How to protect 5v supply from 5v sensor connection?

[MagnusBomba]

I have a board that has a 5v rail for microcontroller (and other electronics). I also have an external sensor that I need to provide 5v to. I am concerned, that if I connect the 5v from main rails directly to the sensor, the entire board will go (in case of fault/short). Previously I have "solved" this by deriving another 5v supply and using only for the sensor (obviously this is crud, expensive and not needed). My question is, what type of circuit can I have, that will allow me to power the sensor from the same main 5v rails, but protect the main board if the sensor supply is shorted?

 

[ScottyUK]

How much current does the sensor require and how sensitive is it to voltage variation? You really haven't given enough information to give a decent answer. And if,say, a 78L05 is 'expensive' then give us a budget to work against. A zener and resistor isn't that much cheaper than a regulator.



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If we learn from our mistakes I'm getting a great education!

 

[VEBill]

A fuse.

Or any more-complicated, more-modern, solid state functional equivalent.


But I'd just use a fuse. Someone needs to repair the shorted sensor and/or wiring anyway. So they could replace the fuse at the same time.

 

[MagnusBomba]

Hi ScottyUK. OK, sorry. I am using a LM2575-5 for the 5v and the sensor does not require much current (I don't know how much, but not much, it is a Hall Effect device). So regardless of budget for a moment, do you advocate adding another regulator? Or would you be comfortable using the same 5v supply as the board?

 

[itsmoked]

You still haven't said enough.

If the sensor is only inches from the board in a benign electrical noise environment you need nothing.

If you want something to protect your supply use a PTC resistor that will operate and limit current if a short occurs. This would require a board power down and up to reset the PTC. (Much better than a fuse hunt!)

If you fear noise entry, include an RF choke(bead) followed by a 0.01uf ceramic cap to ground.

Twist all the leads to the sensor.

Use shielded cable. Ground it only at the board.

Keith Cress
kcress -

http://www.flaminsystems.com