Powering Arduino and Mosfets

Other Hardware General Electronics
1

Hi,

I'm working on a piece of hardware powering the well known Dioder RGB LED strips from Ikea.
74HC595 shift registers and way too big tip122 mosfets are attaced to the SPI bus of a pro mini (3,3v version) to drive the whole thing via the ShiftPWM library.
(see attached foto)

Nothing spectacular so far. And: its working fine with the original 12V power supply connected to mosfets/stripes and 3.3v FTDI (usb) to the arduino.
Removing the usb thing and attaching 12v to the arduino's raw pins makes the µc go crazy. (power led flashes like 50hz)

First obvious assumption: power supply not strong enough. But removing all LEDs doesnt get the arduino to work.

Powering the arduino alone (not connected to shift registers/mosfets) works fine with the 12v power supply, so it can not really be the supply.

So, does shifting and setting mosfet outputs interfere so much with my 12v line that even the voltage regulator on the arduino can't handle it?
Any fixes? :slight_smile:

arduino.jpg
arduino.jpg800×600 93.5 KB

2

The TIP122 is not a MOSFET it is a Darlington npn pair.
http://www.fairchildsemi.com/ds/TI/TIP122.pdf

Your schematic diagram was not visible from this distance. Are resistors on the TIP122 base leads?
Are there resistors elsewhere?

3

Oops. Was about to use mosfets, but .. yeah. npn's :smiley:

No, I missed the resistors. Had them while i was running the setup on a 5v Arduino while prototyping but thought i would not need them with 3.3v.

4

You need the base resistors for 3.3 volt input. Here is a calculation for 1.5 volt base voltage limit:

3.3v-1.5v = 1.8v resistor voltage

2 mA base current is plenty:

v=ir
r = v/i = 1.8/.002 = go figure?

5

Thanks for your hint.
Totally forgot this could be the source of error.
Fixed it!