Diodes

Ideal Diode

The most basic function of a diode is to control the flow of current in a certain direction. In an ideal diode, current can only flow in one direction called the “forward direction”. Using the standard convention of current flowing from a positive source to a negative source, this means current would flow in a diode only if the diode’s forward direction was oriented from positive to negative. (see below)

If the voltage across this ideal diode is negative (so voltage is opposed to the diodes “forward direction”) then no current would be allowed to flow.

A diode’s schematic symbol has 2 connection points called the Cathode and the Anode. The cathode is oriented on the side with the flat line (on the right of the image above) and the anode is oriented on the base of the triangle (on the image above). Using this terminology, if the voltage is higher on the anode than on the cathode, current will flow through the diode.

Non Ideal Diode

Of course, we don’t live in an ideal world and in real life what we deal with are non-ideal diodes. In a non-ideal diode, there must exist a certain voltage across the diode (from anode->cathode) before current is allowed to flow. In most situations, this voltage can be assumed to be around .7V. At the same time, this voltage is dropped across the diode, meaning if you had 12V on the anode side of the diode and some other circuitry on the cathode side, that other circuitry would see 11.3V, not 12.

There are 3 ranges for non-ideal diodes, forward biased operation, reverse biased operation and breakdown. In the forward biased operation, the voltage is applied to the diode to allow current to flow (positive on the anode compared to cathode). In the reverse biased operation, the voltage is swapped so the positive is applied to the cathode compared to the anode. This can sometimes allow current to flow in the reverse direction (but very little). Finally there is the breakdown region of the diode. In this mode, the voltage is applied backwards (as in reverse biased) but the difference in voltage is so high that the diode itself will break (and often times combust). See the image below for a graphical representation of each region.

Choosing a Diode

When choosing a diode, it is especially important to take a close look at the diode datasheet and make sense of the various values. The most important values are often the max current that the diode can handle, the max reverse voltage the diode can handle (in both the forward and backwards direction), the forward voltage needed to activate the diode, and the current needed to reach that forward voltage.

Max Voltage: This is important because you don’t want to apply a voltage higher than the diode can handle and accidentally blow up your diode. This value can be found under “Absolute maximum ratings” or something similar. In the image below it can be seen the max reverse voltage across this diode is 100V (pretty high). This value can also be called the “breakdown voltage”

Max Current: This value is important because it can also cause your diode to blow up. You must be sure to never let your diode have more than it’s max rated current flow through it at any point. In the image below it can be seen that the maximum current this diode can handle is 2A during a surge (can only handle this current for a very short amount of time) and 300mA continuously.

Forward Voltage: This value will tell you how much voltage the diode will take away from what you are applying. Typically this value is somewhere around .7V, however in this case the image below states the maximum expected voltage drop is 1000mV (1V).

Activation Current: This value is important because in order to actually activate the diode, you will need to have a small amount of current flowing through it. Usually this value is so small that it doesn’t really matter (as almost all applications will have a small amount of current flowing through the diode), but it is still important. For example in the image below it states the test condition for reaching 1V forward drop was 10mA. So you would need 10mA flowing through the diode to expect 1V across the diode.

Basic introductory video on Zener Dioes

Contributors: Jorian Bruslind Nick McComb | www.nickmccomb.net

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