12V rail is not on ?

[budm]

Re: 12V rail is not on ?

it looks simple and basic but it's not that simple and it's not completely basic...

Well if you know what diode is then it will be simple to see that diode will only conduct in one direction only, and if you understand the current flow in the circuit then you will know what Voltage you will read between the two legs of the diode when the it is not conducting., same way how you find out when you have 10 lamps connected in series, what Voltage will you see on the lamp that has open filament?

BTW, it is also easy enough for to put GOOD diode in your test setup and see what Vdrops you will get in the forward and in reverse bias, it is that simple! Do it and tell us the result.

Right now it is not simple to you because you do not understand the simple current flow in the circuit so you are wondering why the LED lights up when you test those diodes in the power supply.

Post 56:

I test D22,D20,D17,D18 in circuit with 9V and LED,1K ohm....something is not normal...
It's on a both side of test reverse and forward bias...
Are they all bad ?
or to be sure must be off circuit ?
PCB code Y-B200 ATX Ver 2.6...and they are on 12V and 5V rail...

[bianchi77]

Re: 12V rail is not on ?

it looks simple and basic but it's not that simple and it's not completely basic...

[bianchi77]

Re: 12V rail is not on ?

Ok thanks for the time....sorry if I'm wasting your time....I keep learning

[budm]

Re: 12V rail is not on ?

Just think of the diode as an open switch when it connected in reverse bias, the reading will be the same as the power supply source, if the diode is truly not conducting any current.
Diode connected in forward bias mode will conduct and let current flow, diode connected in reverse bias mode will not let the current flow through, it is that simple! so when there is no current flowing through the diode, what do think the Vdrops on that diode will be when it is not conducting?

Just remove the diode complexly and take the reading between the two wires that were connect to the diode to see what Voltage you will get, that is what it should be when the diode is connected back to the two wires in reverse bias.

Vdrops is the Voltage reading between the two legs of the device, it does not matter if it is a resistor or a diode. it is just a Voltage reading between the two legs of the device.
Just do the search of the word 'Voltage drop'.
You should search he web for 'what is diode' and study it.
If you want to know what the leakage current through your diode in the reverse bias test, all you have to do is to measure the Vdrops on that resistor connect in series with the diode then you can calculate the leakage current. You can also look at the spec sheet of the diode to find out what the leakage current spec of that diode is.
I waste enough time with this simple test, sorry. You are skipping the basic that is why you are not understand the test result.

[bianchi77]

Re: 12V rail is not on ?

are you talking about V drops = V reverse ? so the diode has a leak in this case...1V leak...and getting higher if I have higher voltage and amperage from transformer....

[budm]

Re: 12V rail is not on ?

Rforward =825 ohm, Rreverse = 167K
Vforward = 252mV
Vreverse = 4.7V
Diode = SBL2040CT
R = 41.2 ohm
Vbattery = 5.72V

I want to replace this diode for a long time, not arrived until today, ordered since July..

V drops on the diode should be the same as the supply Voltage because the diode will not conducting due to reverse bias.

[bianchi77]

Re: 12V rail is not on ?

What happened if the 5V or 12V diode sunk to the ground ....
I reckon the diode will get over current....how much current will it be in that 5V and 12V loop ?
I gave a surge current for those SBL1040....but I have no idea how much ?

Is it related with Maximum rating in datasheet :
Peak repetitive reverse surge current per diode at tp = 2.0 μs, 1 kHz == IRRM == 1.0 A


It will die instanly if the limit is only 1 A

[bianchi77]

Re: 12V rail is not on ?

Rforward =825 ohm, Rreverse = 167K
Vforward = 252mV
Vreverse = 4.7V
Diode = SBL2040CT
R = 41.2 ohm
Vbattery = 5.72V

I want to replace this diode for a long time, not arrived until today, ordered since July..

[bianchi77]

Re: 12V rail is not on ?

Here's the observation I've done...do you reckon the diode is faulty ?

[bianchi77]

Re: 12V rail is not on ?

I never use ohm or diode test when there is battery...

[bianchi77]

Re: 12V rail is not on ?

So I will use it on voltage mode when I measure it with battery ( power source )...
I keep learning...
cheers

[budm]

Re: 12V rail is not on ?

Measuring with diode test on dmm or voltage test?

Well, lest see now, you are trying to read the Voltage drop across the diode that is hooked up the power source in reverse bias, so what do you think?
I hope you never ever try to measure any resistance or in Diode mode while the power is still applied to the circuit unless you have lots of money to replace damaged meter, you should know by now on how and when to use meter in Ohm mode, Diode mode, Amp mode.
This thread is getting really long and you do not seems to know the basic of the term Voltage drops across the DUT means, it is really basic electronics terminology that you should know by now when some one ask you to check the Voltage drops on the DUT, at the same time you are trying to troubleshoot the complex circuit.
You need to go by some book for electronics for beginner and study them. It is hard to teach some one that does not have basic knowledge yet, and my patience is running low right now from just trying to explain simple diode testing.

[bianchi77]

Re: 12V rail is not on ?

All you have to do is to flip the diode around then you will see that the DCV across the diode will be at 9V.

Measuring with diode test on dmm or voltage test?

[budm]

Re: 12V rail is not on ?

This is the small that I had done...please correct me if I'm wrong...
That's for forward bias....how about reverse bias ? swap the probe minus to plus and plus to minus with DMM in diode test ?

All you have to do is to flip the diode around then you will see that the DCV across the diode will be at 9V.

[bianchi77]

Re: 12V rail is not on ?

if you check with the 9V and 100 Ohms resistor in reverse, does it show 9V across in reverse bias, it should if it is good diode.

How can I do that ? do you have a schematic ? cheers

[bianchi77]

Re: 12V rail is not on ?

This is the small test that I had done...please correct me if I'm wrong...
That's for forward bias....how about reverse bias ? swap the probe minus to plus and plus to minus with DMM in diode test ?

[budm]

Re: 12V rail is not on ?

the resistance for both diodes, forward = 1K ohm and reverse is blinking between 8K and OL....it's not normal...should be OL in contant for reverse...right ?

Sounds like you are not making good contact in reverse, if you do get 8K then it's bad, and if you check with the 9V and 100 Ohms resistor in reverse, does it show 9V across in reverse bias, it should if it is good diode.
At this point why do you not just change the diodes?
BTW, your hands are not making contact with the probes or the diode legs, right?

[bianchi77]

Re: 12V rail is not on ?

the resistance for both diodes, forward = 1K ohm and reverse is blinking between 8K and OL....it's not normal...should be OL in contant for reverse...right ?

[budm]

Re: 12V rail is not on ?

Less than 100 Ohms in BOTH direction, the really bad one will be <1 Ohm in both direction.
Bottom line, it should only show conduction in ONE direction only.
To truely test the diode you need to use variable power supply with current limiter to test the diode break down Voltage since your meter does not put out high enough Voltage that is supply by the circuit to cause the diode to breakdown and conduct when reverse bias is applied to the diode. The test will also show the leakage current.
https://cdn.badcaps-static.com/pdfs/...95e0f81994.pdf

[bianchi77]

Re: 12V rail is not on ?

If the diode is bad then you should be able to use meter in Ohm mode and it should show low resistance in both direction if the diode is shorted out.

"low resistance" == few kilo ohm ? or few hundred ohm ?