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Posted 20 May 2018
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Practical Electronics For Makers (Part 4 of N)

, 20 May 2018
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Chapter 4 - Resistors and Resistance

Background

It has taken me far too long to complete this chapter.  I began questioning why it was taking me so long. The funny thing is that this chapter which will investigate electrical resistance ended up exposing the internal resistance I had to writing the chapter.  

As I wrote the chapter I noticed I was having to delve deeply into details that I feel have been explained elsewhere more clearly than what I can do. That made me feel like simply quitting.  But I really don't want to give up because there are other things I’m trying to get to that I feel compelled to write about.  As I examined how I felt about it I noticed it was because I want to discuss things at a slightly higher level.

For example, the entire reason I want to explain how resistors and capacitors work is so I can get to the main point which is a circuit that flip flops between two different LEDs turning on and off.

I am more interested in showing you that circuit and talking about it and how it can lead us to doing something a bit larger.  However, I don’t always enjoy explaining exactly every nuance of each component down to the physics level.  Primarily because it has been done before and in many instances far better than I can do.

That makes sense to me too, because this book is named Practical Electronics for Makers, after all.

The strong emphasis is on Practical and Makers.  I want you to understand some basic principles but I’m not trying to turn you into a Physics Professor.  I want you to know the basics of how things work but always with the end result of helping you learn to Make something larger : an amplifier, a bluetooth garage door opener, etc.

Flip Everything Upside Down

So, after this chapter I will flip things upside down a bit and begin building circuits and then discovering why they work.  This makes things much more interesting and is closer to my original purpose: build circuits, display circuits, explain circuits and help you to take these larger pieces and use them in your own work.

Systems View Versus Component View

I recently stumbled upon this concept in a really great book,  Electronics Explained, Second Edition: Fundamentals for Engineers, Technicians, and Makers 2nd Edition[^].

Instead of learning everything first from the component level, we first learn things from the system level and then only if we need to investigate how something works more deeply do we delve into how the components themselves work.  This alllows us to move more quickly into building useful things.

Woody Zuill
It is in the doing of the work that we discover the work that we must do.

Learn By Experience

Besides, I honestly think you only learn when you actually do the work.  And here, I mean build larger circuits that do something interesting (at least in some small context).  As you build the circuits and get them going I believe you will find that they energize your creativity and make you think of other ways you can put the circuits together to do more interesting things.

With That Explanation Out of The Way

This chapter will go into some detail on resistors and how they work and how to read the color code stripes etc.  But you may find gaps or things you specifically want to know more about. In those cases I’ve added links to youtube videos and articles which are great and add to the conversation.  That’s how I will move forward with the rest of the chapters in the this book.   From this chapter onward we will begin to ramp up to building things that are bit larger that will allow you to gain the confidence to create your own things.

Introduction

Now that we’ve looked at some basic circuits and successfully built a few, we have gained valuable experience.  However, now we need to take a step back and examine some of the common components of circuits more closely. As we do, we will better understand how the circuits work.  Understanding how/why circuits work will strengthen our ability to create our own circuits.

So far we’ve used LEDs, resistors, wires, switches and batteries in our circuits.  We’ve looked at all of these components relatively closely, except for the resistors.

This Chapter & Following Chapter

In this chapter we will examine resistors in quite a bit of detail and then in the next chapter we will build a flip flop circuit which willl incorporate a couple of electrolytic capacitors and transistors along with some resistors and LEDs to create a circuit which flashes two LEDs on and off.

Here’s what we will discuss in this chapter.

What We Will Cover 

  • Resistors

    • color coding.

    • Measure resistance with meter.

    • Power (P=EI) and heat dissipation.

    • Combining resistors in series in circuits

    • Combining resistors in parallel (show formula).

    • How resistors can help to divide voltage (to step voltage down)

    • Potentiometers (variable resistors) Show turning off LED by increasing resistance.

    • How your multimeter may trick you (internal resistance of your multimeter)

Supplies

What Do Resistors Do?

Resistors basically have one function : they limit current.  When current is limited (resisted) it also has the effect of dropping voltage.  (We’ll see this more clearly in a moment by analogy of water in a pipe (images below).)  When you keep all things the same in the circuit but you lower voltage current is also lowered.  If you raise voltage then current is also increased.

This takes us back to Ohm’s Law.  

Ohm's Law Again

E=IR (Voltage = Current * Resistance).  But in this case we rearrange the formula to show how changing only the resistance in a circuit affects the calculation.

E/R = I   

That means you divide voltage (E) by Resistance (R) and you get the resulting value of Current (I).  You can see that as the Resistance increases the current will decrease. And inversely, as the Resistance decreases the Current will increase.  

Resistance and current aren’t actually inversely proportional to one another though. Because even though it is true that as Resistance goes up current goes down, it is not true that if you increase current the resistance goes down.  Instead, the resistance in the circuit will stay the same. The point here is that as you increase resistance, you do limit the flow of current.

We’ll do some calculations to see what this looks like in a moment, but first let’s look at an analogy to water flowing (current) in a pipe to get a better idea of how this works.

Water Flowing In A Pipe: Good Analogy

Imagine a pipe of a size where the entire pipe is full and water is flowing.

A cross-section of the pipe with water flowing in it might look something like the following.

 

 

Imagine we have a way to measure the flow of the water and the pressure of the water in the pipe.  We will arbitrarily say that the water current is at 9 and the water resistance (friction of water traveling through the pipe) is at 3.

 

These are analogous to our electric current (water flow) and resistance (friction).  

There’s always some resistance (even when we do not have an obstruction) which is created by the water rubbing along the inside of the pipe (causing friction).  That is also true for our circuits even when there is only a wire and no resistor. A wire’s resistance is a function of it’s length (greater the length the greater the resistance) and the size of its cross-sectional area (the larger the cross-sectional area the lower its resistance).  Of course the resistance will be quite small in the circuits we create with short wires.

Calculating Water Pipe Pressure (Voltage)

Now, let’s multiply the values from our first example to get a basic overall pressure (Electromotive force aka Voltage) that is pushing the water through the pipe.  

E = I * R

E= 9 * 3

27 = 9 * 3

 

So the overall pressure is 27.  Our resistance (of the pipe walls)  is 3 so if we divide the pressure by 3 we do indeed get the current of 9 and that all seems right.

In a pipe there might be a screw device which closes off part of the pipe to limit the amount of water that can get through.  

 

The obstruction blocks a portion of the water that is flowing (current) and limits it.  

Let’s imagine that the obstruction is lowered about half way into the pipe in the previous picture so that resistance is now at 9 units.  You can see that there is less water in the portion of the pipe after the obstruction.

Ohm’s law helps us to see this also.

Ohm's Law Calculations Help Us See This

We now plug in our value for a resistance of 9 (arbitrary value picked by me for the example) and we get the new current value (which was previously at 9).

E/R = I

27 / 9 = I

27 / 9 = 3

Now our current is only moving at only 3 units.   That makes intuitive sense to us with water flowing in a pipe, since we’ve blocked the flow of some of the water.  You can see that as resistance increases, current drops.

Entire Circuit (Pipe or Electrical) Limited To Smallest Opening

Also notice that the max amount of current (water flowing) through the entire pipe is now only 3 units.  Even though the first section of the pipe had 9 units of current the pipe is now limited to only 3 units (at the obstruction ) through the entire pipe.

The smallest opening in the pipe now becomes the maximum flow of the entire pipe.  You can see this more clearly in the following image, marked by the red line:

 In other words the pipe’s maximum current flow is limited by its smallest opening. The current in the pipe cannot be any higher than that anywhere in the entire "circuit" of the pipe.  That is also true for electrical circuits and it means that current flowing in a closed circuit is the same throughout the circuit.

Pressure Drop (Voltage Drop)

However, another thing happens too.  The pressure on the side with more water (left side of obstruction) is higher than on the side where the pipe is less full (right side).  The more current is limited the greater the pressure drop that occurs also.

As If You Have Two Circuits

We can actually see this in Ohm’s law too.  But you have to think about it a little differently.  You have to think about it as if you now have two circuits: one before the obstruction and one after the obstruction.  That way you can calculate them each separately.

 

 

When you placed the obstruction in the pipe you’ve created two different voltages.  The one on the left side that has more current flowing creates more pressure against the left side of the obstruction (indicated by a 1 in the image).  The one on the right side where less current flows (indicated by a 2) cannot create as much pressure further down the pipe so there is a pressure drop (voltage drop) at that location.

How Overall Current Is Affected

Now with the obstruction / resistance added the entire pipe’s current flow has been altered.  Remember current is the amount of moving water through the entire pipe circuit. Now that we’ve created an obstruction the entire pipe is limited to the smallest opening so that the maximum current now is based upon that smallest opening.

The pressure on the left side of the pipe is still at the top pressure and the pressure does drop on the right side.  But the current through the entire pipe is limited.

That is the same for our circuits when we add a resistor.  The pressure on the battery side will still be the source voltage (for example 6V). But depending upon the size of the resistor we add the pressure drop on the opposite side of the resistor (furthest from the battery source) will drop.  Let’s continue with our example.

Calculate Pressure On Right Side

We also know that the original resistance of the pipe (without any obstruction) is actually at 3.  Now we can calculate the pressure that is on the right side of the obstruction by simple multiplication of the current on the right side times the Resistance on the right side.

E = I * R

E = 3 * 3

9 = 3 * 3

There is only 9 units of pressure on the right side of the circuit.  That means there has been a total voltage drop over the obstruction of 27 (pressure on left side) minus 9 (pressure on right side) = 18 units.

27 - 9 = 18 (total voltage drop)

This shows us that as current drops voltage also drops.  With water in a pipe this is intuitive also because we know as you obstruct more of the pipe, current will drop and the pressure after the obstruction will always be lower than before the obstruction since the lower amount of current (after the obstruction) cannot exert as much force (to create pressure).

If we limit the current more, the pressure drop also increases.  Another way to say this might be that the amount of pressure loss increases.  We lose more pressure on the right side as the obstruction (resistance) increases.

Finally, if the obstruction completely closes, no water flows at all (no current).  

 

 

At this point there would not be any water pressure on the right side of the pipe.  All water pressure is on the left side of the obstruction.

The Analogy to Electricity

All of this is directly analogous to the situation with current flowing in a wire with an unchanging voltage, where we continually increase the resistance.  

We can easily prove this by creating a simple circuit and adding in various sizes  of resistors but keeping the voltage the same.

Pre-build Calculations

Just to get an idea of the values that we will see when we measure, let’s calculate a few of these.

We are going to use a 3V battery supply and since we are wanting to calculate current (I) we will use the following Ohm’s law formula: E / R = I

We will use various resistors of the following sizes (all measured in Ohms, of course):

  • 10
  • 100
  • 220
  • 2.2K*
  • 4.7K
  • 10K

 

K* indicates Kilo meaning 1000 which means multiply the base value times 1000.  For example, 2.2K = 2.2 * 1000 = 2,200 Ohms.

47K = 47 * 1000 = 47,000 Ohms.

 

3 / 10 = 0.3  (300mA)

3 / 100 = 0.03 (30mA)

3 / 220 = 0.013 (13mA)

3 / 2200 = 0.0013 (13uA)

3 / 4700 = 0.0006 (600uA)

3 / 10000 = 0.0003 (300uA)

 

Let’s build a circuit and measure current and voltage.

Before We Continue

It was at this point that some things fell apart. I took my meter and attempted to measure the values on the circuit with 3V and 10 Ohm resistor.  When I did that I fell into an issue that is related to the precision and accuracy of electronic meters. I wrote up the question and answer at StackExchange.com and that post will explain everything more clearly.

https://electronics.stackexchange.com/questions/359691/multimeter-measuring-current-calculates-less-can-i-calculate-meters-internal

Here's an image of the first measurement I made with the circuit that has the 10 Ohm resistor.  I was expecting to see a number close to 300mA (0.300 amperes) but as you can see I only saw 157.7mA (about half).

Summary Of Meter Challenges

The summary is that a meter cannot measure the current with the same accuracy or precision that we can calculate it.  The post explains the reason why and explains a way that your meter may allow greater accuracy but at the price of precision.  In the case of my meter I can get the precision of milliamps (3 decimal places) with less accuracy or I can get only within 2 decimal places but more accurate reading.

 

Again, my first current reading with my meter (on circuit including 10 Ohm resistor) displayed 157mA.  

Of course, our calculation showed that we should get 300mA.

The value I saw on my meter was about half of the calculated value.

This means my meter is not very accurate.  Though it’s precision is to 3 decimals (to milliamps).

Let’s assume the battery voltage is 3V and I get 157mA.  What I want to know now is how much resistance seems to be in the circuit?

We just switch Ohms law around and calculate resistance.

3V / .157 = R

3 / 0.157 = ~19.10

It looks like there is something like 19.10 Ohms of resistance in my circuit.

Meter Introduces Internal Resistance

That’s because the meter itself introduces an internal resistance into the circuit and that resistance is consistently around 9-10 Ohms.

What i mean by consistently is that if I do the same current measurement with the 220 Ohm resistor in the circuit I get a value that is 13mA (.013) which is actually the calculated value for 230 Ohms of resistance - again about 9-10 Ohms of extra resistance that isn’t in our actual circuit.

In Theory Things Don’t Work Like They Do In Practice

This is why we do experiments and test things and examine the theory behind them.  You have to know how everything works.

Trade-Off: Accuracy Versus Precision

Finally, if I use the 20A port of my meter (for red probe) on the original 10 Ohm circuit then I see a value of 270mA.  I actually see 0.27. When connected to the 20A port I no longer see three decimal places of precision, but the measured value is only 3mA (.003) off from my calculated value.

That’s because on the 20A port there is less (but still some) internal resistance created by the meter.  We can calculate the internal resistance on this one too.

3V / .27 = R

3 / .27 = 11.11

We are expecting the circuit’s resistance to be 10 Ohms (with 10 Ohm resistor) but we are seeing 11.11 so it looks like the meter is creating about 1.11 Ohms of extra resistance on the 20A port.  That means our displayed value will be more accurate (closer to our calculated value) but it also won’t be as precise.

Resistor Identification

We never have talked about resistor identification yet.  I mean how do you know which size of resistor you have in your hand when you go to use it?

I don’t want to spend a bunch of time on this for a few of reasons:

  1. it’s relatively simple

  2. You can use your meter to measure the resistance

  3. it’s explained in numerous places on the Web ( )

  4. if you bought the suggested resistor pack,they are in packages which are labeled.  Of course as soon as you take the resistor out and lay it down the resistor could become unidentified to you so it is best if you understand the basics of how to identify them

  5. There are online calculators that allow you to easily determine the value of the resistor by selecting colors.


There are some kinds of resistors which have their values printed directly on them.  However, since most resistors are very small most use color code bands (stripes) to let you know the resistance they are rated at.

Two Kinds of Resistors

Color-banded resistors come in two types:

  1. 4 color band

  2. 5 color band

 

On four color band resistors you will see three color bands indicating the value of the resistor and one color band indicating the tolerance.  The last color band indicating tolerance will always be either silver (10% tolerance) or gold (5% tolerance).

On five color band resistors there are 4 color bands indicating the value of the resistor and

Online Calculators

The easiest calculators to use are probably at DigiKey.com (electronic parts seller) who provides this the calculator for free.  I want to give you the links now so you can check values as you go.

 

4 Band Resistor Calculator

https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band

 

5 Band Resistor Calculator

https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-5-band

 

Remember The Rainbow

If you can remember that Black is zero and brown is one then all you really have to remember is the order of the colors in the rainbow (Roy G. Biv from school days) and you’ll be able to identify resistors without having to look them up all the time.  That’s because the way we determine the values of a resistor using the color code works like the following.

First simply understand that each color represents a number.

Black

Brown

Red

Orange

Yellow

Green

Blue

Violet

Grey

White

0

1

2

3

4

5

6

7

8

9

 

Gold and silver bands are not part of the resistance value scheme.  Instead those colors indicate the tolerance (how precisely they will adhere to the coded value).  

Gold stripe : tolerance to 5%

Silver stripe : tolerance to 10%

The precision part of the code is always at the end of the color code.  That means to identify the resistor you start at the other end of the resistor (away from gold or silver color).

Once you do that you will see something like:

Red, Red, Brown

The first two numbers simply indicate the numbers you use to determine the resistance:

Red is equal to 2 so you’d have

22 then the last color is the number of zeros you had after the two numbers you’ve just determined.  In our case the last color is brown which indicates one zero.

So we have:

Red  Red   Brown

2     2     0

220 That’s a 220 Ohm resistor.

 

That means if you had a 2.2K (2,200 Ohm resistor) you’d have :

Red  Red   Red

2     2    00

That’s a 2, followed by a 2 and then two zeros.

 

What colors would a 470 Ohm resistor have?

Yellow   Violet(pink)  Brown

4          7             0

 

An odd looking one might be a 22 Ohm resistor.  What might that look like?

Red    Red      Black

 2     2     (no zeros)

That’s because we have 22 then we have black which means 0, which means we add no zeros (zero zeros) so we just have a 22 Ohm resistor.

The Reality of Color Bands

So, here is a picture of three resistors.  The first two we have seen already and the last one is a mystery.  These are from the Cutequeen pack that I ordered on Amazon so the resistor body is blue. It’s a bit unfortunate because the blue background seems to affect the stripe colors.

 

 

The first one has two red bands and then a black band and then a gold band.

That is a 22 Ohm resistor, but it’s a bit odd that the manufacturer marked it like that. I think they are attempting to let you know that the fourth band is not to be used in the resistance calculation.

 

The next one is problematic.

To me, it looks like Green, Blue, Black, Black, Black

If we plug that into the resistance calculator at the digikey site we would get: 560 Ohms

However, it is actually a 470 Ohm resistor.

That’s because the first band is actually Yellow (4) and the second band is actually violet (7).

Keep Them In Bags and Check With Multimeter

The point here is that most likely you will want to keep your resistors in bags that are marked and then use a multimeter to insure you have the right value.

I also bought some small envelopes and when I find a resistor I’ve been using to experiment with laying around unidentified I will check it with the meter and drop it in a marked envelope.

Some Resistors Do Have Five Bands

If you see a resistor with five bands (4 color code stripes and 1 tolerance stripe) you just use the extra band as another number and the last color code band is always the number of zeros to add.

For example,

Blue   Green   Green  Yellow

6        5      5     0000 (4 zeros)

 

6,550,000  (6.55 million) Ohms

Use Your Meter

If you have a unidentified resistor get into the habit of powering up your meter, changing it to the resistance measuring setting (indicated by the Omega symbol) and clip a probe on each end and read the screen.

Why Are Resistors Important to Our Circuits?

Resistors are important to us because they allow us to control current and voltage.  Controlling current is important for us so we don’t run our batteries down fast for no reason.  For example, if you built a circuit that consisted of nothing but a AA battery and wire the current would be at the maximum the battery can supply since the wire itself offers very little resistance.

 

That could also cause the wire to become quite hot and possibly even melt (depending upon the diameter of the wire).

  Of course controlling current is also important because putting too much current through a particular circuit can damage components in the circuit.  Each component in a circuit will be rated at a maximum current level and can only take so much current before they break down.

Current Creates Heat

That’s because current creates friction in components and friction creates heat.  And if the current creates more friction and heat than the component can dissipate in the time allotted then the component may melt or even catch on fire.  Remember that current is measured by the number of electrons that pass a point in 1 second of time. The more current that is occurring then the more electrons which are speeding past a point within each unit of time, which translates to friction and heat.

Even Resistors Can Breakdown and/or Melt

Resistors themselves are rated with a specific power rating.  That’s because as resistors limit the flow of electrons resistors also heat up.  Again, if you have too much voltage behind the current you are pushing through the resistor then the resistor itself will heat up more -- since current is pushing faster through the resistor and creating more friction.  If the resistor isn’t rated at a high enough power rating it may overheat and become damaged or break or even melt.

There is a formula so you can know how much current you can put through a particular resistor.

That formula allows us to calculate the power in a circuit.

About Power

Power is another variable of electricity and there is a formula we can use to measure power.  

The formula looks like :

P = EI

Power is equal to E (Voltage) multipled by I (Current)

James Watt

Again you can see that if we know two variables from our circuit we can then calculate the power it is using. Of course we know that our Voltage is measured in Volts named for its discoverer (Alessandro Volta) and our Current is measured in Amps (named for its discoverer Andre Marie Ampere).  But what is Power measured in? It is also named for its discoverer and it is named Watts. We’ve all heard of Watts and Wattage but maybe never knew it is named after James Watt.

 

The resistors I bought for these experiments are rated at ¼ W (Watt) or 0.25 Watts.

 

I have to keep that value in mind as I create my circuits.  Suppose I set up a circuit that is using 3V (two AA batteries) and one 30 Ohm resistor.

The current in the circuit would be approximately 0.1A as calculated from:

3V / 30 Ohm = 0.1A

Calculate Power From Voltage & Current

Now to calculate power I take the voltage and current and multiply them:

P = E*I

P = 3V * 0.1A

0.3W = 3V * 0.1A

So I am at 0.3 W which is greater than the rating of (0.25 W) of these resistors and they are not going to hold up over time in a circuit like that.  

If You Only Know Current & Resistance

If you only know the current and resistance values of a circuit you can still calculate the power using the following formula:

P = I<sup>2</sup> * R -- That’s Current squared times the resistance.

The reason we can do that calculation relates back to a reworking of Ohm’s law and the substitution principle of algebra.  

We know two formulas.  

The first one is Ohm’s law:

E = IR

The second one is the formula for calculating power:

P = EI

Now suppose we substitute the E in our Power calculation with the value that it equals in Ohm’s law.

We know that Ohm’s law says that E is equal to IR.

So in our power calculation we replace the E with IR like the following:

P =   EI

P = (IR)I

That means P is equal to I*R*I.

Since multiplication is commutative (numbers can be multiplied in any order), we simply rearrange it to simplify it and we get:

E = I * I * R

E = I<sup>2</sup> * R

Let’s see if the value we get seems correct.

In our case we could have:

P = 0.1<sup>2</sup> * 30

P = 0.01 * 30

P = 0.30 (Watts)

That’s the same value we got when we used the other formula (P=EI).

Heat Dissipation is Important

If we push the voltage up (and the resulting current) then we are at risk of the resistors melting since they cannot dissipate the heat fast enough.

But even at this slightly elevated wattage we should expect the resistors to fail at some time and should not use them in a permanent circuit.

Sparkfun : Great Site and Resource

Now that we’ve talked about all of these ideas with some detail here is another resource from the fantastic sparkfun.com site that even provides some cutaway pictures of the inside of resistors so you can get an idea of what is inside them : https://learn.sparkfun.com/tutorials/resistors

The wikipedia entry on resistors also provides some additional great information:

https://en.wikipedia.org/wiki/Resistor

The last couple of things we should cover about resistors is how they work when placed in series and when placed in parallel in your circuits.  

Resistors In Series

When resisitors are placed in series (one after the other) then you calculate resistance by simply adding the values of the resistors together.  We call this calculating the equivalent resistance. 

That's all there is to it.  Just add up the total resistance of all the resistors that are in series.

Principle of Resistors In Series

You can think of this as a principle that resistors in series increase the amount of resistance in the circuit.

When doing this for resistors in series it is very easy.  However, it's a bit different when calculating for resistors in parallel.

Resistors In Parallel

Resistors in parallel actually decrease the overall resistance.  We need to use a formula to calculate the equivalent resistance for resistors in parallel.  

The formula used to calculate equivalent resistance for parallel is:

That means to calculate total resistance (Rt) you :

  1. Divide 1 by each resistor's value
  2. Add the resulting values
  3. Divide 1 by the resulting resistance total you obtain, to get the total resistance.

In this case you take each resistor's value and divide 1 by that value, then add up all of the results.

For example, if we have two 100 Ohm resistors in parallel, we would calculate total resistance in the following way:

Fractions are very easy when you have a common denominator, because you just add.

1/100 + 1/100 = 2/100

Now take the result and divide 1 by that result.

1 / (2/100)= Rt

Dividing a number by a fraction is as simple as multiplying by it's reciprocal value:

1 * (100 / 2) 

1 * 50

Rt = 50 Ohms

Shortcut When All Resistors In Parallel Are Same Value

There is a very quick shortcut when all resistors in parallel are the same value.  But this only works when all resistors are the same value.  

You simply take the value of one resistor  (in our case 100) and divide it by the number of resistors in parallel (in our case 2).

100 / 2 = 50 Ohms

Online Parallel Resistance Calculator

Here's a great online resistance calculator.  

https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/

 

Here's what our resistors in parallel look like in a schematic and you can see how I've measured parallel resistance using my meter.

 

Here you can see I've clipped my meter probes to two 100 Ohm resistors in parallel and the meter reads (approx 50 Ohms) 49.7 Ohms.

 

Finally, you can see I put 3 100 Ohm resistors in parallel and used a wire at each end to connect my meter probes to so I could get a good connection.  We calculate 33.3 Ohms of resistance but of course the real world with resistor tolerances, meter variance and even ambient temperatures affecting resistance we see 34.9 Ohms of resistance.

 

 

Some Youtube Videos Can Be Great Too

Here’s a great youtube video which shows these resistor principles (basic operation and series versus parallel) and compares them to each other with a nice video graphic so you can see how the principles work:

https://youtu.be/G3H5lKoWPpY

 

Variable Resistance Via Potentiometers

No discussion of resistors would really be complete without mentioning variable resistors (potentiometers).

Potentiometers have a rotary knob and three poles.

These are nice because instead of including a fixed resistance in your circuit as you do with the resistors we've seen so far, potentiometers allow us to add a variable resistance.

The middle post is the common and the left and right posts will create a variable resistance when used in conjunction with the middle post.  The difference is that if you use the left post then min value will be when the knob is turned completely counter-clockwise and the max will be when the knob is turned all the clockwise.  If you use the right post then turning the knob the will have the opposite effect.

If you connect wires (or your meter) between the left and right posts you will get the same effect as if you have a fixed resistor of the max value that the potentiometer is rate at.  

You can see that this potentiometer's max is rated at 2K Ohms.

Multimeter Testing

To get an idea of how you can use potentiometers in your circuits you can set up your multimeter to read resistance and then connect one probe to the left pole and one to the middle pole and then watch the meter as you move the knob.

You will see that various values from something near 0 Ohms (this one has a min around 3.8 Ohms) all the way up to 2000 Ohms.

Simple Circuit To See It Work

Of course, if we create a circuit with the potentiometer in it we can dial up the resistance in such a way that the LED will turn off.

First of all, here is what a potentiometer (variable resistor) looks like on a schematic diagram.

You can see that it is very similar to a fixed resistor.  There is just a additional arrow element which represents the wiper which is the arm inside the potentiometer that moves and increases/decreases the resistance.

Here's the circuit we are going to build to show how this will work.

You can see this is very simple.

When the potentiometer is at it's lowest setting (3.4 Ohms) and turned completely counter-clockwise then the LED is very bright.  That's because we actually only have about 103.4 Ohms of resistance against the 3V battery. 

Keep in mind that the pot (potentiometer) has 3.4 Ohms at its lowest setting and it is in series with the fixed resistor of 100 Ohms which gives us a total of 103.4 Ohms.

That means we have about 3V / 103.4R =  0.029A (or 29mA) of current.  It's actually a bit less because there is a voltage drop over the LED too.

 

Of course, with the low resistance more current is able to flow.  However, when you turn the knob on the potentiometer in the clockwise direction you will see the LED begin to dim as the resistance increases.

Finally, the LED is barely lit at all with the full 2K Ohms of resistance.  Actually 2100 Ohms of resistance since the pot (short for potentiometer) is in series with the fixed 100 Ohm resistor.

You can barely tell the LED is lit at all in that last picture.  There is very little current flowing.

We will use potentiometers in future circuits so we will learn more about them then so that wraps it up for this chapter.

I hope you've found this an interesting an informative chapter on resistance and why resistors are important in our circuits.

You Are Ready, We Are Ready

Now that we have a lot of basics down we are ready to build some far more interesting circuits.

Next Time

Next time we will build our flip flop circuit which will incorporate resistors, capacitors, transistors and LEDs and it will lead us to discoveries of digital logic and great possibilities that are created by ICs (integrated circuits).

History

2018-05-20 : (Finally) released for publication.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

raddevus
Software Developer (Senior) RADDev Publishing
United States United States
The CP editors went to Canada and all I got was this crummy sig line.
My web site, blog and other dev projects including C'YaPass : http://raddev.us^

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Praisevery nice Pin
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QuestionResistors in Parallel Pin
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Suggestion"Two Kinds of Resistors", sentence incomplete Pin
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