So, what is a diode? A diode is an electrical component that allows current to flow in one direction, but not another. On an electrical diagram, a diode is represented by this symbol:
The physical diode may have a black band on it, this band corresponds with the band on the symbol. One use for a diode is to protect against the power source being hooked up backwards. Let’s look at an example.
In this diagram, the diode is hooked up backwards. This configuration is said to be reverse biased. In this configuration, ‘no’ current will flow across the circuit. In fact, if the current exceeds the diode’s Maximum Repetitive Reverse Voltage (VRRM), the diode will be damaged. This value can be found on the diode’s datasheet. When I say ‘no’ current flows in this configuration, I actually mean a negligable amount of current flows. More on this in a minute.
In this diagram, the diode is hooked up in the correct direction. This configuration is said to be forward biased. In this configuration, current will flow across the circuit. How much current? Well, according to Ohm’s Law:
1.5 Volts / 10000 Ohms = 0.15 Miliamps
But let’s check just to be sure… If we set our multimeter to amps, and we measure, we get… 0.1 Miliamps… …that doesn’t seem right…
Well, the bad news is that it is just one post into this series, and we’ve already come across an exception to Ohm’s Law. To figure out the current across a diode, we need to use the Shockley Ideal Diode Equation. Do yourself a favor: don’t waste your time trying to figure this out. Just use the Voltage/Amperage curve graphs provided on the datasheet for your diode and save yourself a headache.
Therefore, you will see a somewhat level amount of current flowing through your diode, then suddenly it will shoot up. If this happens, it’s time to consider that you need a different diode.