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TOPIC: Isolating / Switching to and from USB or External 5v Supply

Isolating / Switching to and from USB or External 5v Supply 9 months 2 weeks ago #17878

  • hop
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Hello All!
I have a simple question that came up while I was getting ready to prototype with a STM32F103C8T6 Minimum System Development Board. The board sports a 3.3v regulator that is tied to the USB 5v line to supply 3.3v to the MCU. The board also has a breakout pin for 5v. I have read stories about smoke and blown devices if both the USB and external 5v is connected at the same time, with ANY board like this, but I have not experienced that with my Pro Mini (ATMEGA328P) boards. This is a new board for me though and don't want to destroy any finding out.

If the schematic poses a hazard with this, what would you think would be a lightweight way of switching or isolating the two power buses from each other?

Normally this would not be an issue with a deployed project, but with programming, I do not want to have to worry about disconnecting the project power before hand.

Any thoughts or suggestions?

AFTERTHOUGHT... Actually, I am programming my STM board using ST-Link and not the USB at all. Still would like to know though.

Thank you for your time!

Isolating / Switching to and from USB or External 5v Supply 9 months 2 weeks ago #17879

  • Jon Chandler
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I'm not quite following exactly how things are hooked up, but let me make a couple comments.

Usually, feeding 5 volts from a USB device back to the host isn't desirable. A series diode in the supply from the host is probably a good idea. Make it a schottky diode to keep the voltage drop low.

Most 3 terminal regulators don't like voltage on the output if the input is open. In fact it's rare to find one that allows this. The solution to this is pretty simple. Install a reverse-biased diode from output to input. When the circuit is powered from other than the regulator, the diode will conduct from output to input, keeping the difference in voltage between input and output within limits.
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Isolating / Switching to and from USB or External 5v Supply 9 months 2 weeks ago #17881

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Yes sir, both excellent and fairly obvious solutions I should know by now. It's one of those things that I do not run into very often and therefor do not have practice in.
I have not measured anything yet, just making assumptions.

So I listed the scenarios I would be encountering...
ST Link connected and supplying power, project board other than the STM MCU not powered externally.
ST Link connected and supplying power, project board is also powered externally.
ST Link NOT connected, project board is powered externally. This is the deployed state.

The WS2812B LED device works using the fastled library routed to the PB5 SPI MOSI pin. 5v supplied through a 1N4148 diode with a voltage drop to 4.3v. This apparently is adequate to allow the WS2812B to read the 3.3v logic level serial signal from the STM32F103C8T6 development board. The rest of the strip is supplied with external 5v (isolated from the first WS2812B) with only the data and ground connected.

What I need to do is to make sure that if the strip is connected but 5v not externally supplied that the board does not try to supply the strip. I want the external 5v supplying the strip to also supply the 5v to the board regulator in the deployed scenario. I will work this up in a schematic before hooking it all up.

Thank you Jon!
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