Mosfet With Middle Pin Missing? - Page 1
Forum section: electronics / beginners

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 02, 2013, 10:17:22 pm
content: Hi guys!I was scraping two computer PSU's, just ripping all the parts off and I found something interesting.There is a ST P10NK60Z N Channel Mosfet mounted to a heatsink and for some reason the middle pin is cut off?The back of the fet has an insulating pad behind it so they arent using that in place of the pin, what could you use a Mosfet for with a pin missing?Thanks!

author: 8086
author_rank: Super Contributor
timestamp: August 02, 2013, 10:26:33 pm
content: A diode

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 02, 2013, 10:31:53 pm
content: Quote from: 8086 on August 02, 2013, 10:26:33 pmWow really?With what sort of specs?

author: 8086
author_rank: Super Contributor
timestamp: August 02, 2013, 10:36:38 pm
content: Quote from: EpicIntelGamer on August 02, 2013, 10:31:53 pmWell that depends.Usually not very exciting, with a slightly higher forward voltage than a "normal" diode.If you look at a symbol for a MOSFET you will usually find the diode in there.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 02, 2013, 10:40:04 pm
content: Quote from: 8086 on August 02, 2013, 10:36:38 pmOh yeah I guess they do. I don't usually look at that little diagram.What would make them use a mosfet like this instead of a normal diode?

author: senso
author_rank: Frequent Contributor
timestamp: August 02, 2013, 11:16:36 pm
content: Wireless gate control

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 02, 2013, 11:17:19 pm
content: Quote from: senso on August 02, 2013, 11:16:36 pm0_0 lol

author: Andy Watson
author_rank: Super Contributor
timestamp: August 02, 2013, 11:36:21 pm
content: If it was the middle pin it looks like it would have been the drain that was cut off. You wouldn't want to leave the gate floating. Still a bit odd.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 03, 2013, 01:36:43 am
content: Quote from: Andy Watson on August 02, 2013, 11:36:21 pmYeah its the middle pin and I'm sure I didn't break it off while removing it because I looked at the PCB and there is actually no hole for that middle pin.Perhaps they needed a diode but had that mosfet sourced for cheap?

author: Towz
author_rank: Contributor
timestamp: August 03, 2013, 05:49:20 am
content: maybe a picture of the setup would be useful. All i can think of at the moment is some sort of peak/transient suppresion since it's the Drain that is cut off. This mosfet has built in zener protection between the gate and the source.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 03, 2013, 06:33:51 am
content: Quote from: Towz on August 03, 2013, 05:49:20 amI'll grab a picture for you tommorrow, it's pretty late right now.

author: amyk
author_rank: Super Contributor
timestamp: August 03, 2013, 06:57:18 am
content: Quote from: Towz on August 03, 2013, 05:49:20 amIt's two 30V zeners back-to-back, so maybe that's what they were using it for. Might've been cheaper if the same part was already used in the PSU for switching, than to add another part to the BOM.

author: smashedProton
author_rank: Frequent Contributor
timestamp: August 03, 2013, 07:07:55 am
content: Wouldn't it be just a low inpeadance connection? In most packages, the center pin is connected to the case. Maybe the case on this fet isn't connected to the substrate.I used to cut off the middle pins of a n channel and p channel fets and solder them directly together for an active breaking system I put in by buddys airsoft gun.

author: Neilm
author_rank: Super Contributor
timestamp: August 03, 2013, 12:06:26 pm
content: I have seen some packages like this that have two matched diodes in them. The case is common and each leg is one connection to the diode.Neil

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 03, 2013, 10:30:12 pm
content: Quote from: Neilm on August 03, 2013, 12:06:26 pmYeah it has some of those dual doide packages but this is not one of those. This is an N Channel Mosfet (details in the OP) that they cut a leg off of.

author: c4757p
author_rank: Super Contributor
timestamp: August 03, 2013, 10:40:48 pm
content: Dear god.... flip it over and trace out that section of the circuit!

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 03, 2013, 11:06:50 pm
content: Quote from: c4757p on August 03, 2013, 10:40:48 pmWill do, just a second...

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 03, 2013, 11:10:41 pm
content: I just tried but really it goes all over the place.The one side of the mosfet goes over to one side of one of the big filter capacitors on the rectified input.The other breaks off to multiple locations including some little 8pin IC and then it apparently also goes to the same place as the other side of the mosfet.

author: Rufus
author_rank: Super Contributor
timestamp: August 03, 2013, 11:53:40 pm
content: This is silly.If the drain wasn't connected you are left with gate to source capacitance and wimpy ESD protection diodes as the only accessible components neither of which are useful or require mounting on a heat sink.How was it fastened to the heat sink? With a screw to make the drain connection I'll bet and the silicon insulator pad was there just for thermal reasons.

author: Psi
author_rank: Super Contributor
timestamp: August 04, 2013, 12:01:47 am
content: Was it a cheap china powersupply? Might be from a batch of diodes that got wrongly stamped as mosfets.They could be using them up since they're a surplus/cheap stock of diodes.This sort of manufacturing mistake happens more often than you know.Someone at the semiconductor plant sets the machine wrong and before they know it they have 100 reels of diodes marked as mosfets.

author: c4757p
author_rank: Super Contributor
timestamp: August 04, 2013, 12:07:58 am
content: Quote from: EpicIntelGamer on August 03, 2013, 11:10:41 pmAll PCBs do. It's good reverse engineering practice.

author: c4757p
author_rank: Super Contributor
timestamp: August 04, 2013, 12:09:21 am
content: Quote from: Psi on August 04, 2013, 12:01:47 amGood point. Probe them dammit! You've got to figure these things out for yourself!