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Topic: Suggestions on a SMD transistor (hardwar) to use... (Read 18313 times) previous topic - next topic


Jan 11, 2013, 05:44 pm Last Edit: Jan 11, 2013, 06:38 pm by xl97 Reason: 1
hey guys.. wondering what suggestions or what you have used in your projects as far as SMD transistors go..


I want to make a simple PCB that has a certain amount of leds in a line across the pcb (pcb is roughly 1.5" long x .5" wide)..
anywhere from maybe 4 leds to as many as can fit across.

most likely the leds will be smd variations of some kind.. (most everything I have looked at as been around 3.1-3.3v and fV of no more than 20)

I want to be able to PWM control this 'strip/pcb' of leds (all at once) from an Arduino pin.

Knowing the current draw will be too much for a single Arduino pin to handle...  I know I need something 'else/more' to achieve this.

Figured using a simple transistor to 'bridge' this gap, (having the transistor supply/control the direct power supply to the leds...andhaving the Arduino PWM the transistor)

I have a few questions that will hopefully help me make a better decision.

1.) the leds are NOT set-in stone yet......  that being said I was thinking about using 5050 SMD leds (one color).. maybe 4 of them..
the 5050 leds are really 3 smaller leds in one package/housing.. (ie; there is 3 GND and 3 POS pads on each 5050 led)..

I dont have a problem 'bridging' all GND pads and all POS pads together in the schematic/board layout.... more or less treating it as 1 led
my question is...  if I use 1 resistor to limit the current it gets (as a whole)..  let say to 30mA for discussion sakes..  (normally it would be 60mA,... 20mA per led  x  3-leds in each 5050 housing  =  60mA total).. but if I put on a resistor that limited this to around 30mA and knowing inside the 5050 unit it truly is 3 separate leds...  will they all split up that 30mA equally? or will one of the leds in the 5050 unit be brighter?.. pulling a full 20?mA while the others split the remaining 10mA?  hope that makes sense!?  LOL

I know I'll need more current than what the Arduino will be able to provide..  but I dont if going with 5050's @ 60mA each is too much?

I'll only have 1 battery pack.. another pcb like this.. (the Arduino itself).. and a servo or two to power..

battery pack will more than likely be a 7.4v li-ion pack of some kind..

so... with all the above explained.. what SMD transistor do you guys recommend?  (I cant seem to find a 2N2222 transistor in SMD package in Eagle?)  can I use another SOT23 package?  like the BC847 transistor series??


I have a question about mine.. and others I have seen..

1.) I have seen some diagrams have the battery+ pad/line (in my schematic) be the 3.3v or 5v lines coming FROM the Arduino?
2.) I have seen some diagrams have the GND line from the Arduino be used and tied into the GND line from the battery source/pack?..
Why?  Im failin to see what part the Arduino circuit plays into this (powering the leds) if it can not give enough current.. 'and' a transistor is being used instead?   (maybe it was showing the Arduino GND to imply they needed to be joined together?)



Most cube projects have a transistor involved...

In this thread CrossRoads gave me some advice for my component needs including a FET.

Good links: Eagle tutorial= http://www.youtube.com/playlist?list=PLDE1858BD83D19C70
General Arduion tutorials = http://tronixstuff.wordpress.com


I'd go with FET with low Rds, otherwise have ~0.7V across the BJT transistor.
LED with resistor each is smart way to go.

You can make a 3-pad symbol quick enough xl97, I've seen you do it before ;)

Power from your battery as shown, adjust the resistors for whatever that voltage is. No point burning off voltage in the regulator just for LEDs.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


Jan 11, 2013, 08:25 pm Last Edit: Jan 11, 2013, 08:30 pm by xl97 Reason: 1
Thanks guys..


I wann amake sure I understand properly.. :)

you'd got with a FET? (MOSFET?) I have never used one before.. (similar to the transistor? in purpose? design?)

what is low Rds mean?

Im 'open' to whatever is best/easiest and cheap..  (as most of my projects.. these pcbs are TINY....and will be used in props/toys/small space applications)..  

but why the deviation from the transistor? and how things are set up now?  (why is one better than the other I guess Im getting after?) :)

still new to transistors.. (know zero about 'fets')  :)

yeah I can make a new symbol (for Eagle).... just not sure why or what for yet? for this FET?  (or for the SMT transistor?)..

I figured they both had the same size SOT23 footprint?  (maybe not)  lol..


1.)  anyone wanna comment on the 5050 leds? and that whole theory above?
2.) if using a transistor (and not the FET yet)... the above schematic 'is' correct?  a 2N2222 transistor is good to use?



<offtopic>crossroads if your come back/read this..  HELP!!!!..  I cant get the 3.3v/8Mhz internal circuit to init/work with the SD card!!</ offtopic>



You mentioned running from 3.3V.
With a FET, the voltage drop across the FET will be Rds * current. With 0.05 Ohm (50 milliohm) FET and 60 mA of current, that will be just .003V, so you have 3.297 to go across your LED & resistor.  With a 3.2V Vf diode, that could be enough to drive it.
With Vsource of 3.3V and a NPN, and Vce = 0.7V, you only have 2.6V for the LED - not enough to turn on.
N-channel logic level MOSFET with low Rds will get you there.

With Vsource of 5V, the 2N2222A will do.

Footprints, read the datasheet. I am not going to guess about sameness or not.

Resistor per LED, what else is there to say? Use one resistor per LED.

Somewhere I commented that your other thread description said
SD PIN        >>>     Arduino
DAT2(9)        >>>      NOT USED
CD/CS (1)      >>>     D10
CMD/DI(2)     >>>      D11
VSS1/GND(3)  >>>     GND rail
VDD/V++(4)    >>>     +3.3v rail
CLK/SCK(5)    >>>     D12
VSS2/GND(6)  >>>     GND rail
DAT0/DO(7)   >>>     D13
DAT1(        >>>     NOT USED

These are not the correct pins for SPI, which should be:
D13 - SCK
D12 - MISO (SD card Dout)
D11 - MOSI (SD card Din)
D10 - SS
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


ahh.. ok I see what the Rds means now.

The leds I sourced are actually greens (and reds) with a fV of around 2.1 I believe off the top of my head.. (yes for the greens too!) :)

but for this project and schematic posted below.. I mentioned using:
"battery pack will more than likely be a 7.4v li-ion pack of some kind.."

the 3.3v mention is from another post/project.  (I think my PS comment was mis-leading.. as it was mention of my other thread..I wont co-mingle anymore) :)

to make sure I had enough current for the leds in the pcb...(there may be a few more leds in the end..maybe 6?)

Also.. I 'do' have a resistor for each led..


(I got you post in the other thread.. I will only post there.. and swap the two pins)


Not really sure how exactly you are trying to do. But a few things:

1) a sot23 bjt is good for maybe 50ma on a good day; and a mosfet of that size maybe 75 - 100ma.
2) sharing one resistor for a rgb led may not work, depending on which type of led it is.


Thanks for the note that the transistor wont have enough current.. 

what RGB?...umm... what?  lol

these are 4 (maybe 6) single color leds.....

I had asked about possibly not using (say) 1206 SMD leds.. but maybe using 5050 leds.. (which are really 3 separate leds under 1 dome/housing)  3 x GND pads / 3 x V++ pads.. for each 5050 led.. 

was asking about if I bridged the 3 GND pads.. bridged the 3V++ pads.. (treating it as one led, pad wise).. how that would work if I use a resistor on it.. that was less than the nominal 60mA the 5050's want/take....if I used a resistor to limit to that say 30mA given tot he whole 5050 led.. how do the 3 separate diodes inside behave? they will all get 10mA equally? 1 will take 20mA and the other will split the left over 10mA?..etc..

"want I want to do" is.. have a strip of leds on a PCB.. (not sure on exact number say 4 to match schematic for now) and be able to PWM the whole strip

.. but its more than 1 Arduino pin can dish out/give (safe to say) so I came here looking for the best way to use atransistor to overcome this obstacle..

Im going to have to read-up on MOSFETS and hwo they work...

but if SMD MOSFET gives more current than a SMD transistor.. then even better..

4 leds.. driven correctly at 20mA each is = 80mA.. so I need that much for sure!.. 

(its going into a 'helmet' for light up eyes)  so there will probably be another identical board on the other eye..

same battery pack going to that pcb as well..

Question:  can the same PWM pin drive 2 transistors/mosfets at the same time?

ie: if I have two identical boards with mosfets on them.. 4 leds each.. can I control them from same PWM pin?


With Vsource of 3.3V and a NPN, and Vce = 0.7V, you only have 2.6V for the LED - not enough to turn on.

Vsat can be much lower than 0.7V - not confusing Vbe with Vce perhaps?
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


No, I was thinking Vce.

I see that low Vce parts, which are also low power handling, are available, with some careful searching:
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


BSS138 is a ~200ma mosfet. Dirt cheap.

It's also the same chip that both Sparkfun and Adafruit use for their bidirectional logic level converter.


I use BC817 for that sort of thing. It's the SMD version of BC337 and good for switching up to 200mA or a bit more. For higher currents, I'd use a mosfet.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.



so a transistor 'will' work?  LOL..

(just swap that out for the 2N2222 in my schematic above?)

of course I'll double check for/on an Eagle symbol for it..or make one from scratch..

@char & dc42-

thanks for the links..  I'll check them out..

so the: BSS138  is a Mosfet.. able to give 200mA of current..(should be more than enough for all the leds on the pcb)

and the: BC817 is a Transistor able to give 200mA of current as well...??

being a noob.. if they both can give me enough current,..why choose one over the other??

and (package/footprint aside).. I can just drop.use any of these in the schematic above, replacing the 2N2222 transistor...correct?


The BC817 (npn transistor, cost 1.9 pence from Farnell UK) is a direct replacement for the 2N2222 in your schematic.

The BSS138 (n-channel mosfet, cost 4.9 pence from Farnell UK) is not. You can omit the series resistor between the Arduino and the gate (because it is a small mosfet with low input capacitance), however a resistor between the output pin and ground is recommended, to ensure the mosfet stays off until you have initialized the pin mode to OUTPUT. The resistor value is not critical; 10K is typically used.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.


Jan 16, 2013, 06:46 pm Last Edit: Jan 16, 2013, 07:01 pm by xl97 Reason: 1

I have updated the schematic/layout to include the BC817 transistor (SMD).. and made a quick board layout for it (simple enough)

I have a couple of 'dumb' questions.. some about transistors in general (and those darn datasheets).. and one about extending this with the Arduino..

as you can see its a fairly easy/simple schematic & board design.. (not much to it).. just a pcb of leds for the Arduino to 'control'..

looking at the datasheet for the BC817 transistor.. (http://www.datasheetcatalog.org/datasheet/philips/BC817_3.pdf)

I get confused at to the 'numbers' given because of the naming conventions used.

basically a transistor will allow a 'higher' voltage/current pass through it (to the target device)....(unaffected..ie: not regulated or anything)...  by allowing a 'smaller' voltage/current to 'toggle' the gate and allow said 'flow'...


Collector Base Voltage Max:
Collector Emitter Voltage Max:
Emitter Base Voltage:

(these names are confusing me)

1.) Emitter Base = Base?.. ie: the Arduino 'toggle/gate' control pin?

2.) Collector Base = Collector?  (out to leds/device)

3.) Collector Emitter = Collector? (GND from battery -)

(Vcbo) Collector Base = 50v
(Vceo) Collector Emitter = 45v
(Vebo) Emitter Base = 5v

Does that mean it can have up to 50v connected (from battery/source)...
output up to 45v to the 'target' device..
and can take 'up to' 5v (max) to toggle/control the gate?

4.) Im also confused about the current ratings given.. it is 200mA available or 500mA available?  what are the difference between the two numbers given?

(Ic) Collector Current = 500mA
(Ibm) Peak Base Current = 200mA

anyways..moving on..

I have a question about usability and the Arduino.... using the same schematic/board files from above...

If I had two of these pcb's.. (each with a transistor, leds..etc)

could I connect this 'second' pcb in series to the first?

basically jumper/bridge from the battery +/- pads from the first pcb to the second.....  
but more importantly.. could I use the same PWM pin to control/fade BOTH pcb's at the same time??

** and lastly (last question)  :)

I had asked above.. but I dont think it was addressed/answered...

the current footprint of the LEDs are 1206 package.. this is not set in stone  (all resistors will be based of the battery source and vF of the leds when finalized.. although Im sure the leds are still just white in color)...

a thought was to use the 5050 package leds.. (these 505 leds are really 3-separate leds under 1 housing/dome.. 
the 5050 leds have 3 GND pads.. and 3 POS pads (ie: set for each led)..

in this application.. the 5050 unit will be treated a WHOLE unit/led.. in respect that the GND pads will be bridged.. and the POS pads will be bridged.. on the 5050 led itself.. (with traces on the PCB)..

still only using "1" resistor per led.. (but now this is a 5050 packaged led)

these 5050 leds (I believe) take 60mA!!  (3 x leds in eahc unit, 20mA each, 3 x 20 = 60mA)..

my question is.. if the one resistor I use, limits the current to, say, ...40mA going to the 5050 led/unit...  how will the leds respond/behave?

Will all 3 leds split the '40mA' equally?  will one leds take more? or full mA leaving the others without full or equal current?

in the current example. if I changed to 5050 leds..  there are 5 leds.. going to all 5 x 5050 leds would be (5 x 60mA = 300mA) neded per pcb/board!!

Im not even sure the transistor can handle/provide that?.. however if @ 40mA each 5050 led looks and acts fine.. it might be a viable option... (or maybe just more 1206 leds!)  lol..  but I 'am' curious.


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