Photon got fried

I can’t explain it yet. Flashed a Modbus firmware and connected Tx - Rx and D7 pins of Photon to the ports of a MAX485 transceiver. MAX485 is mounted on a separate board and supplied by a NCP1117-5V. The NCP is powered from the auxiliary 12V output of a Modbus RTU unit. I plugged the USB cable to PC, without firstly powering the Modbus unit, Photon was powered-up and its tiny voltage regulator ended-up through smoke. Desoldered the regulator, but checking with multimeter +3.3V and GND it shows short circuit. Desoldered and capacitors and short circuit remains. So the P0 of Photon is also damaged and probably caused the short circuit that damaged the voltage regulator.Photon’s P0 I/O pins are 5V tolerant.

Only in specific circumstances, for specific pins, so you can’t just put 5V on just any pin.

Have you got a schematic of how you wired things?

Yes i have, here is a comprehensible block diagram. Do you mean that UART1 pins Tx - Rx or D7 are not 5V tolerant?
If yes a voltage translator or an isolator is necessary? However, in STM32F205 data all I/O pins (except Vdd, Vss of course) are refered as FT (Five volt Tolerant)

Could you please take a look and in this topic? From memory, i had plugged the PHOTON with the USB cable to PC, without connecting the Modbus unit to the mains. And i remember the power led on Modbus glowing for a while, and after the regulator of PHOTON got fire and ended in smoke.

Hi @fotis

I think there are a few possibilities here:

  • You did not have power on the MODBUS Slave and the USB input tried to supply power to the whole thing and it was too much for the regulator on the Photon.


  • You did have power coming from the MODBUS Slave and so there was a power fight of some kind at the Photon.


  • You had a “ground loop” where the USB ground (which is typically connected to earth safety ground) was driven to some voltage by your MODBUS module power.

I did not clear from your diagram which scenario happened but a best practice is to never have more than one active power supply for a device. That can mean removing the Photon from the circuit completely for reprogramming or using a switch to isolate power and ground connections or using a USB isolator (Adafruit sells a low power one) that allows you to isolate from the earth ground.