Question for Pass a40 Builders

[Eric]

I have finally begun stuffing my a40 boards and have run across a problem. I do not know how to identify the Gate, Source, and Drain pins on my 2N5248 N Channel J FET (TO-92 case).

I ordered the FETs from AckSupply.com, who indicates that their supplier is MicroSemi.com (colors painted on the back of the transistor match the logo colors of MicroSemi, so I assume that I have identified the correct manufacturer for this part). Unfortunately, no one at MicroSemi can provide me with a spec sheet or pinout information for this device.

I have found another manufacturer for the part, NTE inc (www.nteinc.com) that does provide the spec sheet and pinout - but that would require purchasing new parts simply to identify the pinout...

So, my question is: can anyone help me locate a spec sheet for the MicroSemi 2N5248 transistor? Alternately, is there some measurement I can make (either with my VOM or LCR meter) to identify the specific pin-out for this device? I understand that source and drain pins are interchangeable, so what I really need to be able to do is to identify the Gate pin.

Any help would be greatly appreciated!

Eric

 

[Eric]

On a related note, how important is it to place heat sinks on Q3, Q4, and Q5 as Nelson recommends? If this is something that I really have to do, where can I find them?? I have not been able to find anyone who supplies heat sinks for plastic TO-92 devices...

Thanks! Eric

 

[rwagter]

Eric,

I've just fished with stacking the board (except R11 which will be a pot.). I just followed the drawing like in the article but I just found the following link:

http:\\[url]www.eletronicaexperimental.hpg.ig.com.br/transistores.htm[/URL]

good luck to us both.

Ralph

 

[Eric]

Ralph: thanks for your reply! I too found the page you referenced, however, it conflicts with other sources for the 2N5248 pinout.

Consider the following pinouts:
1) your source lists pin 1=S, 2=G, 3=D
2) NTE lists pin 1=G, 2=S, 3=D
3) original a40 article (figure 10) lists pin 1=S, 2=D, 3=G

So there we have three different pinouts from three different sources, all for the "same" FET...

I don't really want to add another resistor and cap as Nelson suggested in his recent "update" article on the PassDIY web site.
Eric

 

[Michael Rothacher]

Eric:

What are the color bands on the part? Are you sure it's from MicroSemi and not National?

 

[rwagter]

On mine JFET's there are the following colourbands:

Bleu
Red
Yellow

What vrand is that ????

 

[Eric]

Brand and color bands

mrothacher: I spent a good deal of time on the telephone yesterday talking to people at MicroSemi who informed me that they don't make plastic case TO-92 devices, so I guess its not theirs (despite Ack Electronics listing MicroSemi as their supplier...).

I looked again at my FETs and they too have 3 color bands. From top to bottom they are Blue, Red, and Yellow. Guess I have to do some more digging to do.

At one time I did find information about the 2N5248 on the Fairchild Semiconductor web page, but it is no longer there.

Eric

 

[Michael Rothacher]

Eric:

When I get home tonight, I might be able to provide you with some additional info.

Mike

 

[Michael Rothacher]

The parts I have are marked BLUE-RED-VIOLET. I'm fairly certain they are National parts and the center pin is the gate. I'm not sure that all the bands identify the maker. I believe some fets have color codes to grade the Beta. That may be the case here.

Here's something I clipped from a newsgroup:

Generally, the Source to Drain resistance should indicate about
150-300 Ohms resistance in both the directions. and the remaining terminal should show a diode's behaviour with these two pins. That is, high impedance in one direction and low impedance in the opposite direction.

This check correctly identifies the center pin as the gate in my samples. I believe this only works for JFETS.

You could also wire it up as a current source (see Pass DIY Op Amp article) and check it for proper operation.

Hope this helps a little. Sorry I couldn't be more precise.

Mike

 

[Zappa]

You could also wire it up as a current source (see Pass DIY Op Amp article) and check it for proper operation.
Mike

In doubt, that's the only way to go I think. Even if you have to break one to know... But it shouldn't.

 

[rwagter]

mrothacher,

with your method it seems that with mine JFET the gate is at PIN 2 so I have a little rewiring to do. I also read that it doesn't matter what the source and the drain is since these are interchangeble??
Is this correct.

Ralph

 

[Eric]

Thanks for the tips! I, too, have read that the source and the drain pins are interchangeable, so it would seem that the only important pin to identify is the Gate.

Now I have to wait until I get home to test my FETs

Eric

 

[Zappa]

source / drain interchangeability

NO, the source and drain are NOT interchangeable on a fet, no more than the collector / emitter on a bipolar.

 

[Eric]

Naturally, I don't know very much about this (which is the reason for my post) but the NTE Datasheet indicates that Source and Drain are interchangeable.

Click on the picture at the bottom of the page.

 

[rwagter]

And when you look in a schematic the logo for a JFET is symmetrical not like a mosfet

Ralph

 

[Nelson Pass]

Actually it will amuse you to know that they are occasions
where you can swap emitter and collector and never know
it cause circuit works. I did this on some monolithic diff
pairs once, and it worked great but would break down
when the BE voltage exceeded the 7 volts or so Zener
value for the junction. Took me quite a while to figure
out that the package was tabbing the collector pins,
not the emitters like with t0-5's

 

[Eric]

Thanks to everyone for your help! I think I've got it figured out now- I have to reverse my FET before I give it a try.

Eric