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I'm trying to find which diode in a charger is bad. I took the 1N5819 diode out of the electronics board and tested it with my multi-meter set to "diode". What I got was 0000 in both directions. Shouldn't this diode (or any diode for that matter) only give a reading in one direction? Does getting 0000 reading mean that this diode is bad? What should the reading be?

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  • \$\begingroup\$ What does it say in the manual (or on the web) about testing diodes using your meter? \$\endgroup\$
    – Andy aka
    May 20, 2013 at 17:36
  • \$\begingroup\$ 1N5819 diode is unusual in an iron transformer charger. You would usually find them in a switch mode charger. Yes? If they are short circuit in a SMPS charger it is not uncommon for there to also be other problems. \$\endgroup\$
    – Russell McMahon
    May 20, 2013 at 17:53
  • \$\begingroup\$ What does the multimeter show in diode reading mode, when the probes are not connected to anything, i.e. just left open? If that's a 0000 (as it is with almost all my multimeters), then 0000 is an infinite resistance, not a short. On the other hand, one of my meters shows a "*" for an open circuit, and a "0" for a short, in diode testing mode. \$\endgroup\$ May 20, 2013 at 18:15
  • \$\begingroup\$ Thanks for all the info. My Multi meter is a Greenlee DM-810A so it will read both diode and ohms. And yes I have checked others and they do show OL in one direction and a value in the other. Also, when I put the meter into the Ohms setting this diode shows 000.8 either way. Sounds like a dead diode. \$\endgroup\$
    – user24138
    May 20, 2013 at 18:25
  • \$\begingroup\$ If this is a satisfactory resolution to your own question, please feel free to accept it. A suggestion: Links to your multimeter, for instance, would also enable others to look at the specifications and either concur, or point out any fallacy in your conclusions. Similarly a datasheet link to the diode mentioned in the question, would help others know it is a 1 Amp Schottky, thus saving some speculation. \$\endgroup\$ May 20, 2013 at 18:31

4 Answers 4

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Usually, the multimeter will give you a reading in only one direction: the forward biased direction. Some expensive meters can do reverse biased voltages, i.e. zener diodes, but most multimeters will only do one direction. So, you'd have to connect the positive lead to the anode and the negative lead to the cathode. Then the meter will give you the forward voltage drop of the diode.

If your meter is giving you '0000' both directions, it could mean one of two things: -- The diode is definitely busted. -- The meter is not functioning correctly.

Is it possible for you to test the diode function on your multimeter using a known, good diode? That way you can verify at least the meter is working as expected. You don't have to unsolder or take the diode out of the good test board to do this.

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We can't know what your multimeter does when set to "diode". Unless you explain exactly what the meter does on that mode, there is no way to know what 0000 means or what the reading should be for each direction.

However, you should certainly get some assymetry when measuring a diode. On the Ohms scale, the resistance reading should be significantly higher in one direction than the other. The reading itself won't be meaningful since the diode in effect adds a offset voltage, but you should certainly get different readings depending on orientation.

It sounds like maybe your diode is shorted.

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What is show depends on your meter and your diode. For a silicon diode like 1N.... I would expect the meter to read about 0.300V (for Schottky) to 0.700V for a regular type in forward biased direction and OL (overload, infinity) in the reverse biased direction.

Sounds like the diode is bad, you should check the other ones too.

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As noted - diode test ranges usually inject a small current.
Readings are usually mV drop but no guarantee.

1N5819 is 1A Schottky.
should read around 300 mV forwards and open circuit in revere direction.

If you have another diode see if it shows a differential reading when swapped.

If meter has a low Ohm range (200 Ohm max often) the you can usually see diode forward and back direction difference with that. Also try next 1 or 2 Ohm ranges up as well.

Most modern multimeters use 9V battery.
A few use 2 x AA batteries.
The latter may be less able to give good readings using low Ohms range.

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