What is a Diode?

diode

A diode is simply the main building block of semiconductors. It's a great little device that limits current flow to one direction.

There are 2 sides to a diode; the anode (the positive side) and the cathode (the negative side). To turn on a diode, or to "forward bias" it, the anode must be "more positive" than the cathode. To turn off a diode, or to "reverse bias" it, the anode must be "less positive" than the cathode. Here are some examples:

Forward Biased:



In the leftmost circuit, the diode is in its "forward biased" position where the anode is "more positive" than the cathode. Current is flowing through the diode and the resistor.

The middle circuit illustrates the similarities between a switch and a diode. When the switch is closed, current is flowing through the switch and the resistor.

The rightmost circuit illustrates the similarities between a checkvalve and a diode. The flow of pressure forces the ball "away" from its rest position, thus enabling current flow through to the flow meter.

Note: The voltages in the leftmost circuit was omitted to illustrate the similarities between all three circuits. In general there should be approximately 0.7V dropped across the diode every time it is in its "forward biased" condition.

Reverse Biased:



In the leftmost circuit, the diode is in its "reverse biased" position where the anode is "less positive" than the cathode. Current flow is stopped.

The middle circuit illustrates the similarities between a switch and a diode. When the switch is opened, current flow is stopped.

The rightmost circuit illustrates the similarites between a checkvalve and a diode. The flow of pressure forces the ball "into" its rest position, thus impeding current flow through to the flow meter.

Circuit Isolation

One of the more useful things about diodes is how you can isolate a circuit.



Here we have an electronic blackbox providing 12V to a couple of motors. The motors spin in unison. We don't really care what is in the electronic blackbox, but we want to separately provide 12V to each motor, independent to and without changing the operation of the electronic blackbox. At first thought, we could probably add a couple of switches, each switch to a motor, and provide 12V when needed.



In the above circuit, adding a couple of switches and separately supplying 12V to each motor is great in theory, but it does introduce a few problems:

1. Closing SW1, 12V is applied to M1 and M2 because they're connected to each other.
2. Closing SW2, 12V is applied to M2 and M1 because they're connected to each other.

Essentially, connecting a couple of switches to run the motors separately does not work.

Here's what we want:

1. M1 and M2 need to stay connected in order to keep the electronic blackbox's functionality, which is to spin the motors in unison.
2. When the electronic blackbox is off, closing SW1 provides 12V only to M1.
3. When the electronic blackbox is off, closing SW2 provides 12V only to M2.



Here we have an improved version of our last circuit. We added 2 diodes to provide some isolation.

Here's what happens:

1. When the electronic blackbox is ON, the anodes of D1 and D2 is "more positive" than their cathodes, they forward bias and provide a 12V path to M1 and M2.

2. When the electronic blackbox is OFF, closing SW1 provides 12V to only M1. The anode of D1 is "less positive" than the cathode, it reverse biases and does not provide a 12V path to M2.

3. When the electronic blackbox is OFF, closing SW2 provides 12V to only M2. The anode of D2 is "less positive" than the cathode, it reverse biases and does not provide a 12V path to M1.

Arc Suppression

In any relay or similar device where a coil is energized to open/close a circuit or device, a diode is used in parallel to the coil to suppress its charged voltage.



Above illustrates what happens if there's no diode across the coil of a relay. When the switch is "closed," current flows through the relay coil creating an electro-magnetic field that forces the relay contact to close, thus providing 12V to the load resistor. When the switch is "opened," the current flowing through the coil wants to continue to flow creating an arc across the switch. This "inductive kickback" do not pose any short term problems, but long term effects include pitting and/or carbon deposits developing over the contacts of the switch. Both affect the lifetime of the switch.



Here we have a diode placed across the coil. When the switch is "closed," the relay coil energizes and forces the contact to close, thus providing 12V to the load resistor. Since the anode is "less positive" than the cathode, the diode is acting in "reverse bias" which is essentially an open circuit across the coil. When the switch is "opened," current continues to flow through the coil "forward biasing" the diode.  The "forward biased" diode suppresses the "inductive kickback" to 0.7V preventing an arc across the opening switch.

More to come...