Driving relays with Arduino and UNL2003 darlington array

[sean87]

Hi guys,

I want to drive up to 3 relays using Arduino Due (3.3) digital output pins. There are three signals "REL_MASTER, REL_CH_A and REL_CH_B" coming out of the Arduino. I think UNL2003 has protection diodes but I added my own as well. Is the following wiring schematic looks ok? Can you please check before I blow up something 

[vk2amv]

Make sure you have current limiting resistors in series between the microcontroller and each UL2003 input.
The value is not overly important for a basic project like this, anywhere between 2 to 10k should be fine I would think.

Other than that as far as I can tell your schematic looks fine to me.

[Ian.M]

Its ULN2003A, not UNL2003  or UL2003.

The internal clamp diodes are rated for the full collector current so as long as the COM terminal is connected to the highest positive voltage ral supplying the relay coils your extra diodes are superfluous.

You do NOT need extra resistors - the ULN2003A has integral 2.7K base resistors for operation from 5V logic levels.  It needs at least 2.4V in to guarantee to pass 200mA load current, and if you use more than about 5K as an external resistor you wont get enough input voltage.    If you expect to blow up the ULN2003 badly, 1K resistors *MAY* save your MCU.