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The light emitting diode (LED) is commonly used as an indicator.
It can show when the power is on, act as a warning indicator, or be part of trendy jewelry etc.
It needs to be fed from a DC supply, with the anode positive and the cathode negative, as shown in the diagram.
To calculate the value of the series resistor we need to know the diode forward voltage and current and its connections.
The necessary data can be obtained from a catalogue or data book.
In our example it is 2 volts and 20mA (0.02 amps).
The cathode lead is the one nearest a "flat" on the body.
Since the voltage across the diode is 2 volts and the battery voltage is 12 volts, then the voltage across the resistor is 12-2
= 10 volts.
The diode is in series with the resistor, so the current through then both is the same, 0.02 amps.
We now know the voltage across, and the current through the
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resistor.
From Ohm's Law we can now calculate the value of the resistor.
Resistance = Volts divided by Amps = V/I = 10/0.02 =500 ohms.
Since this is not a standard value we can use a 470 or 560 ohm resistor as this application is not critical of values.
Copyright Graham Knott 1999
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RESISTORS
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Resistors are electronic components which resist the flow of electronic current.
The higher the value of resistance (measured in ohms) the lower the current will be. This was discovered by Mr Ohm.
The simplest resistors are made from carbon rod with end caps and wire leads.
Other types are carbon film which is a thin layer of carbon on a ceramic rod, and metal oxide and metal glaze on glass rods.
Wire wound resistors are used where the resistor has to dissipate a lot of heat.
Faulty resistors have gone open circuit or changed in value. They never go short circuit.
Some resistors are designed to change in value when heated. They are called THERMISTORS and are used in temperature measuring circuits.
Some resistors change in value when exposed to light. They are called LIGHT DEPENDANT RESISTORS.
Most resistors are colour coded to indicate their value and tolerance.
Wire wound resistors have their value written on them. (colours would change with heat).
High stability resistors (marked with a fifth pink band) do not change value easily.
Resistors generate heat. Resistors have a wattage rating. The higher this rating the more heat they can dissipate.
To limit the range of resistor values to a manageable number a preferred range only is available.
These are
1.0 1.2 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2
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This mean that 1 ohm, 12 ohm, 180 ohm, 2200 ohm resistors etc are available.
1000 ohms is 1k, 1000,000 ohms is 1M. 3,300,000 ohms is 3.3M etc.
Decimal points are not used on circuit diagrams (they may be confused with fly specks).
3.3M would be written as 3M3 and 1.8k as 1K8 etc.
On circuit diagrams tolerance is indicated by the following letters.
F=1% G=2% J=5% K=10% M=20%
R22M= 0.22 ohm 20% 4R7K= 4.7 ohm 10% 68RJ=68 ohm 5%
Variable resistors are available. These can be operated by means of a knob on the control panel. Examples are volume and brightness controls.
Preset variable resistors are internal controls which are adjusted in value by means of a screwdriver. Once adjusted, they are never touched
again.
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Copyright Graham Knott 1999
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CAPACITORS
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Capacitors are basically two parallel metal plates separated by an insulator.
This insulator is called the dielectric.
Capacitor types are named after the dielectric. Thus we have ceramic, mica, polyester, paper air capacitors etc.
Capacitors can be charged up and store electricity, similar to a car battery.
This can be a hazard if they are charged up to high voltages.
If it is necessary, capacitors with large charges should be discharged via a resistor to limit the discharge current.
DC current cannot flow through a capacitor since the dielectric forms an open circuit.
Capacitors come in all shapes and sizes and are usually marked with their value.
Values are measure in Farads. Values in Farads are unusual.
Most capacitor values are measured in microfarads,
nanofarads or picofarads. See the page on Value multipliers to find out more about this.
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They are often marked with their maximum working voltage.
The voltage across the terminals must not exceed this value. It is OK to use a voltage below the maximum value.
Some capacitors such as electrolytic and tantalums are
polarised. This means that they must be fitted the correct way round. They are marked to indicate polarity.
Some values are indicated with a colour code similar to resistors. There can be some confusion.
A 2200pf capacitor would have three red bands. These merge into one wide red band.
Some values are marked in picofarads using three digit numbers. The first two digits are the base number and the third digit is a multiplier.
For example, 102 is 1000 pF and 104 is 100,000 pF = 100 nF
= 0.1 uF.
To find the total value of capacitors in parallel (that is connected across each other) their values are added.
To find the total value if they are in series (that is in line with each other) then the following formula is used.
1/C total =1/C1 + 1/C2 + 1/C3 etc
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Variable capacitors are available in which the value can be adjusted by controlling the amount of overlap of the plates or the distance between them.
There is a type of diode called the Varicap diode which similar characteristics.
Copyright Graham Knott 1999
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