the

three bulbs that are connected in parallel.

When the switch is closed, each bulb will have the battery

voltage across it.

If one of the bulbs burns out, the others will still light.

The Voltage Divider

A voltage divider is used to reduce a higher voltage

to a lower voltage.

It is simply two or more resistors connected in series.

The current through each resistor is equal.

The voltage drop across the resistors may be the same

(both resistors have same resistance or different (each

Resistor has a different resistance).

The Voltage Divider

Ein = I x (R1 + R2)

I = Ein/(R1+R2)

Eout = I x R2

Eout = Ein (R2/R1+R2)

IR1 = IR2 because the resistors

are in series.

The Voltage Divider

Assume that R1 and R2 are both 1,000 ohms

I = E/R = 10 /2000 = 0.005 A or 5 mA

Since the resistors are in series, both resistors have

0.005 amps passing through them

E = IR, so E = 0.005 x 1000 = 5 volts

IR1

IR2

IR1 = IR2 because the resistors

are in series.

10 volts

5 volts

VOM Versus DVM

Most DVMs have an input resistance of 10 megohms.

-The readout is digital

Many VOMs have input resistances less than 10k – 100 k ohms.

-The readout uses an analog meter

-Sometimes called a multimeter

Digital Volt Meter (DVM)

VOM or Multimeter

The Voltage Divider

Assume R1 and R2 = 1,000 ohms

Assume the voltmeter you are using to measure Eout has an

input resistance of 1,000 ohms.

R2 is now the parallel resistance of 1,000 ohms and 1,000 ohms

or 500 ohms

Eout = Ein x (500/1000 + 500) = Ein x (500/1500) = Ein/3

With Ein equal to 10 volts, Eout = 3.33 volts, not 5 volts!!!

R of voltmeter

Using Digital Volt Meter

Most digital volt meters have an input resistance of 10 megohms.

So if we measure the voltage divider that should have Vout = 5 v

using a DVM with an input resistance of 10 megohms.

Vout = 10 V x (999/1,999) = 4.997 volts

So the input resistance of your voltmeter will affect the measured

voltage.

Measurement with an oscilloscope will also affect the voltage

measured. Many oscilloscopes have input resistance of 1 megohm.

Cautions When Using a DVM

Always double check that the DVM is switched to the proper

Function (volts, ohms or amps). Also AC or DC, with volts and amps.

Trying to measure voltage in the ohms position may damage the

DVM!!!!!

Trying to measure voltage in the amps position may damage

the DVM and the circuit you are measuring.

Measuring ohms in volts or amps position won’t work,

but normally it shouldn’t hurt anything.

Measuring voltage with the wrong scale (it it’s not auto ranging)

may damage the DVM.

Cautions When Using a DVM

**Never **try to measure ohms when the circuit is powered

or the voltage hasn’t decayed to zero.

Be sure that you don’t try to measure a voltage that is

above the limit of the DVM. Usually over 600 – 1,000 V.

Always make sure that your test leads are in good shape

and the insulation isn’t cracked.

Check your test leads by measuring the resistance when

they are connected together. It should measure very close

to zero ohms.

Using a VOM and a DVM

Measure several resistors using both a VOM and a DVM.

– 100 ohms, 10,000 ohms and 10 megaohms

The VOM will require you to adjust the zero setting after

you change the scale.

Notice how precisely you can measure the resistors with

each meter.

Are the resistors within the specified tolerance?

Notice what happens if you hold the leads of the resistor

with your hands.

Using a VOM and a DVM

Make a voltage divider using two 1,000 ohm resistors connected

in series with the power supply.

-Measure the voltage with a DVM and a VOM

Make a voltage divider using two 1 megaohm resistors connected

In series with a power supply.

-Measure the voltage with a DVM and a VOM

Is there a difference in the voltage you measure?