Protecting Vcc when needed externally?

I have a box that runs on 3.3v and 5v rails and I need to provide these same voltages externally also (via connectors) to power sensors and an auxiliary unit that is connected to the main box and draws power from it. Since this is exposing the Vcc directly to the external world, how do I protect them against accidental things like shorts to ground, or application of a higher voltage? I have attached a drawing to explain the issue. Thanks!

Have a current sensor in the rail and switch off whenever mall function or exceeds rated current.

Protection against...

... shorts between Vcc and GND. Put fuse on a power line before the connector. There are resettable fuses, which reset after some time automatically when over-current condition disappears.

If the supply current is always low, you can put a resistor instead of fuse. Then even if Vcc is shorted to GND at the connector, the power supply is shorted through a resistor, which would be better than a direct 0Ω short. Value and power rating of the resistor is determined by Vcc voltage and current going out of the box. Also pay attention whether by design a resistor fails short or open.

... overvoltage, high voltage transients. TVS or Zener diode upstream of the fuse.

If I needed 5V and 3.3V for my controller and sensors, I might consider 2 power supplies from the 30V feed, one for the controller and another for the sensors, especially if the controller is critical and must never crash. The one for the sensors would have additional current and voltage protection and limiting and have the ability to shut down or reboot. I would say the sensor power supply should only be turned on if the main controller power supply is up and running, and some voltage regulators do have an ENABLE input to turn them on/off to do this.

You might want to look at some USB Power and Protection IC's since USB is designed for 5V bus. This app notes shows the polyfuse and overvoltage protection concept.
https://www.littelfuse.com/data/en/Application_Notes/ec616.pdf

There are some smart power switch IC's available that have current protection and feedback to a microcontroller when the over-current happens. If you sensors are below 50 / 100 / 200 mA, then consider these types of intelligent MOSFET devices or some type of similar concept.

I've used these devices in the past for power to a smartcard, but they are limited to either 50mA or 100mA.
https://www.fairchildsemi.com/ds/FP/FPF2004.pdf

Another device can supply 200mA plus has reverse current protection:
https://www.fairchildsemi.com/ds/FP/FPF2174.pdf

There is a voltage drop through the above smart switches, so you might need to run the power to it a little higher, thus it depends on the operating voltage range you need at your sensors.

You should browse the Fairchild web site for other similar products too.

Hope this gives you some ideas of things to consider.

P.S. You don't specify any details for the data lines: speed, logic type, ... but you should protect them too. You might be able to use IC chips that are meant to protect other common buses, like USB / RS232 / HDMI / Video / eSATA / SmartCards / SD Flash / ...

--- Enlightenment ---

Thanks Enlightenment. Yes I have been wondering myself about drawing from the 30v main supply also for the other units. One of the things I wanted to try and do is reduce the number of cables by passing the power directly from the main unit on the same cable that has the data (although I guess I could pass the 30v that way also... hmmm...).
As for the data lines they are just serial communications, some RS232 and some TTL UART. Oh and also SPI.
By the way any idea about the SP720?

Enlightenment said:

If I needed 5V and 3.3V for my controller and sensors, I might consider 2 power supplies from the 30V feed, one for the controller and another for the sensors, especially if the controller is critical and must never crash.

--- Enlightenment ---

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