Fo rc e -S e n s e S w it c h e s
______________Fo rc e -S e n s e P h ilo s o p h y
When a precise voltage must be applied to a load that
draws appreciable current, the resistance of the con-
d uc tors c onne c ting the s ourc e a nd the loa d c a n
degrade the load voltage. The resistance of the con-
ductors forms a voltage divider with the load, so that
the load voltage is lower than the source voltage. The
greater the distance between the source and the load,
and the greater the current or conductor resistance, the
greater the degradation. The resulting signal reduction
can be overcome and the signal at the load guaranteed
by using a 4-wire technique known as Kelvin sensing,
or force-sense.
4-WIRE RESISTANCE MEASUREMENT (CONSTANT CURRENT)
FORCE CURRENT
VOLTAGE
SENSE VOLTAGE
MEASUREMENT
MEASURED
RESISTANCE
V
SENSE VOLTAGE
FORCE CURRENT
CURRENT SOURCE
WIRE AND TERMINAL RESISTANCE
The basic idea behind the force-sense philosophy is to
use four wires, forcing a voltage or current through two
high-current wires to the load, and measuring (sensing)
the voltage with two separate wires that carry very low
or negligible current. One of two basic configurations is
used, depending on whether or not feedback is em-
ployed:
4-WIRE POWER SUPPLY
FORCE CURRENT
VOLTAGE
MEASUREMENT
SENSE VOLTAGE
1) The sensed voltage can be completely independent
of the forced voltage or current, as in the case of a
4-wire ohmmeter, where a constant current is forced
through one pair of wires and the voltage at the
resistor is measured by another pair.
FEED-
BACK
LOAD
V
SENSE VOLTAGE
FORCE CURRENT
2) The sensed voltage can be part of a feedback cir-
cuit to force the load voltage to the desired value,
as in the case of a 4-wire power supply. (In rare
cases, this method is also used to measure resis-
tance; the source is forced to produce a desired
volta g e in the re s is tor, a nd the s ourc e c urre nt
required to achieve this voltage is measured.)
CURRENT SOURCE
/MAX456
WIRE AND TERMINAL RESISTANCE
4-WIRE RESISTANCE MEASUREMENT (CONSTANT VOLTAGE)
In all cases, the resistance of the high-current conduc-
tors can be ignored and the sensed voltage is an accu-
rate measure of the load (or resistor’s) voltage, despite
appreciable voltage loss in the wires connecting the
source and load.
V
FORCE VOLTAGE
SENSE VOLTAGE
FEED-
There are two limitations to this scheme. First, the maxi-
mum source voltage (compliance) must be able to
overcome the combined voltage loss of the load and
the connecting wires. In other words, the conductors in
the force circuit can have significant resistance, but
there is a limit. Second, the impedance of the sensing
circuit (typically a voltmeter, A/D converter, or feedback
amplifier) must be very high compared to the load
resistance and the sense wire resistance. These limita-
tions are usually simple to overcome. The source com-
pliance is usually required to be only a volt more than
the load voltage, and the sense circuit usually has a
multimegohm impedance. Typical 4-wire force-sense
configurations are shown in Figure 1.
BACK
MEASURED
RESISTANCE
V
SENSE VOLTAGE
VOLTAGE
MEASUREMENT
FORCE VOLTAGE
VOLTAGE SOURCE
WIRE AND TERMINAL RESISTANCE
ARROWS INDICATE SIGNAL DIRECTION, NOT POLARITY
Figure 1. 4-Wire Force-Sense Measurements
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
