Dell 1800FP no 5v standby.

[Agent24]

Re: Dell 1800FP no 5v standby.

I'm guessing with Vt they mean the threshold voltage: http://en.wikipedia.org/wiki/Threshold_voltage

[triplefour]

Re: Dell 1800FP no 5v standby.

i hope one day i will understand all this math but for right now id appreciate some simple way to test and know if this mosfet (or any mosfet) is good or bad

[Agent24]

Re: Dell 1800FP no 5v standby.

There is no basic test to know definitively if your MOSFET or any part is 100% OK.
You would have to set up a test circuit and run the thing through all its paces up to the max ratings and check that everything stayed in spec in terms of what the datasheet said...

BUT you can get a pretty good idea with a couple of basic tests, eg: https://www.youtube.com/watch?v=gloikp9t2dA - if it passes those tests then it's very likely to be OK.

With your specific one, you could also check that the threshold voltage is in spec in terms of what the datasheet says, too.


However... sometimes a transistor (or any other semiconductor) can become leaky or break down under load, and testing with the multimeter won't tell you if it's OK or not because the multimeter uses low voltage and low current and it may not be enough to stress a faulty part. In that case you need a more advanced test equipment\circuit again...

If you're still paranoid that it's bad, replace it. The time\money spent on equipment and testing beyond the basic multimeter test is not worth it if you can buy new parts cheaply.

[triplefour]

Re: Dell 1800FP no 5v standby.

you make a lot of good points and parts may be cheap but they always tack on a few dollars shipping making it about 5$ a piece which can add up pretty quickly. especially if u replace something and then it just gets fried immediately. i'd really like to hope i could scavenge these pieces from other junk PSU's i have laying around but something tells me certain components are going to be hard to find. with so many mosfets out there what are the odds ill run into the same one?

thanks for the answer though. i guess ill have to be satisfied with just the basic tests for now.

[triplefour]

Re: Dell 1800FP no 5v standby.

what do you think the capacitance value my tester gives means? does a mosfet have capacitance like a capacitor?

[Agent24]

Re: Dell 1800FP no 5v standby.

Yes. MOSFETs have a gate capacitance due to the gate being electrically isolated, thus it forms a small capacitor. However the reading on your tester in nanofarads is far too high to be a gate capacitance AFAIK.

It may be that the tester does not test it the same way that it is measured for the datasheet and is giving a wrong value. I do not know. You should read the manual of your tester and find out what exactly the reading is supposed to be telling you. It may not be gate capacitance at all.

[triplefour]

Re: Dell 1800FP no 5v standby.

hah i didnt get any kind of manual with this tester it was a diy kit i bought from ebay that just had instructions on where to solder all the components.
as far as i can tell though, it tests capacitors reliably. the brand new caps i bought all tested right and with low ESR. caps i pull from other things usually test the right capacitance if they are not blown but the ESR does vary quite a bit. i still am not entirely sure what is considered low ESR and what is considered failing ESR

[triplefour]

Re: Dell 1800FP no 5v standby.

http://www.ebay.com/itm/281262386306...%3AMEBIDX%3AIT

it was only 15 bucks and really simple to put together

i would recommend it to anyone whos starting out, wants to test capacitance and ESR and only has 15 bucks

i would imagine there has to be some more technical information somewhere about how it works and what it exactly measures but i havent found it...

[triplefour]

Re: Dell 1800FP no 5v standby.

hmm.. so according to the youtube video in post #83, if you have a Vt (voltage threshold) on your mosfet that is greater than what your multimeter puts out, then the gate test wont work.
and um...according to my mosfet/esr/etc tester, this mosfet has a Vt of 4.76
something tells me my multimeter doesnt put out 5v when its in diode test mode.
so is this basic test not going to work for me? :/ or can i say...hit it with a 9v battery or something?

[triplefour]

Re: Dell 1800FP no 5v standby.

in the datasheet i cant find Vt or pinch off voltage exactly but there are a couple of things that sound like it

Gate plateau voltage V(plateau) VDD=480V, ID=11A - 5.5 - V
and
Gate threshold voltage VGS(th) ID=500μA, VGS=VDS 2.1 3 3.9

that second one really sounds like it.

but 3.9 at the max voltage is still a bit lower than what was measured at 4.76
and 5.5 on the other hand is a bit higher
so um if its supposed to be 3.9 at the max voltage but my meter is saying it needs 4.76, could that be how it has failed? it now requires more voltage than the circuit is set up to give it, so the gate never opens now?

[triplefour]

Re: Dell 1800FP no 5v standby.

i looked it up and the manual for my multimeter says

Diode Test
Test current of approximately 1mA, open circuit
voltage 2.8V DC typical

so its only got 2.8v and wouldnt be enough to open the gate.
i need to build myself a variable power supply already

[budm]

Re: Dell 1800FP no 5v standby.

One thing is this, if the MOSFET was shorted and caused the damage to the new IC, I would expect the 0.33 Ohms Source resistor to burn open, but it is not. How many of the new IC did you get? Can you test pin 6 and pin 7 of the ICs you have. You did not install the wrong IC, right?

[triplefour]

Re: Dell 1800FP no 5v standby.

no i got the right one, but i only got one... live and learn..shoulda got 3 or something just in case. now ill have to wait again. no way ill find this chip randomly, right?

on pin 6 and 7 i get 1.4 on one of them and 3.1 on the other

they are both ICE2AS01
the second set of numbers does vary a little bit but i think these are the same chip

[Agent24]

Re: Dell 1800FP no 5v standby.

I still am not entirely sure what is considered low ESR and what is considered failing ESR

That depends on the specific capacitor. The capacitor's datasheet will say (for low-ESR types) what the ESR is supposed to be. If when you measure it, it is different, then you know the capacitor is bad.

The (Original Dick Smith) manual for the Bob Parker meter has a chart that gives a general idea of the worst case ESR for different values too.

[triplefour]

Re: Dell 1800FP no 5v standby.

thank you, i have since remembered this and now datasheets are my friend

[triplefour]

Re: Dell 1800FP no 5v standby.

how do i find in the datasheet what the resistances between pins should be?
i looked all over in the datasheet and i cant figure out how you guys know what the resistance between certain pins should be??

[Agent24]

Re: Dell 1800FP no 5v standby.

how do i find in the datasheet what the resistances between pins should be?
i looked all over in the datasheet and i cant figure out how you guys know what the resistance between certain pins should be??

Do you mean the MOSFET?

There is no typical resistance specified except the resistance the channel has when the MOSFET is switched on. This is typically very low, eg: 0.003 Ohms. Usually referred to in the datasheet as Ron or such.

With the MOSFET off, this will be infinite (At least until you reach the body diode reverse breakdown voltage) - with the Drain being positive and the Source negative (For an N-Channel). If you reverse the connection, then you get the forward voltage drop of the body diode.

The Gate is insulated by an oxide layer (This is where the 'MOS' part of MOSFET comes from) so should also show infinite resistance between it and either the Drain or Source.

[triplefour]

Re: Dell 1800FP no 5v standby.

seems like the only way to know for sure is to build a test circuit on a breadboard and put it through its paces... but if it wasnt the mosfet that fried the IC, then what else could have done it?

[budm]

Re: Dell 1800FP no 5v standby.

seems like the only way to know for sure is to build a test circuit on a breadboard and put it through its paces... but if it wasnt the mosfet that fried the IC, then what else could have done it?

One possibility is the IC is installed back ward.

[triplefour]

Re: Dell 1800FP no 5v standby.

im sure i didnt install it backward, it is clearly marked on the board which way its supposed to go