Hi,

I wish to re-design an instrument that will be used with in hazardous areas. The device will be powered from a single D cell; probably this http://uk.rs-online.com/web/p/d-batteries/6684550/. Its a simple instrument that is controlled by a pic18F, and has only one input, AI for position sensing.


My instrument shall be intrinsically safe. The current version utilises a similar battery housed in a bespoke battery enclosure, with energy limiting battery pack sticker on it. I presume this is the ia part of it, whereby there could never be a short-circuit current and voltage combination sufficient enough to provide an ignition source for the various gas groups.


I don’t know what circuitry they have used within this battery pack and wondered if anyone could propose an IC or circuit config that would suffice my reqs to limit the energy from the battery for intrinsically safe applications? Would a zenner barrier suffice. battery in > energy limited battery power out to feed my instrument??


Many thanks
G

 

What voltage and current is required by your instrument?

You can limit the current using a constant current source built from a 3-terminal regulator IC.

A simple resistor in series might also do the trick.

 

Hi,

I dissected the battery pack, and it contained only the cell, which was encapsulated in a resin or silicon product. The whole lot was contained in a small metal box just bigger than a C cell. I had presumed, that there would some sort of surge and over current protection contained within this box , however i suspect the purpose of th compound and box was to contain an explosion of the cell its self.
I was jubious to begin with anyway, as the ignition curves within IEC 60079-14 and with the voltage of battery I would be using with respect to a potential shortcircuit current, it did not come close that required to ignite the gasses anyway.

Unless anyone can rubbish my theory??

 

I cannot comment about your particular safety situation, but I want to point out that you could make a perfectly fine spark generator using just a single D cell. I suspect you could also heat a wire to make a glow plug, or to ignite the wire.

My point is, you cannot assume safe operation just because you have a "small" battery. The circuitry matters.