ResistorsThis presentation is an introduction resistors and will help the readerunderstand how and why these are used when we discuss controllingexternal devices using the GPIO port of the Raspberry Pi.These are used to limit the flow of current in a circuit and their resistance ismeasured in ohms. Resistors are a fundamental building block in anyelectronic circuit. They are used widely to set the current, voltage, gain anda number of other important parameters.Resistors come in various shapes and sizes and are mainly defined by theirResistance, Tolerance, Power rating and Packagingwww.sf-innovations.co.uk
Resistance: This can vary from milli ohms up-to Mega ohms. The value ofa typical axial resistor is marked on it using an internationally recognisedcolour coding system. There are some slight variations as one can find a 4,5 or 6 rings as shown below.www.sf-innovations.co.uk
Colour code examples: Below are examples of how a 22 ohm and 6200ohm resistors are marked.www.sf-innovations.co.ukNote:6200 ohm is the same as 6.2 kilo ohm1000 ohms = 1 kilo ohm1,000,000 ohms = 1 Mega ohm
Resistance values: This can be any value in practice. However resistorsare normally available in preferred values in ranges known as E12, E24 andE96 ranges.The E12 range is so called as there are 12 values in every decade.Between 10 and 100 ohms:10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82 ohmsBetween 100 ohms and 100 ohms:100, 120, 150, 180, 270, 330, 390, 470, 560, 680, 820Between 1000 and 10,000 ohms:1000, 1200 etc right up toGreater than 1 Mohm:1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 M ohms.The E24 range has 24 values and the E96 has 24 values per decade.www.sf-innovations.co.uk
www.sf-innovations.co.ukWhen resistors areconnected in series thecombined value is arrived atby adding the value of theindividual resistors.In the above example, thiswould be:Rnew = R1 + R2 + ... + RnExample 1:If a 220 ohm resistor and a 330 ohmresistor are in series, then thecombined resistance is (220 + 330) =550 ohmsExample 2:If three 150 ohm resistors areconnected in series, then thecombined resistance is (150 + 150 +150) = 450 ohmsCombining Resistors: The examples below show how to work out thevalue of the new resistance when resistors are combined in series orparallel.
www.sf-innovations.co.ukExample 1:If two 560 ohm resistors areconnected in parallel, thecombined resistance wouldbe 1/[(1/560) + (1/560)] = 280ohms.Example 2:If three 1000 ohm resistorsare connected in parallel,then using the equationshown here one would workout that the combinedresistance is = 330 ohms.When resistors are connected inparallel, the combined value is arrivedat by adding the reciprocal of thevalue of the individual resistors. Thereciprocal of this is the new value.In the above example, this would be:1/R new = 1/R1 + 1/R2 + ... + 1/RnCombining Resistors: The examples below show how to work out thevalue of the new resistance when resistors are combined in series orparallel.
Tolerance: The accuracy of resistors can be specified when they arepurchased. This can be 1%, 2%, 5% or higher depending on theapplication.Power rating: A resistor may be used in a small signal application where apower rating of 125 mW or 250 mW is adequate. However in someapplications a higher power rating like 1W or 5W may be required.Packaging: Resistors come in different packages. The main ones are axialthrough hole or Surface Mount.www.sf-innovations.co.ukPower resistorsSelection of surface mount resistors(note: the resistor values are printed onSMT components with the last digit is amultiplier.)
Experiments with resistorsYou will need the following:Selection of resistors – 1000 ohm (1 kilo ohm) & 330 ohm9V PP3 battery with a battery clipAn LEDDigital Multi-meterwww.sf-innovations.co.uk
Experiment 1 Resistors in seriesUse the Digital multi-meter on itsresistance range to check the value of a 1kilo ohm resistor by connecting the testprobes on either end of the resistor.Make sure that the meter is set to measureresistance and a suitable range isselected. Here it is set to the 2000 ohmrange as this is more than the values thatwe are measuring.Connect a 1 kilo ohm and 330 ohmresistor in series. Check that the value ofthe 2 resistors combined is 1.33 kilo ohms.www.sf-innovations.co.uk
Experiment 2 Resistors in parallelConnect a 1 kilo ohm and 330 ohm resistor in parallel. The combined valueshould be given by the following equation.1/Rnew = 1/1000 + 1/330Rnew = 249 ohmsUsing the multi-meter check that the value of the 2 resistors combined isaround 249 ohms.www.sf-innovations.co.uk
Experiment 3 Controlling the current through an LEDConnect a 1 kilo ohm (1000 ohm) resistor in series with an LED and thenconnect this to the 9V PP3 battery. Note: The long leg of the LED needs togo to the positive of the battery.The voltage across the resistor should be (9 – 2) where 9V is the voltageacross the battery and 2V is the voltage across the LED. Using Ohms law wesee that the current through the 1 kilo ohm resistor should be:9V/1 kilo ohm which is 9 milli amps.www.sf-innovations.co.uk
Experiment 4 Making the LED brighterTo make the LED brighter, you would need to reduce the value of the reistor.For example if you use a 330 ohm resistor instead of the 1 kilo ohm resistor,the current would be:9V/330 ohm which is nearly 30 milli amps. This is more than 3 times thecurrent compared to when a 1 kilo ohm resistor is used and the LED is muchbrighter.Note: A way to quickly double the current, is to halve the resistor from 1000ohm (1 k ohm) down to 500 ohms. This can be done easily by puttinganother 1000 ohm resistor in parallel to the original 1000 ohm resistor.www.sf-innovations.co.uk
Summary - ResistorsThis presentation covered the following facts about resistors:Colour codingOhms, kilo ohms and Mega ohmsValues – E12, E24 rangeResistors in seriesResistors in parallelExperiments with resistors in series, parallel and LEDs.www.sf-innovations.co.uk