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Hi all,
Something has been bothering me for a while. I know there was recently a question posted about transistor pinouts, but I was still left with mixed answers on this.
I have attached a (crappy) drawing I quickly made in photoshop to go with this question.
The package of 2N2222 transistors I bought from RadioShack has pinouts on the back which seem to go against what actually works for me, and is also the opposite of how Eagle Layout wires the transistor (from a schematic->make board). Eagle wires it the way that is in the picture, which is also how I've been wiring it all along.
It seems these transistor will work in either direction, but as I'm soon to be attempting an audio circuit (as opposed to the usual turning on a relay or whatever), I want to make sure I have it right.
Is this circuit in the attached GIF correct, labelled properly and whatnot? I would assume that the collector "collects" electricity and when the base is positive, the emmitter "emits" electricity towards ground. But it seems like I'm getting confused going from the schematic to the actual circuit.
Thanks for any clarification anyone can offer as to why i see these standard NPN transistors wired differently (ie backwards) sometimes and not others.
Peace
MJ Brush
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part 2 10642 bytes content-type:image/gif; (decode)
After some more research it appears that the GIF I posted is correct for
2N2222.
Aparently there are 3 common configurations for these types of transistors
(and then others as well). One pinout (the one which got me confused in the
first place) is for the BC type of transistor apparently common in Europe,
another pinout is the 2N type (the kind I have) which is more common in
North America, and then another for Japanese transistors or something.
So it appears there is no real standard pinout for NPN transistor in the
TO-92 package and it varies by manufacturer. It's still pretty wierd that
my RadioShack packaging had a different pinout than the regular 2N2222
pinout (as per Eagle Layout Editor). I think the biggest confusion came up
because many of the schematics/pcb layouts I've seen on the net use the BC
type of transistor and suggest the 2N type as a replacement (without any
notice of the different pinouts).
> The package of 2N2222 transistors I bought from
> RadioShack has pinouts on the back which seem to
> go against what actually works for me, and is also the
> opposite of how Eagle Layout wires the transistor
Use the data sheet for the transistor you happen to be using ... in this
case, the Radio Shack package. Not all 2N2222's are created equal. Yes,
Dorothy, some are actually backwards from others. Don't expect that an ITT
2N2222 will have the same pinout as a RS 2N2222. Sorry. This has burned
more than one traveller.
Something has been bothering me for a while. I know there was recently a
question posted about transistor pinouts, but I was still left with mixed
answers on this.
I have attached a (crappy) drawing I quickly made in photoshop to go with
this question.
The package of 2N2222 transistors I bought from RadioShack has pinouts on
the back which seem to go against what actually works for me, and is also
the opposite of how Eagle Layout wires the transistor (from a
schematic->make board). Eagle wires it the way that is in the picture,
which is also how I've been wiring it all along.
It seems these transistor will work in either direction, but as I'm soon to
be attempting an audio circuit (as opposed to the usual turning on a relay
or whatever), I want to make sure I have it right.
Is this circuit in the attached GIF correct, labelled properly and whatnot?
I would assume that the collector "collects" electricity and when the base
is positive, the emmitter "emits" electricity towards ground. But it seems
like I'm getting confused going from the schematic to the actual circuit.
Thanks for any clarification anyone can offer as to why i see these standard
NPN transistors wired differently (ie backwards) sometimes and not others.
Peace
MJ Brush
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You cannot arbitrarily swap pinouts. It will only work in one configuration. Just because the Radio Shack packaging says one thing doesn't make it right. Sometimes when the vendor prints the drawing they may look at it from the top or the bottom. It will only work one way.
Also be aware that the flat side may or may not be the same on different manufacturers package. An example is the BS170 and the BS170P. The lead configuration is SGD or DGS.
> Hi all,
>
> Something has been bothering me for a while. I know there was recently a question posted about transistor pinouts, but I was still left with mixed answers on this.
>
> I have attached a (crappy) drawing I quickly made in photoshop to go with this question.
>
> The package of 2N2222 transistors I bought from RadioShack has pinouts on the back which seem to go against what actually works for me, and is also the opposite of how Eagle Layout wires the transistor (from a schematic->make board). Eagle wires it the way that is in the picture, which is also how I've been wiring it all along.
>
> It seems these transistor will work in either direction, but as I'm soon to be attempting an audio circuit (as opposed to the usual turning on a relay or whatever), I want to make sure I have it right.
>
> Is this circuit in the attached GIF correct, labelled properly and whatnot? I would assume that the collector "collects" electricity and when the base is positive, the emmitter "emits" electricity towards ground. But it seems like I'm getting confused going from the schematic to the actual circuit.
>
> Thanks for any clarification anyone can offer as to why i see these standard NPN transistors wired differently (ie backwards) sometimes and not others.
>
> Peace
>
> MJ Brush
>
> --
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The wierdest part is that the 2N2222's I have work in either direction, that
is, with that LED circuit I posted. That only adds to the confusion. So I
guess from now on, I'll just order my transistors from Digikey and go by the
datasheet of the specific model.
Is it normal for a transistor to "work" in either direction? My assumption
is that they CAN be reversed (C & E) but are supposed to be used in a
specific direction.
If i still remember correctly, a BJT will work in reverse active mode when
you swap its collector & emitter. In this mode, current that powers your LED
still flow into the emitter and out to the collector, but at a percentage of
the base current which now flows to out to the collector too.
Yeah, it's working because it has been reverse biased, but it should not
be as bright as if it were in the correct way.
-Shawn
Tan Chun Chiek wrote:
> If i still remember correctly, a BJT will work in reverse active mode when
> you swap its collector & emitter. In this mode, current that powers your LED
> still flow into the emitter and out to the collector, but at a percentage of
> the base current which now flows to out to the collector too.
>
> Regards,
> Tan Chun Chiek
>
> {Original Message removed}
Actually, yes, they can and do work (sort of) with the
collector and emitter reversed. The difference you
will find is that the "upside-down" connection has
much less forward current gain, sometimes as low as
unity.
In your circuit, I would expect the LED to be just
a little dimmer with the transistor connected this
way; you may need to look closely to see any difference.
> The wierdest part is that the 2N2222's I have work in either direction, that
> is, with that LED circuit I posted. That only adds to the confusion. So I
> guess from now on, I'll just order my transistors from Digikey and go by the
> datasheet of the specific model.
>
> Is it normal for a transistor to "work" in either direction? My assumption
> is that they CAN be reversed (C & E) but are supposed to be used in a
> specific direction.
>
> Thanks for the clarification guys. Peace
>
> MJ Brush
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At 01:19 PM 6/5/2004 -0700, you wrote:
>Yeah, it's working because it has been reverse biased, but it should not
>be as bright as if it were in the correct way.
>
>-Shawn
It will still likely be saturated. "Real" 2N2222/2N2222A transistors are in
metal TO-18 case, and the plastic versions have bastardized part numbers
such as PN2222.
Reverse beta of such a transistor is almost surely enough to saturate
it in the given circuit (forced beta ~5), so it will work either way.
Typically it's probably more like 10 or 15.
Note that the breakdown voltage Vceo is now limited by the Vebo
maximum, which is only 5 or 6 volts. And vice versa. That doesn't matter
in a 5V-only circuit, of course. It can be handy if you need a
transistor with a very high Vebo, and there are also some advantages
in terms of the saturation voltage.
maybe of interest how I find pinout: I have a DMM with a h21e measurement
facility. I measure that of the transistor in question swapping pins until
I read a reasonable (some hundred) value. I never failed with this.
I have not been following this so my apology if I repeat someone. But most
bipolar transistors will oscillate at a low frequency. A simple AF
oscillator circuit will always find the E, B and C. The transistor will only
oscillate if it is correctly connected and will not destruct if not. The
basic IC=beta IB can determine the beta with a couple of meters and variable
voltage in a very simple circuit.
There must be abundant information on transistor testing on the web.
YAHOO.CA>
