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

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MagnusBomba

Nuclear
May 16, 2009
8
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?
 
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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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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.


 

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?
 
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 -
 
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