Common Anode vs Common Cathode
Anode and cathode are necessary for electrical set ups where current flow is involved. Electrochemical cells, cathode ray tubes, and X-ray tubes are some examples where we come across anodes and cathodes. When a current is flowing, negatively charged electrons are flowing. In other words, current is carried by moving electrons. When the electrons are flowing to one direction, we say current is flowing to the opposite direction to the electrons. So we talk about positive current. For a device, when we say ‘current-in,’ that means the current is flowing into the system. ‘Current-out’ means current is flowing out of the system. Anode and cathode are defined by this current flow. In some equipments, we cannot surely say one as the anode and the other as the cathode. According to the circumstances, an electrode once functioned as a cathode can change to work as an anode. For example, when a rechargeable battery is charged, the positive terminal is the anode, but when the same battery is discharged, cathode becomes the positive terminal. However, for non-rechargeable batteries and light emitting diodes, the anodes and cathodes are permanent. However, for the study purpose and for our easiness, we can remember anode and cathode in relation to their functions, not the structure.
Anode is the terminal where current flows-in from outside. If we take an electrochemical cell as an example, anode can be remembered as the electrode where anions in the electrolytic solutions are attracted. So from outside circuit, current flows into the anode, which means that the electrons are moving away from the anode. Normally, oxidation reactions are taking place on the anode. So when anions come into the anode in the solution, they undergo oxidation and release electrons. Therefore, there is electron abundance on the anode compared to the cathode. Because of this, electrons flow to cathode from the anode. Since current flow is in the opposite direction of the electron flow, we see it as current flowing into anode.
Common anode is used in the seven-segment displays. This is an electronic display device which shows decimal numerals. They are widely used in digital clocks and meters, etc. In these displays, all the anodes are connected to one point, and it becomes a common anode. Therefore, instead of seven anodes, there is only one common anode. Power supply’s positive end is connected to the anode. However, power will be supplied to all the seven segments.
Cathode is the electrode where the positive current flows out of the system. In an electrochemical cell, inside the solution, the cations are attracted to the cathode. Reduction reaction takes place on the cathode; therefore, there should be electrons. Since current is flowing out of the electrode, electrons are flowing in. As these electrons are used up to the reduction reactions, there will be more electron deficiencies. This allows more electrons to come into the cathode from the anode.
When all the seven cathodes of a 7-segment display are connected together, it becomes common cathode. When using the seven segments, the common cathode must be grounded.
What is the difference between Common Anode and Common Cathode?
• In seven segment displays, when all the anodes are connected to one point, it becomes a common anode. Common cathode means that all the seven cathodes of a 7-segment display are connected together.
• To function, a positive voltage should be supplied to the common anode and the common cathode should be grounded.