On May 24, 2008, at 5:58 AM, Spehro Pefhany wrote:
> I'd look at approaching this problem backwards and use something
> like 4AA cells to supply 6V and then step the voltage up to 18V for  
> when
> the 4-20mA sensors are required (presumably only briefly and  
> periodically).

I have bought one of those "battery operated irrigation controllers  
and valves", which manages to operate for close to a year off 6V  
worth of AA batteries.  It turns out that these use special valves  
(three wires) that get an "on" pulse and and "off" pulse of 20V for  
20ms or so, and the controller has an internal boost circuit to  
generate the 20V, but only when needed.  I was relatively impressed...

BillW

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