Nonlinear Devices - the Diode

The next important device for circuit analysis is the diode. For a bit of semiconductor background, a diode consists of two different semiconductors layers placed together back to back. When voltage is applied one way, it “turns on”, otherwise we can consider it “off”. It's probably the simplest non-linear element to understand, as it just has two “states”.

The diode has two ends, and unlike the other basic two terminal devices (RLC), orientation matters here. If we apply a positive voltage from anode to cathode, we allow current to flow in the direction with the arrow. If we go opposite, from cathode to anode, the big bar blocks up, and no amount of voltage will let current flow. For an ideal diode, we can simply plot the relationship between voltage and current as so:

This relationship is pretty accurate for our real life use. The real relationship is actually a slightly more complex curve (exponential instead of linear), but the only thing that changes between diodes is that turn on voltage. Smaller diodes that don't have a huge power rating will turn on near 0.2V to 0.7V. LEDs (light emitting diodes) turn on from 1.2V to 3.7V for single colors. Anything higher than that is just a serious power diode, which can handle serious amperage without breaking down. The most common use case for these diodes is to force a voltage to only go one way.

Below, we make a simple rectifier using a diode to ensure we only have positive voltage delivered to our load (here a simple resistor).

* Simple Diode Rectifier

Vin n0 0 SIN(0 3 5 0 0)
d1 n0 n1 DIODE1
r1 n1 0 5

.tran 2m 1
.print tran v(n0) v(n1) i(Vin)
.end

The blue line shows the input voltage, just a quick sine wave. The green voltage is the output voltage across the resistor. It only has voltage across it when the diode is on, otherwise, no current flows and thus no resistance. For the diode we used, the drop was about 0.7V to turn it on, so you'll see in the positive section of the graph that there's that constant gap between the resistor curve and input curve. This circuit is called a half-wave rectifier, as it takes in a full wave (positive and negative), and gives you a positive wave out.

The only thing is, we lost half of our power, as during the negative phase, nothing can happen. Later, we'll talk about how to solve this with a full wave rectifier.