DC ELECTRICAL CIRCUITS VOLTAGE DIVIDERS

DC ELECTRICAL CIRCUITS A voltage divider may appear to be a series circuit but its actually a series parallel circuit when a load is applied. It = 30/2000 = .05 THERE ARE NO LOADS PLACED ON R1 OR R2 IN THIS CIRCUIT.

DC ELECTRICAL CIRCUITS If a 1000Ω load is applied to the R1 resistor it changes the output voltage from 15V to 10V which may not be enough to drive the RL load.

DC ELECTRICAL CIRCUITS In order to design a workable voltage divider the current that flows through the load must be taken into account. If we use the example of 3 loads that must be driven by a 12V power supply; First load requires………12V @ 1A, Second load requires….. 8V @ .6A Third load requires………4V @ .2A The circuit will look like…………..

DC ELECTRICAL CIRCUITS Three resistors are used to develop the three desired voltages, the values are not given, however using ohms law we can determine the values of the resistors that we need.

DC ELECTRICAL CIRCUITS At R1 there is a bleeder current, (10% of the circuit current), 2A x 10% = .2A, the bleeder current does not flow through any of the loads and R1 is called the bleeder resistor.

DC ELECTRICAL CIRCUITS We have two values at R1, R2 & R3, lets plug them into ohms law; R1= E/R or 4V/.2A = 20Ω,

DC ELECTRICAL CIRCUITS The current through R2 is the sum of the bleeder current of .2A and the load 3 current .2A, so .4A flows through R2. Voltage at the bottom of R2 is 4V and the voltage above R2 is 8V, so 4V/.4A = 10Ω.

DC ELECTRICAL CIRCUITS The current through R3 is the .4A plus the .6A load current = 1A. The voltage at the bottom of R3 is 8V and the voltage at the top of R3 is 12V, so 4V/1A =4Ω.

DC ELECTRICAL CIRCUITS DESIGNING A VOLTAGE DIVIDER; Arbitrarily selecting a bleeder current that is 10% of the total current. Using the bleeder current and the lowest voltage required by the load compute the value of the bleeder resistor. Using the total current through each resistor and the voltage dropped by the resistor, determine the resistance values required.

Series Voltage Divider DC ELECTRICAL CIRCUITS Series Voltage Divider VT is divided into IR voltage drops that are proportional to the resistance values. VR = (R/RT) ´ VT The largest series R has the largest IR voltage drop.

Series Voltage Divider DC ELECTRICAL CIRCUITS Series Voltage Divider R3 2 kW V3 = R3 RT 2 kW 6 kW = x VT x 12 V = 4 V 12 V 1 kW R2 V2 = R2 RT 1 kW 6 kW = x VT x 12 V = 2 V Note: the largest drop is across the largest resistor. 3 kW V1 = R1 RT 3 kW 6 kW = x VT x 12 V = 6 V R1 KVL check: 6 V + 2 V + 4 V = 12 V

DC ELECTRICAL CIRCUITS Redesigned Voltage Divider 10 kW 4.62 kW 40 V 10 kW 30 kW 15 kW 10 kW 60 V 10 V 5 kW Two of the divider resistors are smaller and the desired voltages are restored.

Grounded Voltage Divider DC ELECTRICAL CIRCUITS Grounded Voltage Divider C VC 10 kW B VB 15 kW 60 V A VA 5 kW GROUND IS THE REFERENCE POINT

Grounded Voltage Divider DC ELECTRICAL CIRCUITS Grounded Voltage Divider C VC = 60 V 10 kW B VB = 20 kW 30 kW x 60 V = 40 V 15 kW 60 V A VA = 5 kW 30 kW x 60 V = 10 V 5 kW GROUND IS THE REFERENCE POINT Note: VB IS THE SUM OF TWO DROPS AND VC IS THE TOTAL VOLTAGE.

Loading a Voltage Divider DC ELECTRICAL CIRCUITS Loading a Voltage Divider This voltage drops to 38.8 V 10 kW 40 V 15 kW This voltage drops to 7.06 V 60 V 10 V 10 kW 5 kW

DC ELECTRICAL CIRCUITS Another widely used series parallel circuit is the bridge circuit, bridge circuits are either balanced or unbalanced.

DC ELECTRICAL CIRCUITS An unbalanced bridge circuit will not be zero volts or zero amps. Some bridge circuits are; Wheatstone bridge- used to measure resistance. Self balancing bridge- used to control motion at a distance (roof top antenna). Temperature Sensing Bridge- used for measuring temperature.