Drive a relay from an open-drain

Thread Starter


Joined Sep 25, 2014
Hello all,

I am new to electronics and i need some support.

I have a digital integrated chip(shift register) with an DMOS open-drain output (from the datasheet -> 33 V and 100 mA continuous sink-current capability) and I want to drive a relay.

The output of the drain is connected to the relay, and then the relay is connected to the power supply 5V.

When the transistor from the digital integrate pulls the line to the ground, the output drain is 0 logic, but the relay is not switching.

What am I doing wrong ? The schematic I use is recommended for driving relays using this shift register (from the application notes of the shift register).


Joined Oct 11, 2012
You need to give more details like specific part numbers for the transistor and the relay. I would guess your relay is just not being driven hard enough. If you short the relay coil to 0V does it switch?


Joined Aug 23, 2012
RM05-4AS-4/4 need the current 24.6~30mA, and each output of TPIC6C595 is 100mA, I think the output of TPIC6C595 can't handle the RM05-4AS-4/4, when the output of TPIC6C595 output 4 high, you should doing the right operation to operating the TPIC6C595, when TPIC6C595 output 4 high to drive RM05-4AS-4/4, using your finger to sense the temperature, if it's too hot then you should adding the PNP bjt or P type mosfet to drive RM05-4AS-4/4.