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Old 04-28-2022, 01:45 PM   #1
caprider
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Default FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

Hello,

I am troubleshooting a power supply.

When measuring a FCL30A015 diode it shows 35 Ohm when reverse biased and 0.05 forward voltage.

See attached.

The datasheet does say "Extremely Low Forward Voltage Drop": https://www.alldatasheet.com/datashe...FCL30A015.html

But 0.05V seems to low.

Is it gone?

Should I perform some other test?
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Old 04-28-2022, 02:11 PM   #2
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

looks shorted to me, the resistance (in resistance mode) should be very high when reverse biased, 35Ω is way too low. 0.05V forward drop not normal either, though lower currents like your DMM should show lower voltages, but something lower than ~0.15V are suspect.
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Old 04-28-2022, 02:21 PM   #3
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

The diode is shorted. It's just a Schottky rectifier Vf should be ~0.25V up to 0.42V at 10A.
If you are not sure, push some current through it. Use a car light bulb and 12V power supply and see what it does.

Many times a diode or transistor gets damaged/shorted but of course will not read like a 0 ohm short. They will still read like a bit of a diode junction is there.
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Old 04-28-2022, 03:01 PM   #4
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

I am not so sure any more. I have 4 of these diodes in the PSU and all read about the same.

If I create a circuit with my lab PSU at 32V and a 1k ohm 50W resistor (the smaller resistors that I have are too low power) with the diode in series it only conducts as it should: with minus terminal in the middle. And it does not get reverse biased - as it should.

Maybe I am missing something about these diodes?
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Old 04-28-2022, 07:19 PM   #5
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

it's your meter,
i get this with schottky diodes too - infact i get fluctuating readings on one meter
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Old 04-29-2022, 01:44 AM   #6
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

How can I test the diode then?
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Old 04-29-2022, 07:35 AM   #7
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

Simple just use an incandescent light bulb and try powering from both directions in one direction it should light the light bulb and the other direction it should not
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Old 04-29-2022, 10:58 AM   #8
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

That's a 30A, 15V Schottky. It has a very low forward drop, possibly for use as a high current or'ing rectifier. Its normal characteristics could "fool" a DMM diode checker. I'd suggest doing an in-circuit test using a 1K (or similar) resistor and a 5V or 12V source, in both polarities.

Long, long ago, when the Soviet Union was still in existence, IR introduced a series of low reverse voltage, ultra-low low Vf Schottkys for redundant power supply configurations. More modern server Power supplies use MOSFETs to gate or un-gate outputs to the load (with current-sharing circuits). This gate or un-gate function also permits quickly disconnecting a power supply from the load in the event of a fault.
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Old 04-29-2022, 11:44 AM   #9
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

It's a PSU from a Supermicro server that has 3 hot swap power supplies (n+1). It might be a FSP PSU, IIRC, I'm away now and can't check. So nothing Russian in there.

Server is over 10 years old.
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Old 04-30-2022, 02:57 AM   #10
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

OP I changed my mind because the datasheet specs say these are normally quite leaky diodes.
(Nihon Inter Electronics Corporation) FCL30A015 shows 15V 30A rating with Irm max. 15mA leakage at 15V. Compare with say a MBR3040CT 30V 30A the spec is 0.5mA, some companies say 0.1mA, but those have much higher Vf 0.84V vs 0.42V which means much more heat 2x compared to FCL30A015. It's a tradeoff.

High power Schottky diodes have high leakage currents, but these ones are really high? And they can be made by small, dirty semi companies lol.

So in reverse bias, up to 15mA leakage. They'll act like a good diode but with a low value resistor across them which will not read good with a multimeter which usually use around only 1mA test current in Ohms/Diode-test mode. The reverse leakage is more like a constant current if I remember right, and goes way up with temperature.

I would say you need to test them with higher current, like you did 1k resistor and 15V max.

Last edited by redwire; 04-30-2022 at 03:06 AM..
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Old 04-30-2022, 03:56 PM   #11
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

I did test them with a 1k resistor and 32V from a lab power supply.

According to the PSU they take about 0.03 A when forward biased and 0.0 A when reverse biased.

So according to this test, they are working ok.
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Old 05-01-2022, 06:00 PM   #12
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

Quote:
Originally Posted by caprider View Post
It's a PSU from a Supermicro server that has 3 hot swap power supplies (n+1). It might be a FSP PSU, IIRC, I'm away now and can't check. So nothing Russian in there.

Server is over 10 years old.
Itís more likely to be made by Compuware, Delta, or Samsung.
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Old 05-02-2022, 01:51 PM   #13
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Default Re: FCL30A015 diode reverse biased shows 35 Ohm, 0.05V forward. Is it gone?

So with a 21W load (car light bulb) on 12V the diode only passes the right way. With zero Amps leaked when reverse biased.

I think the multimeter is misleaded because it leaks some current on low Amps when reversed biased.

Also, on 12V with a 1K resistor it actually leaks 0.01 A when reverse biased. It's not zero Amps like I've written before. Above I mistakenly tested another component.

So the FCL30A015 look ok and seem to be used as ORing for hot swap.

This diodes design idea was probably to be highly efficient on high Amps and they disregarded any leakage on low Amps.
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