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TOPIC: Externally Powering Device on I2C Bus

Externally Powering Device on I2C Bus 4 years 7 months ago #378

  • jvetter
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I'm working on a simple relay board that is controlled over I2C, so I can have the relays closer to the valves. I have successfully been able to get this to work with a Raspberry Pi, and have added code to the Brewtroller to control it, which works.

I have been testing on a BT 4 using the 5v, Gnd, SCL, SDA pins on of the expansion ports which works, but I'm thinking the 5v doesn't have enough power to drive my board because it drops to like ~3v when 3-5 relays are triggered. My thought was to externally power the relay board with 5V but when I do this, the I2C doesn't work, presumably because they don't share a common ground.

Which leads me to my question on how to properly externally power my device. I've looked at the schematic for the BT 4.0 and it seems that the 12v and and 5v share the same GND, but its unclear to me whether it would be safe to plugin another 5v power supply and connect the V- on that to the ground of the BT board. I'm thinking not, but I'm not an EE. Maybe the solution is to use the same 12V power supply and step it down myself to 5, not sure.

Any guidance on how to do this safely would be appreciated.

Cheers,
Jaime
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Externally Powering Device on I2C Bus 4 years 4 months ago #413

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FYI, I talked to Jeremiah about this and my assumptions were correct, the I2C pins on the expansion ports only have enough power for logic level comms.

I gave up on my board for now and purchases some modbus relay boards from Jeramiah instead
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