Can a FET be backwards?

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.
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Dallas
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Post by Dallas »

Can a FET be backwards?
This build of Tillman's calls for a J201. In the schematic by Norcot, which is linked below, the J201 has it's round part facing left - flat is toward R2 resistor.

After building the board, it would decrease volume instead of increase it. So as I was debugging without the scematic in front of me I tried another J201 [had put it in sockets] and then the original backwards. BOOM - works correctly. I would estimate 3db of gain. So now it sits in there with the rounded part facing the R2 resistor and everything appears to work perfectly.

My J201 reads "J 201A V442AA" on the flat part of the transistor. Could the schematic be wrong? Could me chip work backwards? Should I look for another problem?

https://www.freestompboxes.org/viewtopic.php?f=13&t=4464

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Post by lolbou »

Hi,

Dependin on the manufacturer, pins can be different from one transistor to another... Check yours!
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Dallas
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Post by Dallas »

Wow - googling the above number I get nothing. Have no other way of knowing the manufacturer that I'm aware of. There is a symbol before the "J" - looks like two S's connected or S3 written funky.

Is there a way to check things out with a multimeter?
Sorry for the noob questions... I are one.

Also - thanks for feedback on this build. The encouragement means a lot.

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Post by lolbou »

Like this?

I would go for (with the transistor seen like the one one the pic) :

from left to right -> Drain Source Gate.

To me it's the most common configuration...

Does it meet your observation? Anyway, I would think two identical ones (same batch) are expected to be oriented the same way...

Check this... http://www.effectsconnection.com/oscomm ... 5d63ffd49f
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Dallas
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Post by Dallas »

Thanks so much -
If S is source/signal and D is destination/toward output then yes, that is what I observe. The diagram appear to show the left-right order as Drain, Gate, Source. The data sheet link is dead, but effectsconnection .com is where these were ordered from! [very happy with the order from them too.] So that's good to go.

Much appreciated.

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Post by rocklander »

for more info on pin configuration pinout congig see:
https://www.freestompboxes.org/viewtopic ... fet#p38784
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Dallas
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Post by Dallas »

rocklander wrote:for more info on pin configuration pinout congig see:
https://www.freestompboxes.org/viewtopic ... fet#p38784
:D
The more I study this the more I'm a bit confused. These seem to all say that the configuration of the J201 should be different than it appears on the effectsconnection site and different than it is in this circuit... unless I don't understand what Source Gate and Drain mean.

Diagram, and your statement lolbou seem to indicate the gate is at the end, source in the center. But this schematic shows the input coming into the first pin - end pin. Am I missing something?
That would indicate this is setup Source, Gate, Drain. But that isn't what any of those linked schematics shows except the one at effectsconnection.

So I guess I'm wondering if I can trust the tables in that thread from the end of last year - or is it component by compenent via manufacture. Or perhaps I misunderstand drain and gate and source.
Or better yet, what's the easiest way to test one and see?

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Post by rocklander »

well, whilst the link to the external site shows a much more comprehensive list of different FETs, the pic I posted shows both the pinout relationship to the DSG of a J201, but also the GSD as shown in a schematic:

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Post by JiM »

A field-effect transistor (JFET or MOSFET) can usually be reversed and still work, as the drain and source have the same structure. But they may have different dimensions or composition, and this leads to different performance.
http://en.wikipedia.org/wiki/Field-effe ... istor#Uses

Concerning pinout, there are more often than not build reports telling to reverse some FETs on a given PCB. So i guess the "pinout varies with manufacturer" assumption is valid.
You can check for datasheets there : http://www.datasheetcatalog.com
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