Schottky Rectifier (3 pin device with center pin being common cathod) shows resistance value between pin 1 and 3

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Joined Apr 7, 2013

Long time reader, First time post.

My desktop power supply stopped working. On diagnosis, it seems that the power monitor chip is not generating Power Good Signal as expected, though all the inputs required to generate it are good.

I started testing the power supply with dummy load and suddenly the +5 and +3.3 V leds on dummy load started blinking. Since there was no other semiconductor between the +5 and +3.3 V outputs from their respective Schottky rectifiers, the usual suspects in this case were the rectifiers themselves.

+12 V supply leds were rock steady. (This uses S30D150C Schottky -2 rectifiers in parallel).

I also have one of those Chinese - semiconductor pin configurations finding device with me that also provides values such as forward drop, hfe ( in case of transistors) etc. When I connected the rectifier to this device, it correctly identified it as dual diode with common cathode (pin 2) and respective anodes to Pin 1 and 3).

What has foxed me is the device also showed a resistance value of about 25K between pin 1 and 3. This is for 3.3 V rectifiers (SBL3040PT).

Similarly, the rectifier (SBL6040PT) for 5 V line shows a resistance value of about 35K between pin 1 and 3.

So, thinking that these rectifiers were kaput, I ordered replacements.

To add to the insult, the new rectifiers also show a resistance value between pin 1 and 3, albeit a bit higher one, i.e. around 55 to 60K for each new rectifier block.

To the defense of the semiconductor testing device, that type of display did help me to find a faulty optocoupler because it showed the device as plain transistor. For new optocoupler, it always displayed error message "Device Faulty?".

So, I am looking for some help on understanding why these blocks are showing resistance between pin 1 and 3. I have gone through their datasheets and they do not mention about any parasitic/ghost resistance etc. Hence, I am not sure whether this is parasitic/ghost resistance.

Will certainly appreciate any help in understanding this issue since both (old and new) rectifiers are showing similar resistance between pins 1 and 3.
Have confirmed that the 12 volt rectifier (S30D150C) does not show such parasitic/Ghost resistance value between pin 1 and 3 on the same semiconductor testing device.

Warm Regards