BC337 not working - darlington???

[DanPBP]

Hi Guys,

I'm using a pic12f675 to drive 5 groups of LEDs like the schematic bellow:



As a driving transistor I'm using a BC337 but it looks like there's not enough current comming from each output of the PIC to drive the transistors (the LEDs are lit so poorly).

If I connect the base of the transistor (with the 1K resistor) directly to the positive they lit very bright. I also tried with 470 ohms and less.

So, I'm thinking that maybe a darlington transistor could help me with this.

I already have the PCB done (see bellow) and I don't want to do it again, just replace the BC337 with something that might work. Any ideas?




Thanks!

Daniel.

[mackrackit]

Use a N channel MOSFET. You may have to bend the pins some to fit.

Open question --- Why are so many people making the PCB and then trying to get things working? What happened to prototyping on a test board (bread board) to identify components needed?

[Acetronics2]

Hi,

Instead of placing the 337 between 100R and the anodes of your leds ... place it between cathodes and Ground ...

so simple as that !!! ... for next time you draw your PCB




not to change the existing PCB ... use a P-channel Mosfet and invert your pic output level ...

Alain

PS: Hi, McCrack ... your N channel won't turn on the leds : its gate might be at a ~ 8.4v level ... far from PIC output limits !!!

[mackrackit]

PS: Hi, McCrack ... your N channel won't turn on the leds : its gate might be at a ~ 8.4v level ... far from PIC output limits !!!

I guess I should have stated a 5volt or less saturation on the gate.

[DanPBP]

First, thanks for you answers!

And one more question, could this darlington transistor work?

MPSA14 http://www.onsemi.com/pub/Collateral/MPSA13-D.PDF

Daniel.

[T.Jackson]

My first bit of advice would be to learn OHM's law. If you have a collector resistor of 100 ohm, and VC (voltage collector) is 7.2V, the most amount of current you'll squeeze out is: VC - (0.6 + 3.2) / 100 = ~34mA The 0.6 is the emitter drop on the transistor while the 3.2 is the drop on the LEDs.

[Acetronics2]

Hi, Trent ...

There are TWO leds in series ...

7.2 - 3.2 - 3.2 - .6 = ... .2v

.2 v / 100 ohms = ... 2 mA

and the light was ... NOT !!!

Alain

[T.Jackson]

Must be those new white LEDs. Never used them.

[Acetronics2]

Hi, Trent

Buuuuuut,

you forgot the BC 337 base is @ 5v level !!!

so, Emitter is @ 4.4v !

4.4 - 3.2 -3.2 = - 2v !!! LOL

The leds NEVER give any light !!! ...

Alain

[T.Jackson]

It's an NPN transistor, meaning the base must be at 0.6V greater than the emitter in order for it to turn on. I too fail to see how it would ever switch on if those LEDs are in fact 3.2V types.

[DanPBP]

Hi Guys,

The circuit as I posted works fine with more than 5 volts on the base of the transistor, fully bright LEDs.

Yesterday, I did a new PCB with the BC337 placed between cathodes and ground and now they work just fine with the PIC.

Thanks for your help! Btw, I'm a software guy, sometimes electronics beats me.

Daniel.

[DanPBP]

Hi Guys,

I owe you a pic and a video of the final product, a "neon light" for my street R/C car...

I made two of them, one for each side... Maybe I will make one for the front and one for the back with less LEDs...



The software is not finished yet, just a test to see how it works... The idea is to generate different patterns according to the throttle/break signal given to the servos...



Regards!

Danie.