Diodes!


Diodes are nonlinear circuit elements.
andreas zlr

$I$ vs $V$


Sparkfun


Diode - The arrow points in the forward direction.

A simple model of diodes:

  • $V_\text{F}\lt V$: Current flows readily with little or no resistance.
  • $-V_\text{BR}\lt V \lt V_\text{F}$: No current flows.
  • $ V < -V_\text{BR}$: Current flows readily with little or no resistance.

Maximum current: 20 mA. (If you know the voltage to be supplied, you can size a resistor to limit the current through the diode.)


Light Emitting Diode (LED)

$V_f$ depends on the color: 1.6 V (Red)..(Orange)..(Yellow)..(Green) 2.5 V (Blue)..("White") 4 V

Operating current ~10 mA.


Zener diode - These are intentionally operated in reverse bias. They use the breakdown voltage, $V_\text{BR}$. A "noisy" voltage source can be clipped to a smooth$V_\text{BR}$ DC voltage level.

The breakdown voltage can be tuned by "doping" (chosen chemical impurities) and maximum current can be several 100 A.

Current rectifying diodes - High current diodes can be put together into a "Graetz bridge" to convert AC to DC

WS2 Diodes

Here is WS2 - Diodes.

On page 2, parts L and M, the breakdown voltages of the zener diodes are not specified. Let's take them to be $V_\text{br}=3.0$ V.

Our assumptions about diodes

  • If the voltage in the circuit across the diode would be more than +0.5 V (in the "forward" direction), then the diode will conduct, and enough current will flow such that the voltage drop across the diode will stay at 0.5 V.
  • If the voltage in the circuit across the diode is less than +0.5 V (including any negative voltage), then no current flows through the diode.

A zener diode:

  • will conduct if a forward voltage of 0.5 V or greater is applied. Just like a regular diode,
  • It also has a reverse "breakdown" voltage. If the breakdown voltage is 3.6 V, then if the voltage in the circuit across such a zener diode is less than -3.6 V in the forward direction (or, if you like, more than 3.6 V in the reverse direction) then the diode will conduct and stabilize at its breakdown voltage.

Hint: It's useful to:

  1. start by pretending that the diode is not conducting. Calculate what the voltage would be on either side of the diode.
  2. (for zener and regular diodes) If the voltage difference exceeds the forward voltage drop of 0.5 V (both zener and regular diodes), then you know it *will* conduct, and you can set the voltage drop across it to 0.5 V

    Otherwise assume it's not conducting.

  3. For the zener diodes, if the voltage difference exceeds the breakdown voltage in the reverse direction, then you you can set the voltage drop across it to its $V_\text{br}$.

    Otherwise assume it's not conducting.