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stefuel
Joined: 07 Mar 2006
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Location: Green Harbor MA USA
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| Posted: Thu Dec 25, 2008 10:27 pm Post subject: Tech question, Mosfet |
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Can a mosfet be tested in circuit or does it need to be removed first? Surface mount mosfets ain't so easy to pluck off without damaging them. I'm trying to locate a dead short in a LCD monitor/touch screen back light power supply.
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r.bauer
Joined: 08 Apr 2006
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Location: The Netherlands
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| Posted: Thu Dec 25, 2008 10:39 pm Post subject: |
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Sometimes they can be tested in circuit, especially when they are completely shortcircuit.
But you have to be carefull, sometimes when using your multimeter you can 'charge' the Gate enough so that it will simply conduct between the Source and the Drain, and the multimeter shows shows a nice short-circuit!
You first have to remove the charge from the gate and measure between the Source and Drain again.
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stefuel
Joined: 07 Mar 2006
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Location: Green Harbor MA USA
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| Posted: Thu Dec 25, 2008 11:06 pm Post subject: |
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OK, I want to make sure I'm doing this correctly Note: I'm not sure my DMM's diode test puts out the required 3-4 volt required to charge the gate. If not and I discharge the gate by touching it along with the source and drain with my finger, I should read open circuit between the source and drain, correct? With the gate charged, source to drain should be closed.
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Tom.W
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stefuel
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| Posted: Thu Dec 25, 2008 11:50 pm Post subject: |
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Thanks Tom, that is the one I'm using.
I see no difference between the charged state and discharged state. Source to drain I read 3.76K
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Nashou66
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Heywood Jablome
Joined: 12 Mar 2006
Posts: 1548
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| Posted: Fri Dec 26, 2008 12:27 am Post subject: Re: Tech question, Mosfet |
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| stefuel wrote: | | Can a mosfet be tested in circuit or does it need to be removed first? Surface mount mosfets ain't so easy to pluck off without damaging them. I'm trying to locate a dead short in a LCD monitor/touch screen back light power supply. |
Finally trying to fix my touchscreen that you blew up??? 
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-- Baron Alexander von Humboldt: 1769-1859
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stefuel
Joined: 07 Mar 2006
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Location: Green Harbor MA USA
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| Posted: Fri Dec 26, 2008 12:33 am Post subject: |
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It's actually a dual output power supply. Power is entered in the middle, goes through a 2 amp smt fuse (which is blown) then a capacitor and splits from there for the upper and lower back light power supplies. With the back lights unplugged, I still show a short within the supply. The LCD panel is producing a image as I can see it with a flashlight.
I have e-mailed the manufacturer for a replacement supply but thought I might kill some brain cells and find the short my self 
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Heywood Jablome
Joined: 12 Mar 2006
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| Posted: Fri Dec 26, 2008 12:35 am Post subject: |
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Any markings on the power supply board? I may be able to google up the spare...
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"Those countries which lag behind in industry, in the application of mechanics and technical chemistry, in the careful selection and utilization of natural products, where the respect for such activities does not permeate all classes of society, will unfailingly decline in prosperity. They will sink faster when neighbor states, with an energetic exchange between science and industry, go forward with renewed vitality."
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stefuel
Joined: 07 Mar 2006
Posts: 3353
Location: Green Harbor MA USA
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| Posted: Fri Dec 26, 2008 12:47 am Post subject: Re: Tech question, Mosfet |
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| Heywood Jablome wrote: | | stefuel wrote: | | Can a mosfet be tested in circuit or does it need to be removed first? Surface mount mosfets ain't so easy to pluck off without damaging them. I'm trying to locate a dead short in a LCD monitor/touch screen back light power supply. |
Finally trying to fix my touchscreen that you blew up??? |
I knew eventually I'd be asked something like that 
If I can get it going, it would solve a lot of yout local monitor issues and give you a touch panel in the equipment closet for your Crestron work. It's only a 1024X768 panel but you could scale up 480I from your sources for the local stuff.
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macgyver655
Joined: 22 Aug 2007
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| Posted: Fri Dec 26, 2008 1:01 am Post subject: |
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On SMD's you can sometimes just unsolder 1 leg and lift it slightly to test the component.
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stefuel
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| Posted: Fri Dec 26, 2008 1:03 am Post subject: |
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On the back there are two white stickers E153405 and l1502 lL 9912001906A
And on the front a board marking IL SSAN. ISI1502 REV.A
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stefuel
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| Posted: Fri Dec 26, 2008 1:07 am Post subject: |
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| macgyver655 wrote: | | On SMD's you can sometimes just unsolder 1 leg and lift it slightly to test the component. |
Would it be a safe assumption that the one leg would be the gate? That way something could not back feed and charge the gate closing the connection between source and drain?
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macgyver655
Joined: 22 Aug 2007
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| Posted: Fri Dec 26, 2008 1:36 am Post subject: |
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| stefuel wrote: | | macgyver655 wrote: | | On SMD's you can sometimes just unsolder 1 leg and lift it slightly to test the component. |
Would it be a safe assumption that the one leg would be the gate? That way something could not back feed and charge the gate closing the connection between source and drain? |
Actually the correct way to test a Mosfet is to charge and unchagre the gate to make sure the device is working properly. I have found many a Mosfet's that were not shorted or open but the gate was not working properly.
Just for some FYI, some of these were on Ampro horz outputs........... 
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Heywood Jablome
Joined: 12 Mar 2006
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| Posted: Fri Dec 26, 2008 2:07 am Post subject: |
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Finding a lot of nothing so far... Can you post the manufacturers name and model number?
-edit: that was make, model of the finished LCD: most websites search for parts that way.
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"Those countries which lag behind in industry, in the application of mechanics and technical chemistry, in the careful selection and utilization of natural products, where the respect for such activities does not permeate all classes of society, will unfailingly decline in prosperity. They will sink faster when neighbor states, with an energetic exchange between science and industry, go forward with renewed vitality."
-- Baron Alexander von Humboldt: 1769-1859
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Curt Palme
CRT Tech
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| Posted: Fri Dec 26, 2008 4:56 am Post subject: |
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You should also be able to short the gate to the drain and measure continuity between the source and the drain, If you get a dead short across it, then it's shorted.
Mind you, if a bypass capacitor on the power supply rail has shorted out, it will seem like almost everything on that line is shorted. I really doubt that surface mount MOSFETs are the cause of your dead short. Check bypass caps, I've had those short out on boards, and when they short, they really do get very close to 0 ohms. Usualy a bitch to find. Start with the large semiconductors and work down to the smaller ones is my rule of thumb.
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stefuel
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| Posted: Fri Dec 26, 2008 8:44 am Post subject: |
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| Heywood Jablome wrote: | Finding a lot of nothing so far... Can you post the manufacturers name and model number?
-edit: that was make, model of the finished LCD: most websites search for parts that way. |
Advan Model #AGM15TK-TE Ser#H99520196 (1011U) P.D. Dec 1999
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Heywood Jablome
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| Posted: Fri Dec 26, 2008 12:54 pm Post subject: |
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These guys have a number of universal inverters for industrial LDs:
http://www.lcdparts.net/UInverter.aspx
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"Those countries which lag behind in industry, in the application of mechanics and technical chemistry, in the careful selection and utilization of natural products, where the respect for such activities does not permeate all classes of society, will unfailingly decline in prosperity. They will sink faster when neighbor states, with an energetic exchange between science and industry, go forward with renewed vitality."
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stefuel
Joined: 07 Mar 2006
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| Posted: Sat Dec 27, 2008 11:25 pm Post subject: |
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| Curt Palme wrote: | You should also be able to short the gate to the drain and measure continuity between the source and the drain, If you get a dead short across it, then it's shorted.
Mind you, if a bypass capacitor on the power supply rail has shorted out, it will seem like almost everything on that line is shorted. I really doubt that surface mount MOSFETs are the cause of your dead short. Check bypass caps, I've had those short out on boards, and when they short, they really do get very close to 0 ohms. Usualy a bitch to find. Start with the large semiconductors and work down to the smaller ones is my rule of thumb. |
I lifted the mofsets off the board to test them out of circuit. They seemed to be doing just what they're supposed to do. Then I put my dmm on the output of the blown 2 amp smt fuse and ground with the mofsets still off the board, I still had a dead short. WTF. Then I remembered what Curt said about bypass caps. I popped the first cap in the chain and short gone. I didn't have the right size cap to replace it with so for $hits and giggles I put in the closest one I had and it worked.
Thanks Curt
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Heywood Jablome
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| Posted: Sun Dec 28, 2008 11:56 am Post subject: |
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Lemme know the size of the cap (and fuse) and I'll raid the office stores.
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-- Baron Alexander von Humboldt: 1769-1859
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