Thursday, June 28, 2012

Anode vs. Cathode in a High Voltage Diode

SMF6533 High Voltage Diode with Cathode Band
SMF Type High Voltage Diode w/ Cathode Band
VMI’s high voltage diodes almost always include a marking of some type to indicate the cathode end of the device. Sometimes the cathode is indicated by a band, or one or more dots.

Distinguishing between the anode and cathode end is important when the diode is under test or part of a circuit.

Formed Lead High Voltage Diode with Cathode Band
Formed Lead High Voltage Diode w/ Cathode Band

There are two main conventions when dealing with current. They are the engineer’s convention, and the physicist’s convention. Neither is incorrect, it just depends on your definition of ‘current flow’. For the following discussion, we assume the engineering convention - that is, current will flow from anode to cathode or from positive to negative. A diode will block current flow when the cathode becomes more positive than the anode.

High Voltage Glass Body Diode with Cathode Band
Axial-leaded High Voltage Diode w/ Cathode Band
If you want a diode to normally conduct in a positive current, but insert the diode so that the current sees the cathode side first, the device will block current and it will look like you have an open circuit.

Likewise, if you want the diode to normally block, but insert the diode so the anode side sees the current first, the diode will conduct, and you might think you have a short.

 Just a few tips to the wise….contact VMI if you have any questions about high voltage diode anode/cathode conventions.  It can be a little confusing sometimes.    
For a quick reference, feel free to refer to the figure below that ties anode/cathode to blocking and conducting states.

Conducting and Blocking High Voltage Diodes

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