ABE 404/504: Resistive Circuits Basic Laws of Resistive Circuits Circuit Components: Power Supplies Circuit Components: Resistors Basic Electrical Circuit Example • Example: A light bulb has a resistance of 3 ohms during normal operation. The vehicle electrical system operates at 13.8V. How many amps are required by the light and how much total power is consumed? Kirchhoff’s Current Law • KCL: Total current entering a junction or node is equal to the current leaving the node. • In practical use this law allows us to solve for the total current in a circuit by summing the current in each leg. Kirchhoff’s Voltage Law • KVL: In any closed loop network, the total voltage around the loop is equal to the sum of all voltage drops within the same loop. • In practical use this law allows us to solve for the voltage drop across individual elements placed in series in a circuit. Resistive Circuit Example Resistive Circuit Example Circuit Components: Diodes Why do we care about resistance measurement circuits? Consider an example of an LM335 temperature sensor: Consider an example of an LM335 temperature sensor: Continued… Consider an example RTD temperature sensor: Consider an example RTD temperature sensor: Continued… Consider an example of a strain gauge sensor: Consider an example of a strain gauge sensor: Continued… Best Practices for Resistance Based Sensors Resistance Bridge Circuits Wheatstone Bridge R1 R2 Meter R4 R3 Why is a Wheatstone Bridge Circuit better than a Voltage Divider? Why is a Wheatstone Bridge Circuit better than a Voltage Divider? Wheatstone Bridge Principles • Resistance Relationships: When R1/R4 = R2/R3 then the bridge is balanced and Vm = 0. • Bridge Sensitivity: How much does the output voltage Vm change per unit change in resistance? • Depends on the values of R1, R2, R3, and R4 • You can check the sensitivity numerically by applying known ranges of sensor resistance change. • Advantages to Using Bridge Circuits: • Change in resistance is converted into change in voltage which is easier to measure. • Allows for null balancing and zero reference measurements. • It is much easier to measure small changes in voltage from a null voltage (balanced bridge) than from an elevated voltage which can be as much as 1000 times greater than bridge voltage. • Null Method of Measurement: • Rather than using a voltage measurement device, a variable resistor (precision potentiometer) is used to adjust the known resistance in one leg until the Vm reading is zero. • Null balancing is independent of input voltage, so this should be used when the input supply is susceptible to drain.
