Diodes have the following properties:
A diode is represented with the following symbol:
The diode above will pass current flowing from left to right, but block current trying to flow from the right to the left-hand side. The voltage drop over diodes will vary from diode to diode, the most common ones are:
diodes: Have a voltage drop between 0.6V and 0.7V
Using diodesBelow is a simple circuit with a diode. You must always place a resistor in series with a diode to limit the current flow, if you don't do that the diode will act as a short circuit and draw current until the supply or the diode breaks.
The voltage drop across the diode is 0.6V (this is a normal diode and 0.6V is close enough.) The diode and resistor is in series, thus the current through the diode is equal to the current through the resistor.
Thus the current through the diode equals 9.1mA
Here is an example with an LED (you will notice the symbol of an LED has arrows pointing away from it, this is to show that it emits light)
The more current an LED's conducts, the brighter it will be. To increase the current you can either increase the voltage or decrease the value of the resistor. Just remember that there is a limit to the amount of current a diode can conduct before it breaks and for LED's this is rather low (about 20mA typically)
diodes are mostly used to create a constant voltage. Say we want an LED
to be the same brightness, no matter what our input voltage is (the
input voltage could be from a battery that starts out at 14V and goes
down to 9V before it is charged again)
will determine the amount of current flowing through Z1 (for Zener
diodes to work correctly, you need a minimum current flowing through
them, about 10mA is a good value)
If the input voltage is 14V:
Because the Zener diode will always have a constant voltage drop over it, the potential at Vzener will always be 4.7V. The current through D2 is (assuming the voltage drop over D2 is 2V):
And this current will always be the same, whether the input voltage is 14V, 9V or anything in between. Of course when the input voltage drops below 4.7V, the current through D2 will drop.
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