Understanding electrical resistance

One of the base level concepts I’ll need to learn more about is resistance.  As you likely know, when talking about the flow of electricity you typically talk about conductors and insulators.  Conductors are materials that allow the free flow of electricity whereas insulators limit or prevent the flow of electricity.  A good example of a conductor would be a metal like copper or gold.  An insulator would be something like wood or plastic.  Different materials conduct different amounts of electricity.

When talking about resistance in electronics one often talks about resistors.  A resistor is a component that (you guessed it) resists the flow of electrical current.  The best way to show this is a quick example.  Let’s use a multimeter to test the resistance of some resistors that came in our kit and see how they work.  The first thing you need to do is make sure that your multimeter can measure resistance.  I haven’t seen one that cant but I guess it’s possible.  Your meter should have a position that looks like a Greek Omega symbol (Ω).  Like the one shown in the picture below…

Above you can see that I have my multimeter set to measure resistance and that I have the two leads (extended with alligator clip cables for the sake of convenience) connected and the meter is measuring 0 resistance.  Resistance is measured in ohms (Ω).  Since I have the two test leads directly connected getting a reading of 0 makes sense.  Now let’s try to connected the leads to a 100k resistor (note: I was initially leaving the resistor on the adhesive strips to keep them separated which is why the picture shows 10 of them.  Im only hooking the leads to the bottom resistor)

Here you can see that the meter is registering 98.9K (so 98,900) ohms.  Pretty straight forward right?  Resistors can also be chained together and their resistance is additive.  For instance, below I’ve used a spare alligator clip to connect a 220 ohm and a 330 ohm resistor and then I measured their cumulative resistance.  As expected, I end up with 550 ohms total…

One last thing to point out is that resistors are supposed to be color coded.  In my above examples you can see that they came in a sheet of 10 that happened to be labelled.  If they weren’t you should be able to look at the color bands to determine the resistance.  Lets zoom in on a couple and see if we can figure it out.  The first we’ll look at will be a 330 ohm resistor….

If you look closely (sorry for the poor pictures, I’ll need to work on the lighting down here) you can see that the resistor appears to have several bands of color.  To decode this, we can use a resistor color band chart like the one shown below (shamelessly stolen from the DigiKey website)…

There should be one end that has a band that’s thicker than the other.  In our case, the band at the top of the picture looks slightly wider.  The wide band should be on the left when you analyze it against this chart.  If we start from the left, the bands appear to be orange, orange, black, black, brown.  If we compare that to the chart, we should see that it yields a value of 3,3,0, 1x multiplier, 1% tolerance.  So that would 330 times 1 gives us a 330 ohm resistor with a 1% tolerance.  Make sense?  Even though it’s labelled, I still think it would be a good idea to check each resistor you want to use to ensure that it’s labelled correctly and that its working as it should.  That and it took me some time to find a chart that appeared to be accurate.  Other charts only referenced 3 bands and talked about having a silver or gold bar to discern if it was within 5 or 10% tolerance.  So that’s another reason to check each one to make sure its the one you want.

So now that we know how to measure resistance, what do we do with it?  Let’s look at a quick example of trying to light an LED to see why we need resistors at all.  All electronic components should come with what’s called a ‘data’ or ‘spec’ sheet that outlines the components electrical characteristics.  If we look at the sheet for one of the LEDs that came in my kit we can see if has a lot of information on it.  This is a snippet just from one page…


W
hile there’s a lot of information here, the items we really care about right now are the ‘Forward Voltage’ and the ‘Continuous forward current’.  In our case, the typical (TYP) forward voltage for the LED is 3.6 volts with a maximum of 4 volts.  We can also see that the LED is rated for 20 milliamps of current.

Now – let’s say that Im using a 5 volts power supply.  What’s going to happen when I plug the LED directly into the 5 volts power supply?  Well, in theory, it could (should) burn out.  In reality, since my power supply is 5 volts, and the limit is 4 volts, I haven’t yet managed to burn one out by hooking it straight to the power supply.  All the same – if we want to play by the rules, we need to figure out how to only apply 4 volts of power to the LED.  To do this, we can lower the voltage by introducing resistors into the circuit path.  To figure out what resistor to use, we can use what’s called Ohm’s law…

Ohms law states that the voltage(V) will equal the intensity (I) times the resistance (R).  Intensity happens to be measured in amperage so ‘I’ is really amps in this equation.  So in our case, we know the voltage of the LED and what it can handle as far as current.  So we shift our equation around to solve for resistance instead of volts.  Then we just plug in the values making sure that convert milliamps to amps.  In this case, we solve R for a value of 50.  So let’s go back to the lab…

Here we’re using our breadboard to make a simple circuit.  On the top left I have power coming into the board from a USB cable.  I took an old USB cable, cut the end off, and soldered a header onto it so I could plug it into the board.  Then I extended the positive lead to the positive side of the right power rail (top green wire) and the negative lead to the negative side of the right power rail (orange wire).  So now our board has power.  From there, its just a matter of plugging the positive side of the LED (longer lead) into the positive side of the rail and the negative side (shorter lead) of the LED.  At this point, our USB port which supplies 5 volts of power will be delivering all of that voltage to the LED.  Despite being over the rating for the LED, it works and doesn’t show signs of burning out…

If we plug the USB power in and check the circuit with our meter we’ll see we’re getting almost 5 volts into the LED.  (Note: Theres an upcoming post on how to use the multimeter for checking voltage and current, don’t worry about that for now.  Just know we’re measuring the voltage that the LED is receiving).  So at this point everything is working, but its not quite ideal since we’re over voltage.  To get us in spec, we’d need to only deliver 4 volts to the LED.  To do this we’ll insert resistors into the circuit…

If you’ll recall from above we came up with a number of 50 ohms when we finished our math equation.  Since I didn’t have a 50 ohm resistor I had to use five 10 ohm resistors in series.  Its sort of hard to se but each end of each resistor is connected to a different row in the breadboard putting them in series.  Now when we plug in the power we should see the LED is only getting 4 volts…

Prefect!  So this shows us how we can use resistors to impact the voltage of a circuit.  Next up I’m going to try and solder my first circuit.  We’ll see how that goes…

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