Diodes allow electricity or current to flow in one direction, and stop current flow in the other direction. The arrow points in the direction of flow for conventional flow of electricity, from positive to negative.
When a diode is connected to a voltage source so that current flows, it is called "Forward Bias". In this direction, theelectrons and "holes" are pushed into the middle boundary layer so the electrons can flow through the "holes".
Although we often say electricity flows from positive to negative in conventional theory, it is also true that the electrons actually move from negative to positive. This is called electron flow theory. While the positive charge of the missing electrons, or "holes" are pushed from positive to negative, the electrons ars pushed from negative to positive and can conduct through the "holes" that have a missing electron, and so attract an electron.
This enables electricity to flow through a diode when it is forward biased.
When a diode is connected to a voltage source in the other direction current does not flow. This is called "Reverse Bias".
Once current starts to flow through a diode, there is not much resistance to current flow. There is some voltage drop caused by the electromagnetic push required to get the electrons moving. And there is heat created by the small resistance to current flow. Diodes used in a circuit will usually have a resistor in series to limit current flow.
Measuring Diode Resistance with Ohm (Ω)
To check a diode, some will try to use an ohmmeter to check its resistance. A diode should flow in one direction, and not in the other direction. So maybe you can see this with an ohmmeter? The problem is most digital ohmmeter do not put out enough voltage to reliably get through the boundary layer. In other words, they don't put out enough voltage to "turn on" the diode. Older analogue ohm meters often will put out enough voltage in the ohm tset position to be able to "turn on" the diodes.
With a proper analogue meter in ohm position, the resistance will be low when the meter is flowing in forward bias direction, and the resistance will be very high or infinite in reverse bias direction.
Measuring Diodes with "Diode Test"
The most reliable way to check a diode with a digital meter is with the "diode test" position. In this position a higher voltage is used and the meter shows the voltage drop required to push through the boundary layer.
In the forward bias direction, the meter will read the voltage required to push through the diode and "turn it on". In the reverse bias direction, the meter will read the maximum voltage it can put out since there is no flow. If the diode is broken, creating an open circuit, the voltage rises to the maximum output of the meter in the diode test position.
1.Identify the direction of flow through the diode you are given by drawing a simple picture and the symbol for a diode. Identify the ends as anode and cathode on both the picture and the symbol. Also show the direction of current flow.
2. Measure the Resistance ( Ω) of the diode in both directions using the 2KΩ position on the meter:
Anode to Cathode: Infinity
Cathode to Anode: Infinity
2.1 Check the voltage supplied at the meter probes in the ohms position? With another meter set om DC volts while the meter is set on 2K ohms position.
Record here: 0.25V
2.2 Is this enough voltage to theoretically push through the bindery layer of the diode and get an accurate reading? NO
2.3 Did the ohm measurement work? Describe how effective this was in testing the diode in both direction:
No, the ohm measurement didn't work, this was not enough supplied voltage to theoretically push through the bindery layer of the diode and get an accurate reading on the meter.
3.0 Use Diode Test position to measure the diode in both directions:
Anode to Cathode: 0.642V
Cathode to Anode: Infinity
3.1 Explain what the Diode Test position readings mean when you test the diode in both directions and describe whether the diode was good or bad:
When I did the Diode Test position, Anode to Cathode, the reading on meter was 0.642V and the reading on the meter of Cathode to Anode was infinity, that's means this diode was good.
4.0 Build a circuit with a diode and resistor. Build the circuit below with a 1KΩ (1000) resistor and use a 12 volt supply, unless told otherwise.
4.1 Measure the voltage drop across the resistor (R) 11.27Vd
4.2 Measure the voltage drop across the Diode (D) 0.69Vd
4.3 Measure amp flow through the Diode 12mA
4.4 Measure the available voltage at Supply (Vs) 11.96V
4.5 Add the voltage drop across R and D. VDr+VDd= 11.96V
5.0 Apply the Rules of Electricity to these readings above and describe how these readings demonstrate the rules of electricity in action:
When I tested the voltage drop across the resistor it was 11.27Vd and the voltage drop across the Diode was 0.69Vd, the available voltage at supply was 11.96V, the total voltage drop across R and D was same as the available voltage. Which means the resistor and the diode share the supply voltage and when the voltage drop value more 0.6V, the diode got some current flow.
6.0 Change the resistance by replacing the resistor with a higher value resistor to the circuit.
6.1 What size resistor did you put in? 270KΩ
6.2 Measure the voltage drop across R. 11.50Vd
6.3 Measure the voltage drop across D. 0.44Vd
6.4 Measure amp flow through D. 0.04mA
6.5 Describr how this change of resistance lead to changes in your volts and amp readings. Discuss how this demonstrates how the rules of electricity work.
When I used 270KΩ's resistor the voltage drop across R was from 11.27Vd to 11.50Vd and the voltage drop across D was from 0.69Vd to 0.44Vd and the amp flow through D was 0, which means when I used higher value resistor, the current flow through diode also getting smaller.
7.0 Test an LED (light Emitting Diode) with a meter in the Diode Test position to measure the LED in both direction:
Anode to Cathode: 1.674V
Cathode to Anode: infinity
7.1 Compare the voltage drop of a normal diode and an LED. What does this tell you?
The LED got higher voltage drop than a normal diode, that's means LED require higher resistance than normal diode.
7.2 Build a circuit with an LED in the diode position, as shown below. Use a 1KΩ resistor and a 12 volt supply, unless told otherwise.
7.2 Measure the voltage drop across D. Record here: 2.03Vd
7.3 Measure amp flow through LED. Record here: 12.0mA
7.4 Measure the available voltage at Vs. Record here: 11.95V
7.5 Add the voltage drop R+D. Vr+Vd= 1.95V
8.0 Apply the Rules of Electricity to these readings and compare how these readings are different than the readings for the diode above. In other words, how does the difference between a diode and an LED result in different readings for each part?
When I builded a circuit with an LED the voltage drop across R was lower than diode above, the voltage drop across D was higher than diode above. The amp flow through LED, available voltage and total voltage drop R+D were same as diode above, that's means LED require higher resistance than diode and the voltage also require higher than normal diode.
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