Diodes

Diodes

Diodes are semi conductors, which are designed only allow current flow in one direction only normally. There are many types of diodes. In this topic we will only talk about simple diodes such as rectifier diodes.

They will conduct only in one direction only when the knee voltage is exceeded. Otherwise it won’t really conduct electricity. This knee voltage is normally quite low. This is why when we connect an ohm meter to a diode in its forward bias direction, the reading is over limit. The ohm meter could only output 0.24v, this is below the diode’s knee voltage. This diodes later tested with diodes test mode shows a knee voltage of 0.67. The output voltage of an ohm meter is measure by connecting to a voltmeter.

Anyway, once the knee voltage is reached in the forward bias direction diode will conducts with almost no resistance. Diode will not normally conducts in the reverse biased direction until its breakdown voltage is reach. Then obvious the diode is destroyed. Zener diode is an exception though.

In this lab we first connected a diode of 1.66V knee voltage in series with a 1kohm resistor. The supply voltage is 13.3V. We did the following measurements:

The V_drop of resistor                             13.14V

The V_drop of the diode                         0.66V

Amp flow through the diode             0.01A

The available supplied V                    13.82V

In this cct due diode’s low resistance in its forward bias, only very little V is used over the diode to push current through.

We then replaced the 1kohm resistor with a 10kohm resistor and got the following results:

The V_drop of resistor                             13.3V

The V_drop of the diode                         0.50V

Amp flow through the diode             0.001A

The voltage drop across the diode dropped below its knee voltage there is almost no current is flowing anymore. Perhaps this was an good example because 10kohm is already too high resistance for this cct.

Finally we replaced this diode with a LED and replaced the 10kohm resistor with a 1kohm resistor. The following is our test result:

The V_drop of resistor                             11.6V

The V_drop of the LED                                      2.1V

Amp flow through the diode             0.01A

The available supplied V                    13.82V

Compare with the very first cct. The LED required more Vdrop this is because it has a much high knee voltage before it will conduct in the forward b