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Save Old Iron

How To - Test a Diode

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Save Old Iron

Please note the position of the "band" on the diode and what color lead (positive / red or negative / black) is connected banded lead of the diode. Diodes are the equivalent of check valves in the world of plumbing or hydraulics. Current should only flow in one direction thru the diode. Current flow should be present when the negative / black lead of the multimeter is connected to the banded lead of the diode and the positive / red lead is connected to the opposite lead. 

 

(click on image to enlarge)

diodetesting_zpsfb82f8c3.gif


The diagram below shows a defective diode that has an internal short circuit in the body of the diode. The diode no longer has a "one way" characteristic and acts as if it were a piece of wire - current will flow in either direction thru the shorted diode so meter lead orientation makes shows no difference. In the world of plumbing or hydraulics, the check valve would be "stuck open".

diodetestingshorted_zpsca06286e.gif


The defective diode below is the equivalent of a burned out fuse. No current can flow thru the diode in either direction. The meter readings show a high resistance to current flow in either lead orientation. The hydraulic equivalent would be a check valve was "stuck closed".

diodetestingopen_zps50f158bd.gif

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Tinhorse

Just think of a diode as a one way check valve for the flow of electrons.

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Thunderlizard

Great post!  and to add...

 

...which is why LED's are polarized (only work when hooked in correct polarity). ( Light Emitting Diodes. :-)  )

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