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'[EE] Small Transistor for LED Multiplexing'
2010\01\11@153816 by Josh Koffman

face picon face
Hi all. I'm doing a multiplexed LED display for the first time in a
long time. I'm using small displays (0.5" height) that are common
cathode. I haven't decided yet, but I might have a 74HCT4511 decoder
in there, or I might do the decoding in the PIC itself. It'll depend
on how many pins open I end up with.

My problem is that the transistor I used to use for this sort of thing
(2n3904) doesn't seem to come in an SMT variant at Digikey. I find
this pretty amazing, I wonder if I'm searching for it badly.

In any case, does anyone have any favourites you'd recommend that I
_can_ get at Digikey?

Thanks!

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

2010\01\11@155135 by ivp

face picon face

> In any case, does anyone have any favourites you'd recommend that I
> _can_ get at Digikey?

BC846, BC847 ?

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=1376376&k=bc847


2010\01\11@155138 by Vitaliy

face
flavicon
face
Josh Koffman wrote:
> Hi all. I'm doing a multiplexed LED display for the first time in a
> long time. I'm using small displays (0.5" height) that are common
> cathode. I haven't decided yet, but I might have a 74HCT4511 decoder
> in there, or I might do the decoding in the PIC itself. It'll depend
> on how many pins open I end up with.
>
> My problem is that the transistor I used to use for this sort of thing
> (2n3904) doesn't seem to come in an SMT variant at Digikey. I find
> this pretty amazing, I wonder if I'm searching for it badly.
>
> In any case, does anyone have any favourites you'd recommend that I
> _can_ get at Digikey?

You can use their parametric search, or use MMBT3904TPMSCT-ND.

Vitalij

2010\01\11@161715 by Robert Young

picon face



> Date: Mon, 11 Jan 2010 15:37:41 -0500
> Subject: [EE] Small Transistor for LED Multiplexing
> From: spam_OUTjoshybearTakeThisOuTspamgmail.com
> To: .....piclistKILLspamspam@spam@mit.edu
>
> Hi all. I'm doing a multiplexed LED display for the first time in a
> long time. I'm using small displays (0.5" height) that are common
> cathode. I haven't decided yet, but I might have a 74HCT4511 decoder
> in there, or I might do the decoding in the PIC itself. It'll depend
> on how many pins open I end up with.
>
> My problem is that the transistor I used to use for this sort of thing
> (2n3904) doesn't seem to come in an SMT variant at Digikey. I find
> this pretty amazing, I wonder if I'm searching for it badly.
>
> In any case, does anyone have any favourites you'd recommend that I
> _can_ get at Digikey?
>
> Thanks!

Try
mmbt3904

Rob
                                           

2010\01\11@162024 by Josh Koffman
face picon face
On Mon, Jan 11, 2010 at 3:51 PM, Vitaliy <piclistspamKILLspammaksimov.org> wrote:
>
> You can use their parametric search, or use MMBT3904TPMSCT-ND.
>
> Vitalij

The parametric search is so cold though...there's no "feeling" in it
(ie I _like_ to use this one because the part number has sentimental
value to me).

:)

Thanks for the tip! That's likely what I'll go with.

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

2010\01\11@204857 by Xiaofan Chen

face picon face
On Tue, Jan 12, 2010 at 4:51 AM, ivp <.....joecolquittKILLspamspam.....clear.net.nz> wrote:
>
>> In any case, does anyone have any favourites you'd recommend that I
>> _can_ get at Digikey?
>
> BC846, BC847 ?
>
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=1376376&k=bc847
>

BC846/BC847 and BC856/BC857 seem to be the most popular
(European) transistors. We used millions of these transistors in
my previous job (a Germany based company).

In the US based companies, they are not as popular.


--
Xiaofan http://mcuee.blogspot.com

2010\01\11@213403 by Stephen R Phillips

picon face
MMBT3904
http://search.digikey.com/scripts/DkSearch/dksus.dll?lang=en&site=US&WT.z_homepage_link=hp_go_button&KeyWords=MMBT3904+&x=0&y=0

Lots of manufactures looks like.

Have fun!

Stephen


--- On Mon, 1/11/10, Josh Koffman <EraseMEjoshybearspam_OUTspamTakeThisOuTgmail.com> wrote:

{Quote hidden}

> -

2010\01\11@222859 by Spehro Pefhany

picon face
At 03:37 PM 1/11/2010, you wrote:
>Hi all. I'm doing a multiplexed LED display for the first time in a
>long time. I'm using small displays (0.5" height) that are common
>cathode. I haven't decided yet, but I might have a 74HCT4511 decoder
>in there, or I might do the decoding in the PIC itself. It'll depend
>on how many pins open I end up with.
>
>My problem is that the transistor I used to use for this sort of thing
>(2n3904) doesn't seem to come in an SMT variant at Digikey. I find
>this pretty amazing, I wonder if I'm searching for it badly.
>
>In any case, does anyone have any favourites you'd recommend that I
>_can_ get at Digikey?
>
>Thanks!
>
>Josh

MMBT3904 (or MMBT4401 for something a bit beefier and still very cheap).
Or maybe 2N7002.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
KILLspamspeffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2010\01\11@224658 by PICdude

flavicon
face
Quoting Spehro Pefhany <RemoveMEspeffTakeThisOuTspaminterlog.com>:

> MMBT3904 (or MMBT4401 for something a bit beefier and still very cheap).
> Or maybe 2N7002.
>
>> Best regards,
>
> Spehro Pefhany --"it's the network..."            "The Journey is the reward"
> spamBeGonespeffspamBeGonespaminterlog.com             Info for manufacturers: http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:  http://www.speff.com
>

2N7002 is a FET, and some of those (like Fairchild's variants) handle  
only a bit over 100mA.  I use these, but the ones from NXP that handle  
300mA.  I can't remember the specific suffixes/suffices offhand.

Back to the transistors -- did anyone mention the MMBT2222A yet?  
600mA Ic IIRC, and I go through a bunch of those.

Cheers,
-Neil.


2010\01\12@040658 by Alan B. Pearce

face picon face
>My problem is that the transistor I used to use for
>this sort of thing (2n3904) doesn't seem to come in
>an SMT variant at Digikey.

If you are searching for '2n3904' you probably won't find it, but OnSemi
seem to do the jellybean ones as MMTxxxx (we get 2907 and 2222s like this),
so just searching for '3904' would probably find what you want.

2010\01\12@075938 by Russell McMahon

face picon face
> Hi all. I'm doing a multiplexed LED display for the first time in a
> long time. I'm using small displays (0.5" height) that are common ...

> My problem is that the transistor I used to use for this sort of thing
> (2n3904) doesn't seem to come in an SMT variant at Digikey. I find
> this pretty amazing, I wonder if I'm searching for it badly.

> In any case, does anyone have any favourites you'd recommend that I
> _can_ get at Digikey?

I usually use BC337/327 in TO92 and the equivalent BC807/817 in SOT23
for almost all jellybean use. Available from Digikey and also in China
- either as above or ar LRC807/817 (Leshan Radio) or others. .

For extra points use the -40 parts with guaranteed highty
Beta.(250-600, mean 400)
Pricing usually about the same.

Cost about 1 cent in volume in China fwiw. (Installation typically
costs more than the part).

These are typically as cheap or cheaper than most alternatives and
have better current rating, beta and saturation than most.


   Russell McMahon

2010\01\12@114726 by Dwayne Reid

flavicon
face
At 01:37 PM 1/11/2010, Josh Koffman wrote:
>Hi all. I'm doing a multiplexed LED display for the first time in a
>long time. I'm using small displays (0.5" height) that are common
>cathode. I haven't decided yet, but I might have a 74HCT4511 decoder
>in there, or I might do the decoding in the PIC itself. It'll depend
>on how many pins open I end up with.
>
>My problem is that the transistor I used to use for this sort of thing
>(2n3904) doesn't seem to come in an SMT variant at Digikey. I find
>this pretty amazing, I wonder if I'm searching for it badly.

As others have undoubtedly mentioned by now, replace the "2N" portion
of the part number with "mmbt".

I'd also consider moving up to mmbt4401 instead of the '3904 - its a
beefier transistor that costs about the same as (or even less than) the '3904.

Its hard to put into words, but the 2n3906/06 transistors just feel
'softer' than the 2n4401/03.  Its really quite noticeable on a scope
when you feed a ramped current into the base.  I also find that the
'4401 survives "Oopses" better than the '3904 - they appear to have
much higher peak current ratings.

On the other hand, the 2n3904/06 seem to have better high-frequency response.

Your mileage may vary <grin>.

dwayne

--
Dwayne Reid   <TakeThisOuTdwaynerEraseMEspamspam_OUTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2010\01\12@135855 by Vitaliy

face
flavicon
face
Dwayne Reid wrote:
> Its hard to put into words, but the 2n3906/06 transistors just feel
> 'softer' than the 2n4401/03.  Its really quite noticeable on a scope
> when you feed a ramped current into the base.

We discovered the same thing: 3904s tend to require more time to switch off.
In places where it matters, we use MMBT3646.

Vitaliy

2010\01\17@010645 by Josh Koffman

face picon face
Thanks for the help everyone! MMBT4401 on order.

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

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