
Find strain gauges for your measurement here.
By the parallel connection of a resistor Rp to a strain gauge, a defined detuning of the measuring bridge can be brought about.
This can be used for a functional test.
Alternatively, this method can also be used to adjust the output signal of the Wheatstone bridge to 0 mV. For this purpose, the shunt resistor is permanently inserted into the strain gauge bridge circuit, or alternatively into the connection terminals of the measuring amplifier.
The parallel connection of a shunt resistor Rp to one of the four bridge resistors R results in a resistance change ΔR:
Reshaped as a relative change in resistance you get:
With the bridge equation for the strain gauge quarter bridge
Setting equation 3 into equation 2 results in:
and reshaped as Rp:
With Equation 5, it is now possible to calculate the required shunt resistance for a given bridge detuning Ud / Us.
For a negative effect of the shunt resistor set it parallel to R1 or parallel to R3.
For a positive effect of the shunt resistor, it is arranged parallel to R2 or parallel to R4.
Rule of thumb: for approx. 1mV/V bridge detuning on a 350 ohm bridge you need approx. 100 kOhm.
R in Ohm | Ud/Us in mV/V | Rp in Ohm |
∼ Rp in kOhm from E12 |
---|---|---|---|
350 | 0,5 | 174650 | 180 |
350 | 1,0 | 87150 | 82 |
350 | 2,0 | 43400 | 47 |
350 | 4,0 | 21525 | 22 |
R in Ohm | Ud/Us in mV/V | Rp in Ohm | ∼ Rp in kOhm from E12 standard series |
---|---|---|---|
120 | 0,5 | 59880 | 56 |
120 | 1,0 | 29880 | 27 |
120 | 2,0 | 14880 | 12 |
120 | 4,0 | 7380 | 8,2 |
R in Ohm | Ud/Us in mV/V | Rp in Ohm | ∼ Rp in kOhm from E12 standard series |
---|---|---|---|
1000 | 0,5 | 499000 | 470 |
1000 | 1,0 | 249000 | 270 |
1000 | 2,0 | 124000 | 120 |
1000 | 4,0 | 61500 | 68 |
When calculating the shunt resistance in equation 5, the linearized form of the bridge equation was used:
or for the quarter bridge with only one active strain gauge R = R1 = R2 = R3 = R4
The exact solution for the quarter bridge is:
The additional term 1 / (1 + ΔR / 2R) takes into account the nonlinear component.
or with c = 1/(1 - 2·ΔUd/Us) and eq. 4 and resolved according Rp:
The bridge detuning calculated with the linearized bridge equation is too large by the factor c.
For a linearized calculated strain of 1000 μm/m, the exact strain is 999 μm/m. The error is about +1 μm/m (+0.1%).
With 87150 ohms in parallel to a 350 ohm strain gauge, the bridge is detuned by 0.998 mV/V. This corresponds to an elongation of 2000 μm/m with a k-factor of 2.
Rv is composed, e.g. from 2x 1 ohm line resistance plus 2x 20 ohms nickel resistor plus 2x 10 ohms fixed resistor = 62 ohms. Taking into account the additional voltage divider, which leads to a reduction of the supply voltage Us at the Wheatstone bridge, results for the bridge equation of the quarter bridge (with R = R1 = R2 = R3 = R4):
or c = 1/(1 - 2·ΔUd/Us) and eq. 4 and eq. 10 and resolved according Rp:
The red components take into account the non-linearity bridge circuit, the blue components take into account the influence of the series resistors.