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Checking Quench Diodes by Norman Crane

How to do all those tricky XJ40 repairs
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Checking Quench Diodes by Norman Crane

Postby j_r » Mon Sep 02, 2013 8:18 pm

In an effort to explain how the famous quench diodes can be checked for functionality.

You need a multi-meter to check this and if you haven’t got one then I strongly recommend buying one, they are not expensive and are an invaluable addition to the tool kit.

A diode is a discrete electronic component that lets electrons flow in one direction, but effectively blocks their flow in the other direction. Failures can be either short circuit (allowing electron flow in both directions and open circuit (effectively blocking flow in both directions)

The generic standard diodes as used in numerous circuits on the 40 can be checked as follows.

The first two photos show the testing of a diode (out of circuit) on Ohms range, and the other two photos shows the same test but using the special “diode” position on the multi-meter. Should your diode be short circuited then it will pass current in both directions, conversely should it be open circuited then it will block electron flow in both directions. The diode could be checked in circuit (with no power applied), the short circuit reading will be the same, whereas the open circuit reading made differ from an “out of circuit”

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