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Topic: NPN Transistors controlling 12v LED stirps (Read 2 times) previous topic - next topic

ibourdon

I am currently running 8 strips of blue LEDs each with 12 LEDs on the strip.
The current pulled by each strip is 200mA @ +12V. This setup is working perfectly fine with an external +12vDC & NPN 2N3904 Transistors but not with NPN 2N222A or NPN 2N4401 Transistors.

I only have 5 3904's on hand so the other three remaining strips I used two 4401's and 1 222A
They all have a 1k resister between the base and arduino pin but the 3 strips using different transistors are FAR dimmer than the others.
The 3904's are full brightness while the others are hardly lit.

What would cause this?
Should I lower the R between the 222A, 4401's and my arduino pins?

ibourdon

So I went ahead and lowered the R values on the 222A & 4401 pins to 470 Ohms and it made no difference in the brightness.

What I don't understand is that the 3904 transistor is only rated for 200mA where as the other two are rated 600mA.
I would think these to stronger transistors would have no problem switching my 200mA load.

retrolefty


So I went ahead and lowered the R values on the 222A & 4401 pins to 470 Ohms and it made no difference in the brightness.

What I don't understand is that the 3904 transistor is only rated for 200mA where as the other two are rated 600mA.
I would think these to stronger transistors would have no problem switching my 200mA load.


A proper analysis of using a specific transistor requires that you consider it's current gain spec (beta). The fixed series base resistor you are using may be what's limiting your collector current flow, or maybe not.

While the transistor is turned on, take a voltage reading from the collector lead to ground, if it's not at 'full saturation voltage' (.2vdc or so?) then you are not turning it one hard enough.

Lefty

ibourdon

I took the measurement reading you suggested and the reading was the same between all transistors with a variance of .1 volts.

The voltage in is +11.97vDC (+12vDC) @ Source and the voltage at the collector to ground is +11.70 give or take the .1 between the different types. That reading is with a 470 Ohm R on the 3 "odd balls" and a 1k R on all others.

I would still like an explanation to this problem even though I have solved it by switching to a ULN2803A. Knowing why the NPN setup was doing that will help in future projects I'm sure.

retrolefty


I took the measurement reading you suggested and the reading was the same between all transistors with a variance of .1 volts.

The voltage in is +11.97vDC (+12vDC) @ Source and the voltage at the collector to ground is +11.70 give or take the .1 between the different types. That reading is with a 470 Ohm R on the 3 "odd balls" and a 1k R on all others.

I would still like an explanation to this problem even though I have solved it by switching to a ULN2803A. Knowing why the NPN setup was doing that will help in future projects I'm sure.



Sounds like you were not wiring up the transistors correctly? NPN transistor should be wired as a 'low-side' switch, with emitter lead grounded, base lead via resistor to arduino output pin, collector to load (LED negative) and LED positive to + external voltage source. External negative must also wire to arduino ground, as well as to emitter lead.

Is that how you wired it up?

Lefty

ibourdon

This is why I am stumped. My wiring was exactly as you just stated.

The FAR RIGHT little red wire the leaves the image is the common ground for all emitter pins, arduino ground, and external power supply ground (for 12v LEDs).


retrolefty

Quote
This is why I am stumped. My wiring was exactly as you just stated.


Except you misidentifed the proper emitter/base/collector leads for that package type?


http://www.bytesonic.com/sonic/2N2222.pdf

ibourdon

Nope, it is proper according to the datasheet for this transistor. MPS2222A.


I figured it out. It was an ID10T Error (idiot)


In my sketch I did not specify pinMode(pin, OUTPUT) for the pins those were on.

Thanks for the help! Maybe next time it will be for an actual hardware problem..  :smiley-red:

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