Embedded Files
Boulder, CO (November 2024) - This experiment created a Solderless Bread-Board prototype of an instrument droid, a device used to measure the Thevenin voltage and resistance of voltage sources as a function of output current. The prototype consists of an external DAC, an opAmp, a MOSFET, and an ADC configured as shown in Figure 1 and 2.
Figure 1: The wiring diagram for the instrument droid.
Figure 2: The prototype circuit for the instrument droid.
The device can measure the voltage drop on a voltage source, referred to as a voltage regulator module (VRM), with a known current load provided by the circuit. Without the current, the voltage on the VRM is the Thevenin voltage. Knowing the voltage drop and current, the Thevenin resistance can be calculated.
The external DAC (MCP4725) generates a voltage equal to the voltage across the sense resistor which controls the current through the MOSFET. Two channels of the external ADC (ADS1115) create the differential voltage measurement across the sense resistor. The ADC also creates a differential measurement for the voltage of the VRM. However, since the ADC can only measure up to 5V while the device needs to be able to measure voltages up to 12, a 3:1 voltage divider was added using a 10K and 5.6K resistor, similar to the wiring diagram shown in Figure 1.
The current across the sense resistor and the VRM voltage are measured with the MOSFET off and active. This provides the Thevenin voltage and the measurements required to calculate the Thevenin resistance. Using a 1Ω resistor, the characteristics of a function generator were measured and compared to the known value of 50Ω to confirm the device was working properly. Figure 3 shows the voltage drop over the sense resistor incrementing upward as the application increases the current. Table 1 summarizes the application output for several devices using a 10Ω sense resistor. The data acquisition data points for these devices are plotted in Figure 4.
Figure 3: Voltage across the sense resistor increases as the application runs indicating that it works as expected.
Table 1: Thevenin voltage and resistance for four devices using a 10Ω sense resistor. The function generator was used with a 1Ω sense resistor and was treated as a control device with a known Thevenin Resistance of 50Ω. The measured Thevenin resistance for the function generator indicates the instrument droid is working.
Figure 4: Current versus Resistance data points for the instrument droid across three devices. The current is increased at each step. The final values indicate the Thevenin Resistance.
Page updated
Google Sites
Report abuse