The document discusses the electrical connections of force transducers, specifically strain gauge load cells. It explains that strain gauge load cells use a Wheatstone bridge circuit to convert variations in electrical resistance caused by applied force into proportional voltage signals. A four-wire system connects power and output signals, while a six-wire system adds sense wires to compensate for voltage drops in long connecting cables. Key electrical specifications for load cells include excitation voltage, full-scale output, input/output resistance, and temperature effects. Proper electrical connections and signal conditioning are required to accurately measure force with load cells.

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Contact Email: contact@tacunasystems.com
Website: https://tacunasystems.com/
All about electrical connections of
force transducers
Introduction
A force transducer is a sensor that transforms the physical parameter –
force, load, weight, into an equivalent analog electrical voltage signal that is
proportional to the magnitude of the force. Force transducers are also
called load cells.
There are different types of load cells, each with its unique mode of opera-
tion and construction.
These types include:
 pneumatic load cell
 hydraulic load cell
 strain gauge transducer
 piezoelectric transducer
The pneumatic and the hydraulic load cells are force balance devices that
indicate the size of the force by the pressure it exerts on a transmitting me-
dium like fluids or gases. Hence, they are not really transduction devices,
except a pressure transducer is fixed to the pressure output thereby mak-
ing the whole system bulky and expensive.
The piezoelectric transducer uses the piezoelectric effect that makes a ma-
terial generate electric charges when subjected to a stress. This device,
however, can only support dynamic loading conditions; this is because the

2. device works like a capacitor, but this capacitor has a large internal re-
sistance, thereby causing rapid charge decay over a period of time.
The most commonly used of force transducers is the strain gauge load cell,
as it has a moderate combination of the good characteristics of the force
balance devices and that of the piezoelectric transducer. Therefore, the
thesis of this article is centered on its electrical connections.
The Internal Electrical Setup of the Strain
Gauge Load Cell
The major element working this transducer is the strain gauge which is a
length of a flexible conductor – metallic or semiconductor, attached or mi-
cro-machined to a substrate which may be a polyester or any non-conduct-
ing layer.
Together, the conductor and the substrate form a flexible material – the
strain gauge that is then bonded to an elastic structural element – like a
beam that makes up the force transducer. The bonded strain gauge is fixed
such that its long length lies along the direction of application of the force to
be measured.
There are strain gauge designs that use multiple strain gauges with each
attached to different parts of the structural element; the gauges could also
be fabricated on a single substrate and then placed on a central location on
the structure as shown in figure 1 below. In general, multi-gauge systems
have improved sensitivity because the number of the strain gauge is di-
rectly proportional to sensitivity.
Figure 1. A Multi-Strain Gauge Setup

3. Under the influence of a force applied to the loading point of the transducer,
a stress is exerted on the elastic structural member that structure deflects;
the structural member acts as a primary sensor.
The deflection then creates a local strain along the length of its body. The
strain gauge is bonded such that it lies around the region of maximum
strain on the body of the structural member. The developed strain causes
the strain gauge to deform in geometry hence causing variations in electri-
cal resistance.
It is the variation in the electrical resistance that is then measured and
simply read out by a sensitive voltage meter or manipulated into variations
in voltage that can be further processed to calculate the magnitude of the
applied force. A Wheatstone bridge is the common approach to manipulat-
ing this changes in resistance. The Wheatstone bridge is more like a volt-
age divider that has four arms as shown in figure 2 below.
Read:
 Load Cell FAQ
 The Essential Guide to Load Cells
 How to Test for Faults in a Load Cell
 Can you use a Single Load Cells to Measure Constant Weight?
 What is the Minimum Weight a Load Cell Can Sense
 How to Calibrate the Measuring Chain
Figure 2. A Wheatstone bridge setup.

4. Consider a case: Measuring the changes in the resistance of a single
strain gauge. The strain gauge is simply fixed into one arm of the Wheat-
stone bridge; hence it is no different from a resistor, in fact, it is a variable
resistor.
It should be noted that even though the strain gauge changes in resistance,
it normally has a base resistance value; to make design easier, the resis-
tors in the other arms of the bridge are made to have the same resistance
as the strain gauge.
This type of Wheatstone bridge configuration is called the Quarter-bridge
configuration.
There are other configurations based on the number of strain gauges pre-
sent in the arms of the bridge; multi-strain gauge electrical configurations
are half-bridge configurations and full-bridge configurations.
Half-bridge
The half-bridge configuration consists of two strain gauges fixed into two al-
ternating arms of the Wheatstone bridge. The strain gauges can be
mounted on the structural element at high strain location to measure axial
or bending strain. It gives a double sensitivity value.
Full-bridge
The full-bridge configuration has four strain gauges fixed into the four arms
of the Wheatstone bridge. They are connected such that two strain gauges
measuring the compressive strain are fixed on alternating arms while the
remaining two strain gauges measuring the tensile strain are fixed into the
other two arms of the bridge.
The electrical resistance and the voltage output sensitivity of a strain gauge
varies with temperature, hence, there is a high possibility of measurement
errors. Therefore, the designs need to minimize the effects of these tem-
perature changes on resistivity. Some designs make the strain gauges ma-
terials to be self-temperature compensating; some make use of dummy
gauges – especially in the Quarter-bridge configuration; here the dummy
gauge is placed in the arm alternate to the strain gauge. The other designs
use a temperature compensated multi-strain gauge bridge configuration to
cater to temperature shifts.

5. Electrical Wiring of Force Transducers
Intuitively, looking at the underlying Wheatstone bridge connection of a
strain gauge transducer, one would see that the minimum number of elec-
trical wires coming off the device is four; that is by default, it is a four-wire
system.
In the four-wire system, there are two wires for power supply – positive and
negative terminals, to the Wheatstone bridge; the other two wires are the
signal output terminals of the bridge – the positive and negative output ter-
minals. This wiring system is shown in figure 3.
Figure 3. A 4-Wire Circuit
Now, most textbooks and articles always show the lumped parameter mod-
els of the strain gauge Wheatstone bridge and its connections. The lumped
parameter model ideally assumes that the connecting wires to all these ter-
minals on the Wheatstone bridge have zero resistance, hence zero voltage
drop.
However, in practice, what is applicable is the distributed parameter model:
here, the connecting wires actually have resistance to the flow of current,
hence, there are voltage drops across the wires. Also, since the connecting
wires have resistance, then it means their length, cross-sectional area, and
resistance variation with temperature needs to be considered and catered
for by design
The distributed parameter model is why there is a six-wire strain gauge
system. The six-wire strain gauge load cell has the normal power sup-
ply/excitation and signals output terminal wires; in addition, there are the
sense terminals – positive and negative terminals. The sense terminal
wires are connected at one end to the nodes where the power supply wires
are also connected as shown in figure 4 below.

6. Figure 4. A 6-wire Circuit.
It is set up so that the other end of these sense terminals are connected to
the input port of an amplifier, the output of the amplifiers is then connected
to the power supply terminals of the load cell – Wheatstone bridge.
This way the actual voltage powering the load cell is detected and the am-
plifier makes the necessary adjustment to keep the voltage supply at the
desired operating level irrespective of the voltage drop across the cables.
So, there is a simple voltage controller design integrated with the six-wire
system.
This shows that a four-wire strain gage load cell is easily affected by the
varying resistance of the supply wires; the six-wire type is not affected be-
cause it is possible to adjust the cable length without creating any error; the
amplifier will make the needed gain adjustments to the power supply.
In conclusion, an additional wire may be present in both types, this wire is
called the shield wire. The shield wire is not hooked to the strain gauge ra-
ther it is connected to the body of the transducer. It protects the internal cir-
cuitry from electromagnetic interference.
Important Electrical Characteristics
The various electrical characteristics for a strain gauge transducer can be
found in the product’s datasheet. The following are some of these specifi-
cations:
 Recommended Excitation Voltage: this is the electrical voltage that
should be applied to the transducer. It usually has a range of about
10V to 15V max. therefore, a 6 wire load cell will maintain this level of
voltage regardless of cable voltage drops
 Full-Scale Output: this is the specification that determines the out-
put voltage for a certain excitation voltage and applied load. It can
also be called the sensitivity of the transducer. The unit is mV/V and
common value of 2mV/V at rated capacity.

7.  The Zero Balance: this is the value that determines the transducer
output voltage when no force is applied. The unit is also mV/V.
 Input Resistance: this is the resistance value obtained at the supply
terminals of the force transducer at standard test condition, zero
loads, zero excitation and with an open circuit output terminals.
 Output Resistance: the resistance value obtained at the signal out-
put terminals under standard test conditions, zero load, zero excita-
tion, and open circuit input terminals.
 Insulation Resistance: this is the resistance measured between the
body of the transducer and the interconnecting node of all the de-
vice’s wires. A good transducer should have an insulation resistance
that is greater or equal to Two Gigaohms.
 Temperature Effects: the temperature effects on the sensitivity and
zero balance is a measure of how the output voltage changes with
respect to temperature variations when the rated load and no-load is
applied respectively. There is also the temperature range value which
shows the range of temperature within which measurements must be
taken to ensure accuracy.
Conclusion
The various issues relating to the electrical connections of a strain gauge
transducer have been shown in the body of this article. Although it was
specifically for strain gauge load cells, the conceptual understanding can
also be applied to other transducers like the pressure and piezoelectric
transducers; for example, the compensating techniques for the influence of
power supply cables.
Furthermore, a force transducer always needs output signal amplification
because the output electrical signal is always small – mV for strain gauge
load cells and pC for piezoelectric transducers. Instrumentation amplifiers
are used for strain gauge transducers while charge amplifiers are used for
piezoelectric transducers. Other signal conditioning processes – such as
noise filtering and isolation – are also carried for better accuracy.
Finally, every transducer needs to be calibrated so that systematic errors
eliminated for improved system accuracy.
Sources
 The Instrumentation Reference Book Edited by Walt Boyes
 Force Measurement Glossary by Tacuna Systems.

8.  Load Cell Material by Tacuna Systems.
Phone Number: 1-800-550-0280
Contact Email: contact@tacunasystems.com
Website: https://tacunasystems.com/