Mospec rectifier info.

[jhh]

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

 

[(*steve*)]

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.

 

[jhh]

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

 

[(*steve*)]

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.

 

[jhh]

Re mospec rectifier

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

 

[Resqueline]

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.

 

[jhh]

re mospec

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

 

[(*steve*)]

I never came across an open-circuit diode btw., they always failed short-circuit for me.

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.

 

[jhh]

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 ?

 

[(*steve*)]

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.

 

[jhh]

This setup is in a 550watt ChiefMax and I think it's a design flaw from the information I've seen on it (all bad). That why I'm using it to try to learn how to trouble shoot power supplies. I would never use it again haha

 

[(*steve*)]

heh - all bad huh.

The device is probably either underrated, insufficiently heatsinked, or (as I suggested) suffering from 1 diode taking all the current.

Do you know if it is connected as Resqueline suggests? (with both diodes in parallel)

Does it have a heatsink?

Do you know what the purpose of this device is? (or do you have the circuit diagram)

OK, some googling reveals that they're on the secondary, so probably they are fast rectifiers. It also reveals that you have some insulation melting from a transformer?

Sounds like there's more than one thing going on. It may be that the fault in the rectifier initially caused the transformer to heat, then finally blew the diode.

I'd be looking for faster, higher current rectifiers for a start.

Some possible replacements (check the specs to be sure) are:

BYV32E-200,127
GI2404-E3/45-ND
U30DCT-E3/4W
MBR20H200CT-E3/45
VF30200C-E3/4W

I got these by looking on digikey for TO-220 fast recovery dual diodes with common cathode, 200V PIV or greater, and 10A to 20A per leg.

p.s. Hope your mum gets better soon.

 

[jhh]

Even to me it's the cheapest made thing I've seen and I have about 10 old supplys here. ( I go to the trash recycle place and get old computers for the magnets in the hard drives for my wind generators that's why I have so many.) Some faults I think it has is no pri. filtering coil, tiny transformer, the smallest heat sinks I've ever seen and the soldering looks awful also transistors set sideways and the power Fet's and rectifiers are mounted at an angle on the tiny heat sinks.This thing looks like it was assembled by a kid. It ia dual fast recovery power rectifier and I don't know about parallel. Mom's not to good and thanks for your intrest. Well to bed I go, have to get over to see her tomorrow. Thanks for possible replacement ideas. Thanks Jim

 

[(*steve*)]

You may be right about it being assembled by a child (sad to say).

Those diodes are rated at 6A continuous per leg, so if in parallel, 12A per package. It would be interesting to note the rated current from the 5V rail of that power supply and what (and how many) rectifiers are used. I would not be surprised at all if they are grossly under-rated.

 

[Mitchekj]

I'd hate to see the numbers for efficiency and power factor on that supply. Actually, no, I'd like to see them.

 

[Resqueline]

Hehe, as for my lack of experience with open diodes, I'm not sure if I would have counted any "craterized" parts - as I wouldn't even have used a meter on them.

A shorted diode in the secondary side should activate the short-circuit protection in the primary side, making it go "tick-tick-tick". If there is no protection, well, then...

I'd expect two diodes on the same chip to be well matched to begin with, and to have very good thermal coupling so runaway would not pose a real threat. All PC-PSU's I've seen has this configuration. I've not done any scientific study on this though.

I saw some Asian manufactured AT-PSU 20 years ago that had no particular creepage-distance between primary & secondary btw.. They were interweaved with just 1mm clearance.. Creepy..

 

[(*steve*)]

I'm in the middle of repairing the power supply of an LCD monitor.

I just glanced at the diodes used to rectify the output and one of them is a MBR10100CT, the other is a BBL1045CT.

the MBR10100CT is rated at 70V 10A, the BBL1045CT I can't find anywhere (It's hard to read, I may have it wrong).

This power supply is rated for 12V at 400mA and 5V at 3.5A. It may give you an idea of how generously rated the rectifiers in some power supplies are.

As Resqueline suggests, these diodes are used in parallel.

The circuit diagram I have (which varies in several details from the board I have) notes the rectifiers as FME-210B (12V rail) and FMB26L (5V rail). The former is rated at 10A 60V, the latter at 10A 50V.

Perhaps these devices are cheap, or perhaps they have other characteristics that make them particularly suitable, but they certainly have current and voltage ratings far in excess of the rating of the supply rail.

It would be interesting to look at the ratings of rectifiers in "good" power supplies that are closer in rating to yours.

 

[Resqueline]

steve, the BBL1045CT sounds like it's a 10A (2x5A) 45V Schottky rectifier.

One medical grade PSU rated at 13V & 10.8A have a S25SC6M (25A 60V Schottky) out from the transformer, and a S20LC20U (20A 200V Super Fast) to ground before the coil & the 2 x 3300uF 25V Elna RJH caps. (Forward topology.)

Schottky's have very high reverse bias power dissipations at elevated temperatures & voltages so I guess it's advisable to overrate them sustantially.