Voltage Drop problem with 6 sided LED panelling

Power supply can output 3.3v, 5v, 12v, and has up to 30A available at 5v.

Transistors are BC557, rated at 100mA (200mA peak).

Each RGB LED in the schematic represents 8 LEDs in parallel rated at 20mA. There are 6 panels, each with a set of 8 LEDs, 3 Transistors, and 3 data pins from 2 more shift registers.

So total LEDs would be 144 x 20mA = 2.880A. Total mA through each transistor would be 160mA.

My problem is that my LEDs are significantly dimmer than they should be. At 3.3v (within voltage range for each colour, a little high for red) they are barely on. At 5v they give off a decent glow, but are about half as bright as I would expect. I don’t have any resistors wired in, as things just get dimmer. I tried including a 100 Ohm resistor between the base of the transistor and the shift register pin, as I read somewhere this prevents voltage leaking into the register instead of the LEDs, but this seemed to make no difference.

So I’m hoping someone can help me isolate if the problem is with the transistors limiting the current, or if there is a current sink limit happening with the shift registers or arduino and I need to wire differently to prevent this? Or some thing I am not aware of with transistors and the relationships between base and emitter voltages? or who knows. I know the transistors are rated slightly lower than they should be, but I’m hoping to isolate if they would be 100% the culprit before replacing them.


You're driving current thru the pictured transistors backwards.
LED cathodes go to collector, emitter goes to battery-.

If you're using PNP, you want them between +supply and LED anodes, with cathodes to Gnd.
Once you get the wiring straight, don't forget the current limit resistors.

Also, BC557 data sheet only shows currents to 100mA.
At 160mA, the amount of base current needed is not specified - 74HC595 may not support whatever that current might be.

I see IC max as 100mA Absolute Max - what datasheet are you pulling 200mA from?


Here's the datasheet showing peak collector current of 200mA http://www.nxp.com/documents/data_sheet/BC556_557.pdf

If the schematic shows the transistors backwards, it's cus it's my first time doing a schematic, sorry... no such backwardness in the real one. They are NPN.

The Shift register has 20mA output on the pins, according to this: http://www.nxp.com/documents/data_sheet/74HC_HCT595.pdf
I thought 5v at 20mA should be well above what's needed to saturate the transistor? Does the current needed to do so vary that much when there is more than standard current running through it?

And could you let me know where I would need the 'current limit resistors' since the transistor wiring is straight in real life? Are these the resistors between the base and the shift register?

These are just to protect my transistors and shift registers from exploding, they wouldn't effect the LED brightness, correct?