Mospec rectifier info.

Hi, is there any way to test this rectifier with a multimeter if not how can you tell if it's ok?MOSPEC F12C20A Dual Fast Recovery Power Rectifier
Common Anode. 12 Amperes / 50-200 Volts. TO-220AB Case I tried to test it same as regular 3 leg rectifiers and got wierd readings, sort of like a capicator, ( high ohm and bleeding repidly down to no ohm's or 0.5 ohm's. This is one of 3 rectifiers on the secondary side of a ATX switching power supply. Thanks for any info. Jim

Firstly a capacitor will look the opposite to your example. It will start with a low resistance and ramp up as the capacitor charges.

If you have a diode test mode on your multimeter, it should tell you if the diodes are behaving like diodes, however it won't tell you if the diode is damaged.

In all likelihood though, if this failed in a switchmode power supply, it would probably be open circuit. And also smell pretty bad.

If you don't have a diode test mode, connect a 9V battery in series with a 1K resistor and place this across the diodes. Measuring the voltage across the diodes you should see 9V one way, and 0.6 to 0.8 volts the other way.

Hi Steve I think I understand now, as the indicator swings to the right and stops thats where charge starts and when it swings back to the left (or infinity) thats when it's full charged??? But back to the rectifier I don't understand the test you said to do as it has 3 legs, on a regular diode I could do this test with a resistor and the diode hooked in series but on a TO-220AB with 3 legs I'm not understanding.??? Jim WOW WOW thats what it was, a bad mospec F12C20C rectifier. Runs like a clock, now I can junk it!!! Thanks Jim

For a device like this, the first step is to place its part number into google and fins a datasheet. If you're lucky (and quite often you are) you will get one and be able to see where the diodes (in this case) are connected.

Then it's a simple matter to test each one of them.

But it sounds like you've diagnosed the fault.

Back to the capacitor thing. If it is a large value capacitor, your meter will swing very quickly to the right (same as if you shorted the test leads) and then swing back to the left initially quite quickly, then slower and slower... If it is a very small value capacitor it will just show as an open circuit. Intermediate values may give just a small kick of the meter, or swing only part way up the scale before settling to the left. In all cases the actual effect is the same, however it may be far too fast for your meter to respond to.

I got the data sheet for this rectifier but in my limited studies I've not seen anything like this, the other rectifiers had the legs marked ( cathod , anode, cathod, this I know how to test) but this says (cathod, anode, D= double) ?? On the drawing it shows 6 diodes with the common cathode on 1 leg, the common anode on the middle leg (this I understand how to test) but on the 3 leg 1 diode goes one way and the other diode goes the other way and this is what I've never seen and don't know how to test this leg?? I had another mospec Rectifier but it was only 150v, 10a and it tested the same as the regular rectifiers so I put It in and it worked, didn't run it long though. I wouldn't put this supply in anything but am using it to learn on. It's light as a feather and no wonder, no filtering coil, very small transformer and the heat sinks are so small I don't know how they could work and I can see that with my very limited experience. Thanks Jim

I was very confused by your description, until I read the datasheet. That's not 6 diodes drawn in one package, it's 2 diodes each - in each of the 3 versions of the device. You have the first Common Cathod - Suffix "C" version.

In-circuit the outer legs are usually shorted, so the diodes are parallelled.
I never came across an open-circuit diode btw., they always failed short-circuit for me.

Oh that makes perfect sense now. I set and looked at this thing and nothing made sense, why I couldn't see ( leg 1-2-3 ) I don't know. I've only been fooling with electronics for 2-3 months and than only what I can find on google, not like having a person to ask when you don't understand. I sure appreciate all the help from everybody here.This is a very good forum and helps just reading all the other post. Thanks Jim

Hahaha. I've seen one that was 2 leads and a smoking void between them. I suspect that it initially failed to a short circuit, then continued to a more ummm... "catastrophic" failure mode.

I'm actually surprised that these double diodes are used in parallel. Doesn't Vf fall with temperature? Would that not lead to thermal runaway as one diode takes more and more of the load? I guess having them in the same package (maybe on the same piece of silicon) prevents their temperature from deviating too much.

It did fail with maybe 6 hrs. total computer use, ( was in cp longer but the cp was not used much) so maybe your theory is right ?

It is not overly unusual for electronic components to fail early in their life.

Most failures follow a "bathtub" curve where the failures are typically seen early or late in the life of the component, with very few in the middle.

It may simply be that this component had a defect that was not seen during manufacture, and which exhibited itself as an early failure.

It may also be that the design is flawed, and that this particular power supply is prone to this failure.

The easy way is to replace this component and ignore "design issues" unless it happens again.