does anyone know of any 'hobbyist' type priced (cheap) 2 or 4 wire RTDs 
available from somewhere?   i don't need them to measure exact temperature 
(i use a 1-wire ic for that) what i need the RTD for is to give me an 
indication of if the 'wind is blowing' & how much (only relatively 
speaking - not mph - just an absolute indication / variance from a 'no wind 
reference base-line') via the wind's effect on the physically exposed 
bridge.  this is in order to avoid having to use any type of mechanical 
spinning device.  the idea was to be able to apply some course measurement 
of wind loading effect on a building's heat loss vs a 'no wind day'.  maybe 
someone has another way to approach it.

 thanx

m

----- Original Message ----- 
From: "Mike Harrison" <mike@whitewing.co.uk>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Thursday, January 03, 2008 1:43 PM
Subject: Re: [EE] RTD - 3 wire and 4 wire types


> On Thu, 3 Jan 2008 09:55:38 -0800 (PST), you wrote:
>
>>I've used the 2 wire RTDs  before...essentially measure the voltage across 
>>the device for the temperature, but I've been asked to look at 3 wire or 
>>full bridge RTD's.  Now, they talked like they are one and the same but I 
>>think a full bridge is a 4 wire?  4 wire seems simple enough in that it 
>>has a constant current source to get better accuracy but what about 3 wire 
>>devices?  A current source that dumps into a common ground?  Just need 
>>some clarification on that.
>>
>
> With 4 wire, you apply current on one pair and measure voltage 
> differentially on the other. Easiest
> way is to apply a constant-ish current, and take the ADC reference as the 
> voltage across a precision
> series resistor in the excitation leg - this gives a direct reading of 
> resistance independent (
> within reason) of the excitation current, avoiding the need for expensive 
> voltage references.
>
> 3-wire makes the reasonable assumption that the voltage drop on both 
> excitation wires is the same.
>
> Assume the wires to one end are A,B and the other is C
>
> Apply current to A, with C grounded
> Measure voltage across B & C - this is the RTD voltage plus the voltage 
> drop on the C wire
> Measure voltage between A and B - this is the voltage drop on the A wire. 
> Subtract this value from
> the BC reading to get the 'real' RTD voltage value.
>
>
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