Q: When is a Diode not a Diode?

April 2015

A: When it’s a Transistor or a Zener diode.

What is a Diode?

A diode allows an electric current to pass in one direction while blocking current in the opposite direction. It works using a doped silicon P-N Junction.

PN diode with electrical symbol

If you put two diodes in combination, it can be used to a positive effect; for example, a diode bridge uses an arrangement of four diodes to convert alternating current to direct current.

However, when you put the diode components, two P-N junctions back to back, rather than controlling the direction of current flow, you can get an amplification effect.

In addition, A Zener Diode is a single P-N junction that allows current to flow in the forward direction in the same manner as an ideal diode, but also permits it to flow in the reverse direction when an avalanche breakdown occurs. Note the term Avalanche – a sudden and overwhelming effect.

Data Diodes

How does this relate to Data Diodes? Data Diodes are designed to do the data equivalent of silicon diodes: A data diode allows data to pass in one direction while blocking data flows in the opposite direction.

If you start to put combinations of data diodes together (like a diode bridge), you can get a very positive effect, as Nexor demonstrate in our remote camera control solution.

However, if you put diodes back to back, you may get a different effect to the one you expected.
Beware of diodes that are not really diodes, you might just get a transistor and amplify your data leakage or a Zener diode where you get sudden and overwhelming data loss.

This is sometimes referred to as the Lirpa Loof effect.


Author Bio – Colin Robbins

Colin Robbins is Nexor’s Managing Security Consultant. He is a Fellow of the IISP, and a NCSC certified Security and Information Risk Adviser (Lead CCP) and Security Auditor (Senior CCP). He has specific technical experience in Secure Information Exchange & Identity Systems and is credited as the co-inventor of LDAP. He also has a strong interest in security governance, being a qualified ISO 27001 auditor.


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