There are two types of electronic or electrical devices: those that supply power and those that use power. On a device that supplies power, the cathode is the positive terminal, and the anode is the negative terminal. For devices that use power, the cathode is the negative terminal, and the anode is the positive terminal. A positive cathode current is a current that flows from the cathode, and a negative cathode current flows into a cathode.
Electrons have a negative charge and are attracted to positive charges. It is the flow of electrons through a conductive material that makes up electrical current, and electrons always flow from negative to positive. Therefore, electrons tend to flow out of a cathode, which is a negative terminal, and into an anode, which is a positive terminal. This makes understanding the cathode current of some devices such as diodes and batteries somewhat complicated.
Batteries are labeled with positive and negative terminals, often using a plus sign (+) or a minus sign (-). These labels can be misleading. The current flow from a battery is a positive current — meaning a forward-flowing current — that leaves the battery’s positive terminal, flows through the circuit and back to the battery’s negative terminal. The battery’s positive terminal actually has a negative charge and is the cathode of the device. The cathode current flows from the battery’s positive terminal.
Electronic devices called diodes have polarized terminals. The cathode of a diode is made up of a negative material, which means it has more electrons than protons. It resists the flow of electrons through it because the material already has a surplus of electrons. The anode is the opposite and is lacking electrons, which makes it easy for electrons to flow into it. As the number of electrons in the anode’s positive material reaches a certain level, they overcome the resistance of the negative material, and the forward or positive cathode current flows out of the diode’s cathode terminal.
A diode will begin to conduct in the reverse direction if the voltage becomes high enough. On a standard diode, this reverse current quickly causes permanent damage to the diode. Special-purpose diodes such as Zener diodes and tunnel diodes are designed to conduct after the reverse voltage reaches a specific threshold. The reverse or negative cathode current flows into the diode through the cathode and out of the anode.
Vacuum tubes are somewhat different because of how they operate. Electrons flow into the device’s cathode and gather on the electrode within the tube’s vacuum. As the negative charge on the electrode grows, the electrons leave the electrode and flow to the more positively charged anode. This causes a positive current flow from the anode terminal of the tube. In this case, the cathode current is a negative current and flows into the device, rather than out of it.