ILX Lightwave RTD Series Tech Note

PURPOSE
This tech note presents the proper method for
using three-wire RTD sensors with the ILX
Lightwave LDT-5900 Series temperature
controllers.
BACKGROUND
Three-wire RTD temperature sensors are
commonly used in applications where the RTD is
located a substantial distance from the
temperature controller. Three-wire RTDs can be
configured so that the resistance from the long
leads is effectively cancelled, but a specialized
measurement system is required. Figure 1 shows
a typical three-wire RTD measurement system.
FIGURE 1 – Three-Wire RTD System
Two matched current sources are required for this
application, I and I . To measure the actual
1 2
voltage drop across the RTD, I
the voltage between points A and C is measured,
(V ). Next, I is switched off and I
AC 1
on; the voltage between points B and C is
measured, (V ). Assuming that R
BC
subtracting the V from V
BC
drop across the RTD and effectively cancels the
lead wire resistances, R
1-800-459-9459 U.S. and Canada
Using Three-Wire RTDs with the LDT-5900
is switched on and
1
is switched
2
= R = R ,
1 2 3
gives the voltage
AC
, R , and R .
1 2 3
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Series Temperature Controllers
V = V + V
AC R1 RTD
V = V + V
BC R2 R3
V = V – V
RTD AC BC
= V + V
R1 RTD
(Assume V
R1
= V + V
R1 RTD
Once V is known, it is a simple matter to
RTD
calculate R using the RTD exitation current I
RTD
and Ohm's Law. This method is conceptually
simple, but difficult to execute in practice because
the two current sources must be precisely
matched.
The LDT-5900 Series Temperature Controllers
have four-wire sensor measurement systems, and
so a two- or four-wire RTD is the most logical RTD
to use. Some lab equipment is designed with an
integrated three-wire RTD, and there is no easy
way to replace it with a four-wire type. In these
cases, following the method described in this
document will result in the lowest possible
measurement error.
MEASUREMENT SETUP
Figure 2 illustrates the recommended wiring
connection configuration for three-wire RTD
sensors. This method takes advantage of the
LDT-5900 Series four-wire voltage sensing
capability to reduce the error cause by the RTD
lead-wire resistance.
+ V
R3
+ V – V – V 1
R3 R2 R3
= V = V )
R2 R3
+ V – V – V
R3 R1 R3
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TN#5900-2
1