Instructables

Can I use USB to power a Relay??

I have purchased a simple 4 channel relay (http://www.fort777.co.za/index.php?main_page=product_info&cPath=800_801_946&products_id=10357&zenid=fvokss9s9ucgakrrn9ged441hs1oushq) My Aim is to use the USB port on my TV to control the relay.  The intention is to have it switch on a lamp when the TV is powered up.  I have run into an issue and being a bit of an electronics novice I am not sure what to do.  The Relay kit as both a 12v input (I assume that is to actually power the relay coil) as well as a connector that has pins for Ground, 5v and Signal for each relay (S1 - S4). If I attach the red wire from USB (5v) to S1, and the ground from the USB Cable (Black) to ground, the indicator LED flickers and then dies.  at no point does the relay click, and the light seems unaffected whether the 12v power is connected or not.   I feel I might be missing something simple, but as an utter novice in this area I am not sure.  Any help. 

mpilchfamily7 months ago

The coil on the relay is drawing more power than the USB port can support. The port on your TV is only able able to supply the minimum spec which is 200mA. It's not meant to supply power or charge things. What you can do is take the power from the port, use it to trigger a transistor which triggers a stronger power source to trigger the relay to turn on the lamp.

The image i've added here will give you an idea of what to do. In it J1 will be the +5 from the USB port to the transistor. The 9V will be whatever voltage and power source you'll need to trigger the relay. You'll need to take the ground wire from the USB port and tie it into the ground wire of the other power source so everything will work correctly. When the TV comes on and power is sent to the USB port the transistor will trigger allowing the higher power source to flow through and trigger the relay allowing the lamp to get power.

Horn .jpg

If you look at the PCB in the OP's question, you can see the drive transistors and a 12V input - we can only presume, but that the 12V feeds the relays, and the transistors switch them on, just like your circuit diagram

The link wasn't showing when i posted.

Lylec (author) 7 months ago

Thank you all for the input. I will see if I can get this working.

robo103027 months ago

hi mpilchfamily I thought about your answer but I was able to power my relay with an arduino's 5v 150 milliamps output.

steveastrouk7 months ago

Have you wired a 12V supply to it ? DId you connect the USB 0V to the 12V supply 0V ?

Ignore the comments about diodes from robo - there is no need for ANY extra components, and certainly none on the mains side.