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Old 05-16-2013, 01:11 AM   #1
pk7tanner
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Default Questions about FETs

If I replace a bad one with one from another board that has the same ratings as the bad one is supposed to have but I the amperage wasn't high enough, will I damage more than the fet I just replaced?
The stated ratings (in the LG training manual) for the fets are almost all a little higher (about a tenth of a volt) than mine checked out at. Why is this and and does it mean I need to replace them
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Old 05-16-2013, 02:09 AM   #2
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Default Re: Questions about FETs

I think if what your saying is when you following the tests in cct in the training manual and they don't match exactly but are close then they're probably ok and the results are indicative. Probably depends on how accurate your meter is too.

Maybe list the results in question and hopefully somne can make sense of it, but generally if a FET has failed I usually find they fail shorted.

Not quite following this
"one from another board that has the same ratings as the bad one is supposed to have but I the amperage wasn't high enough"

Are we talking replacing the transistor like for like i.e. the same part number?
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Old 05-16-2013, 02:49 AM   #3
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Default Re: Questions about FETs

Sorry I am a crap typer and have mild dyslexia. What I am trying to ask is I swapped out a shorted fet with one from another board. It matched what was listed in schem, but is a different brand/type (it came off of a Visio psu) I'm just hoping it wont blow anything, but at this point I'm willing to risk it. I'll let you know how it goes.
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Old 05-16-2013, 03:00 AM   #4
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Default Re: Questions about FETs

If it's the same board but they've used a different part, maybe? Why don't you list the part numbers and we may find some datasheets to verify the specs??

I know with some Panasonic boards they use the same PCB in various models but the parts are not always the same spec in the same locations.

I hate typing too, single finger touch for me. easy to interpret wording a number of ways
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Old 05-16-2013, 07:18 PM   #5
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Default Re: Questions about FETs

That attitude is what gets me in trouble sometimes, but luckily nothing happened. It was a transistor off of a Vizio power supply and I put it on the ysus of the LG I've been trying to get working.
Here is one with numbers: The chip/fet is 30f124 and I'm get drastically different readings. I know is 1 of the five is bad for sure because I get OL in the spot I get a reading on the other 4. But one pair gives me a reading almost double what the other do. And the two giving me the high reading don't don't have a pin that touches ground

Last edited by pk7tanner; 05-16-2013 at 07:41 PM..
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Old 05-18-2013, 07:44 PM   #6
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Default Re: Questions about FETs

On a ysus board out of a 50pq30 the fet's at the same spot on the same board (the boards are interchangeable) are 45f122. Does that mean that the 30f124 on my board should have the same output as what's listed for the 45f122?
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Old 05-18-2013, 08:02 PM   #7
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Default Re: Questions about FETs

Voltage they're close but current rating no.

maybe post some clears shots so we can see what exactly we're talking about. 2 pin sounds like a diode unless it's a surface mounted FET?
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File Type: jpg Tohiba IGBT mark.jpg (33.3 KB, 57 views)
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File Type: pdf GT30F125.pdf (919.0 KB, 68 views)

Last edited by tw2005; 05-18-2013 at 08:05 PM..
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Old 05-18-2013, 09:57 PM   #8
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Default Re: Questions about FETs

Quote:
Originally Posted by tw2005 View Post
Voltage they're close but current rating no.

maybe post some clears shots so we can see what exactly we're talking about. 2 pin sounds like a diode unless it's a surface mounted FET?
Man you guys are awesome! Now if only I can figure out what that all says. Here's the pics. It's in a difficult spot to take a picture and that's why I hadn't included one before
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File Type: jpg IMG_0325.jpg (457.9 KB, 50 views)
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Old 05-19-2013, 01:49 AM   #9
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Default Re: Questions about FETs

Looks all original, Ok so I understand a little clearer. The PQ30 has the 45Amp and your original has the 30amp?

Same voltage, maybe a production upgrade to improve reliabilty. I don't see any 2 pin but 3 pin RF2001T3D which is a double diode. RF2001 means 20amp the suffix is important too 3 means 300V, D means double diode.

So T4S means single 400V,T4D 400v double etc, T2 200v etc
Attached Files
File Type: pdf rf2001t3d.pdf (201.4 KB, 32 views)
File Type: pdf ROHM fast recovery diodes 2009.pdf (661.9 KB, 23 views)
File Type: pdf rohm 2012.pdf (2.62 MB, 21 views)

Last edited by tw2005; 05-19-2013 at 01:54 AM..
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Old 05-19-2013, 12:30 PM   #10
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Default Re: Questions about FETs

Here's what I believe to be the same PDF, but in English
http://www.semicon.toshiba.co.jp/dow...10_catalog.pdf
But I'm having problems deciphering what the correct outputs should be. Do they increase when they have a common contact point?
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Old 05-19-2013, 03:20 PM   #11
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Default Re: Questions about FETs

I don't understand, usually they have them working in parallel, if there's nothing shorted I'd move on or remove from the board and test them individually.
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Old 05-19-2013, 04:44 PM   #12
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Default Re: Questions about FETs

OK I didn't know if it mattered that on one pair I get a reading of .043 and at the same contact points on the 3 I have pictured I get a reading of .896. They're all 30f124 fet's but the two with the low readings are the one's I'm most uncertain about because of http://www.badcaps.net/forum/showthr...ight=lg+plasma
And then confusing me even more is that I have the same fet's on a zsus that read .434.
So as long as they don't read OL, I'm fine?
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Old 05-19-2013, 08:45 PM   #13
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Default Re: Questions about FETs

Quote:
Originally Posted by pk7tanner View Post
OK I didn't know if it mattered that on one pair I get a reading of .043 and at the same contact points on the 3 I have pictured I get a reading of .896. They're all 30f124 fet's but the two with the low readings are the one's I'm most uncertain about because of http://www.badcaps.net/forum/showthr...ight=lg+plasma
And then confusing me even more is that I have the same fet's on a zsus that read .434.
So as long as they don't read OL, I'm fine?
OL= open circuit, 0.043 looks like a short
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Old 05-20-2013, 06:56 PM   #14
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Default Re: Questions about FETs

I'm having a heck of a time finding a place that will sell these fet's inn\ a small quantity. Any suggestions?
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Old 05-21-2013, 01:32 AM   #15
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Default Re: Questions about FETs

Ebay, UTSOURCE although anything from china takes weeks to arrive. Always a chance of fakes especially the 2 pin 30F124 items i saw ?

FGPF4633 I was looking at this for 30F125 sub

http://au.mouser.com/ProductDetail/F...L4w9lb0h0yc%3d

http://www.digikey.com.au/product-de...3TU-ND/2269819

I purchased a TNPA5082 SS board from a panasonic purely for the components, $11 + postage on ebay and yielded 4 x 30F125, 2x 45F128, 2 x RF2001T3D .
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Last edited by tw2005; 05-21-2013 at 01:33 AM..
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Old 05-21-2013, 07:52 PM   #16
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Default Re: Questions about FETs

If I'm reading the "part numbering scheme" section correctly, I should be able to sub the 45f122, 45f127, 45f128 or 30f125 for the 30f124, correct?
Is it that these were weak or why does it seem that out of three boards (2 zsus & 1 ysus) it appear that 2 of 10+/- are ok?
How did you know that the board you were getting already had those and I thought they were only supposed to cost <$1.25? I should've specified this point in my previous post. I found them on ebay and in China. They seemed overly expensive on ebay, and the places I found in China, I was skeptical of. I guess that the conundrum
Mine are all 3 pin

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Old 05-22-2013, 01:41 AM   #17
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Default Re: Questions about FETs

They should be 3 pin, that's my point. I own a panasonic which has those boards and I've spent a lot of time trying to rebuild the SC boards with some success.

The only thing I'm not 100% on is the how important Vce sat is? Otherwise as far as ratings goes they'd all do.

IGBTs can be pricey, but that Fairchild is not a bad price and should work.

Last edited by tw2005; 05-22-2013 at 01:42 AM..
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Old 05-22-2013, 02:08 PM   #18
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Default Re: Questions about FETs

Yeah, I thought I was slick and tried to get fairchild to send me some free samples, but I didn't have verifiable business email and so no luck. Their stuff is hard to figure out. Is there a way to cross reference?
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Old 05-23-2013, 01:30 AM   #19
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Default Re: Questions about FETs

I check voltage, current, peak pulsed current rating , case construction, pinout configuration and Vce sat which I'm thinking you would want equal to or less than what was in there but I can't quote from experience.

If it was me chasing those specs and I could not get exact replacement I'd give those fairchild ones a go. Does not mean I'm right but you should be ok.

Hopefully some other opinions may come along.
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Old 06-01-2013, 11:00 AM   #20
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Default Re: Questions about FETs

Well we had company stay over and I had been using the living room as my work space. That meant everything has been put up, but now that they've left, I'm free to resume my crash course in electronics.
Your previous two posts used acronyms and other terms I'm unfamiliar with, and therefor, don't know how one would test them.
  • SC Board
  • Vce sat
  • peak pulsed current rating
  • case construction (it varies?)

I'm also still confused about the reason why all of the N-channel fet's that I have removed, test OL on all the pin combos. But attached to a board, most measure correctly? It'd be one thing if they all came from one tv, but I'm taking parts from two other tvs and from an extra zsus board. Plus in the LG guide it lists the fet's by the num/letters printed on the front, but some also have a subset that I don't understand? For example fet RF2001 has D301-306, D311 & D312. What do the D___ mean?

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