Diodes are semiconductor devices that allow current to flow only in one direction. Diodes look like this:
The diagram is labelled with an anode and a cathode. The voltage drop accross the diode is from anode -> cathode, and the current is conducted in the direction pointed by the really big black arrow.
The type's of diode's we're concerned with are silicon diodes, which have a forward voltage of about 0.7V. This is only an approximation, but is the value to use in calculations.
Diodes are non-linear components:
- When current is flowing from anode to cathode, the diode is forward-biased, and will conduct current
- When the current is flowing backwards (the wrong way), the diode is reverse-biased.
- At a large negative voltage, the diode will break down, and start to conduct current again
- Don't let the voltage get this high, you wont like what happens.
For the diode to conduct, it must have a minimum voltage accross it, known as the forward voltage. This is also always the total voltage drop accross the diode. For a silicon diode, this is 0.7V, which is why the I-V graph does not go up from zero. The diode can be said to "open" or "switch on" at about this voltage.
- If there is a voltage of 0.2V accross a diode, no current will flow
- If there is a voltage of 0.6V accross a diode, a tiny amount of current may flow
- At >0.7V, the full current will flow with no resistance.
Find the current and the voltages accross each component in the circuit below.
By Ohm's law, the current is:
Thefore, the voltages are
Find the current through each resistor in the circuit below.
Doing KCL around node :
The three currents are then: