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I am trying to figure what is with my broken power adapter (230V AC -> 12V + 5V DC at both 2A).

After checking the fuse, now I extracted two heavily cooled diodes, which could be the cause. But I simply can't figure out what diodes that are.

They are standard shaped (black cylinder, one end has the silver mark), 9mm long and 5 mm diameter, the connector wire is 1.1 mm diameter. On it I find two lines:

SR 8D

SB 54.0

I have performed some quick measurements, which are confusing me. I found a 3300 ohm resistor and plugged them at 2.7V. The voltage drop at the diode was 0,16V ( 2.7V if reversed) With another 220 ohm resistor it was similar.

Maybe the resistor values I am using are too high (the voltage is too low) to make useful tests...

How can I find out if they are OK or broken, and what type of diode do I have here?

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The diodes are SB540 schottky diodes.

If you look at the graph, in the link I posted, named: FIG. 3, you can see that the forward voltage of 0.16V is correct for the ammount of current through it: \$I=\frac{2.7}{3300}\approx 1mA\$.

And when you reverse the polarity of the voltage source, all of the voltage (if withing the maximum reverse voltage spec of the diode) should be across a working diode. So 2.7V across the diode when reverse-biased is expected.

You can do another faster/easier test to see if the diode is working. Do a continuity test on it. Most multimeters have this option on them (its a picture of a diode). So switch your multimeter to the continuity test and place the positive probe to the anode of the diode and negative probe to the cathode (side with the silver band) of the diode. If the multimeter shows some small value (some multimeters even beep) like 0.2V or similar and when you switch the probes and multimeter shows OL or value of 1, then the diode is a working one.

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  • \$\begingroup\$ I performed that "continuity test" and my multimeter gave on the one diode 167 (Q: which means 16.7 V?) and on the other 348. Does that mean that the 2nd diode is broken, or is that still OK? \$\endgroup\$ – tverrbjelke Feb 25 '15 at 8:42
  • \$\begingroup\$ @tverrbjelke It shows these values when positive probe is on anode and negative on cathode? If it is, it should probably read 0.something. \$\endgroup\$ – Golaž Feb 25 '15 at 21:23
  • \$\begingroup\$ Multimeter (PeakTech 2010DMM) shows this reading when "place the positive probe to the anode of the diode and negative probe to the cathode (side with the silver band)". Otherwise it shows "OL". I can tomorrow perform some more measurement-tests (plugging some more resistors in). \$\endgroup\$ – tverrbjelke Feb 26 '15 at 18:03
  • \$\begingroup\$ @tverrbjelke The diodes are OK. \$\endgroup\$ – Golaž Feb 26 '15 at 22:53

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