This is the method we use at work for pressure sensors.
:-)
_______________________________________
Roy
Tauranga
New Zealand
_______________________________________

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
Of Marcel Duchamp
Sent: Wednesday, 22 December 2004 5:53 p.m.
To: Microcontroller discussion list - Public.
Subject: Re: [EE] 4-20mA -> 0-5V + comparator

Jinx wrote:
>>If they put it near the ground end, it's possible it will run out of 
>>voltage headroom
> 
> Can you expand on that please ? I have limited experience with
> 4-20mA sensors

The current sensor you mentioned (MAX472 I think?) has a spec in the 
data sheet of 3-36Volts of power supply.  Your circuit (I'm at home now 
and don't have the email in front of me) showed, what, 10 Ohms? for the 
sense resistor?  So... 10 Ohms * 20mA = 200mV at full scale.

The pressure sensor you mentioned needs a power supply that can range 
from 10 to 36 volts.  So if you put your device at the output of the 
power supply then the pressure sensor then back to the supply, you are 
always sitting high up in the food chain.  So you would see a voltage 
signal across your sense resistor that runs from 40mV @ 4mA to 200mV @ 
20mA and is always well inside of the power supply spec for the Maxim 
current sensor chip.

That *should* (fingers crossed) take care of most problems.
MD

> 
> I think you're saying that the final (earthy end) sensor in series
> configuration might not have enough V ? If so, would the fix
> be to use multiple A->V convertors with the sensors in parallel
> across the 12V and 0V ? What about upping V ? Could do that
> failry easily
> 
> The one that he's trialling with my unit is the 15ft version of
> 
> http://www.globalw.com/products/levelsensor.html
> 
> I haven't actually sighted this sensor yet, but will be able to when
> he gets back from holiday with it (he's taken the family on holiday,
> not the sensor BTW)
> 
> If there are some gotchas we need to know about it would be helpful
> to plan for them as we progress with the project


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