1. Introduction In engineering practice, stress change measurement is a crucial and demanding field. Since strain measurement involves measuring weak signals, requiring the detection of changes in tens of microohms, differential bridges, constant current sources, or constant voltage sources are often used to reduce nonlinear errors. The precision of the constant voltage/constant current source directly determines the precision of the strain measurement. This necessitates the use of high-precision constant voltage/constant current sources. 2. Strain Measurement Principle and Requirements Strain gauges measure strain by utilizing the relationship between the resistivity of the resistance wire and its deformation, converting mechanical parameters into proportional electrical parameters. The strain gauge induces a change in resistance during operation. This minute resistance change in the bridge is converted into a change in voltage or current, amplified, and then, based on a certain proportionality constant, transformed into the strain surface of the specimen. The instrument that performs this function is called a strain gauge. Its principle is shown in Figure 1. The mechanical strain sensed by the strain gauge is generally 10⁻⁶-10⁻², and the resulting resistance change rate is also approximately in the range of 10⁻⁶ to 10⁻². Such a small resistance change is difficult to detect with ordinary resistance measuring instruments, so a certain form of measuring circuit must be used to convert the tiny resistance change rate into a voltage or current change so that it can be recorded or displayed by electronic instruments. The measuring circuit must meet at least the following two requirements: (1) The measuring bridge and its excitation should have sufficiently high accuracy and be able to flexibly control the excitation size according to different test requirements. (2) It should be able to convert the minute resistance change rate into a voltage or current change and have sufficiently high sensitivity. This article discusses in detail the design of the generation, control, and drive of a precision programmable constant voltage and constant current source. 3 Excitation Source Scheme Design The strain gauge has very high accuracy requirements for the bridge source, so it is important to ensure that the accuracy of the power supply meets the requirements during the power supply design process. In addition to meeting the accuracy requirements, in order to meet different application scenarios, the strain gauge also requires the power supply to provide two different bridge forms: voltage source and current source. When analog signals output by instruments need to be transmitted over long distances, current signals are generally used instead of voltage signals because current signals have strong anti-interference capabilities, and the resistance of the signal line does not cause signal loss. The current magnitude is determined by the load size, typically ranging from 0 mA to 20 mA. When analog signals output by intelligent instruments need to be transmitted to multiple instruments, DC voltage signals are generally used instead of DC current signals. This is because if DC current signals are used, their input terminals must be connected in series to ensure that multiple receiving devices receive the same signal. This introduces an unreliable factor: when any receiving device experiences an open-circuit fault, the other receiving devices will also lose their signals. Moreover, the individual receiving devices connected in series will also lose their signals. Furthermore, the unequal potentials of the individual receiving devices to ground can also cause problems. To increase the flexibility and versatility of strain gauge applications, a programmable power supply was designed and developed. A programmable power supply is one that can automatically control the output (voltage or current) of a power supply device according to a pre-set program and values, stabilizing it at a given value. To meet the different requirements of the resistance strain gauge for the bridge source in this design, the following excitation source design parameters are given: (1) Constant voltage source: 0V～8V, output current stability better than 0.01%. (2) Constant current source: 0mA～20mA, output current stability better than 0.02%. 4 Programmable constant voltage source reference circuit As shown in Figure 2, in this module, in order to ensure the design accuracy, we selected a 16-bit programmable D/A converter, so that we can make its step size 10W (216) ≈ 0.152mV. Therefore, when our voltage excitation is 2V, the maximum error of the 16-bit DAC voltage output is, so it fully meets the system design requirements. The DAC we selected is DAC 7731EC, and it is accompanied by a MAX 6225 2.5V external reference voltage source to minimize the nonlinear error. The maximum error is only 1LSB. At this point, we have obtained a high-precision constant voltage from the DAC. However, due to the DAC's poor load-driving capability, it cannot drive the subsequent V/I selection circuit and constant voltage/constant current source drive circuit. Therefore, we need to add a drive circuit after the DAC output. Here, we choose the high-precision operational amplifier OPA 227. As can be seen from the diagram, we are using an open-loop connection, which achieves both maximum voltage to control the gate (G) terminal of the MOSFET and voltage isolation. 5. V/I Selection Circuit and Drive Circuit Since the same load bridge requires either a constant voltage source or a constant current source under different conditions, we need a resistorless (to ensure no voltage drop occurs on the load bridge source) and precise V/I selection switching circuit in the bridge source section. Simultaneously, when using a constant current source on the load, a low-drift, high-precision, and high-power drive circuit is required, which is difficult for general precision instrumentation amplifiers to simultaneously meet. Therefore, in this paper, we adopt a novel method to generate a high-precision, high-current constant current source. 5.1 The constant voltage source drive circuit is shown in Figure 3. Assuming the standard voltage value of the DAC output is 8V, a 12V voltage is output from the output terminal of the precision amplifier OPA 227 (connected in an open loop) to the gate (G) terminal of the field-effect transistor IRF 840 to control its switching. Simultaneously, the same standard voltage value of 8V is output from the negative terminal of OPA 227 to the COM terminal of the relay. The ON/OFF control signal connects the COM and ON terminals of the relay. Since the internal resistance of the relay is very small and negligible, it can be considered to have no voltage divider effect. Therefore, the voltage at one end of the external load bridge is the standard 8V, while its other end is grounded through the relay. Meanwhile, for the G and S terminals of the IRF840, their voltages are VGS=4V and VDS=4V. Based on the output characteristics of the MOSFET, a maximum current of 1 A can flow through the MOSFET at this voltage. However, in this design, the current magnitude is determined by the precise, low-error voltage value generated by the DAC (8V in this case) and the resistance of the load bridge. The +12V power supply and its protection resistor R1 only serve to provide the current. Through this setup, we can provide a precise, programmable constant voltage source for the external load bridge while avoiding noise introduced from the strain gauge's own power supply. This is because strain gauges typically use an external 220V AC power supply, which is then converted to a DC voltage source (such as +12V) internally. However, this usually leaves strong power frequency noise in the strain gauge's power supply. For a bridge testing minute strains, this noise is clearly unacceptable. The advantage of this design lies in the fact that we provide a high-precision constant voltage reference through the external load bridge generated by the DAC, and the instrument's own power supply provides the drive current through the MOSFET. This significantly improves the strain gauge's load-carrying capacity while effectively minimizing noise. 5.2 The constant current source drive circuit is also shown in Figure 4. When the COM and OFF terminals of the relay are in contact, terminals 2 of the external load bridge are grounded through a 400Ω precision resistor connected to the relay. Simultaneously, terminals 2 are also connected to the precise voltage value V-COM output by the reference voltage circuit through the relay. Terminal 1 of the bridge remains connected to the S terminal of the MOSFET. Assuming V-COM = 8 V, the MOSFET's VGS = 4V and VDS > 0V, so the MOSFET is in the ON state. According to Kirchhoff's laws, the current flowing through the load bridge, I1, is equal to the current I2 flowing from terminal 2 through the precision resistor into the ground. At this point, the magnitude of I2 can be controlled by the precise voltage value V-COM output from the voltage reference circuit and the precision resistor R2. Similarly, like the constant voltage source, the strain gauge's own +12V power supply and MOSFET only provide current to the load bridge, but cannot determine its magnitude. Therefore, we have also achieved the goal of providing a precise, low-error constant current source to the external load while effectively isolating the noise introduced by the instrument's own power supply. 6. Conclusion Through the effective combination of DAC, MOSFET, and relay, a precise, low-error, and low-noise programmable constant voltage/constant current source that meets the requirements of strain testing is formed. The DAC generates a precise reference voltage or current value for the load bridge, but the drive current for the load is provided by the instrument's own power supply. This solves the problem of weak driving capability of general bridge sources and effectively isolates the noise introduced by the strain gauge's power supply itself. At the same time, through FPGA control of the relay, we flexibly switch between the constant voltage source and the constant current source, ensuring that only one excitation source is in use at any given time, effectively reducing the circuit's power consumption. In summary, compared to other bridge sources used in strain gauges, this design is smaller, consumes less power, offers flexible configuration, and still meets measurement requirements. This design has been successfully applied to a practical strain gauge, and through actual testing and use, it has met design standards.