EECE 312L Notes Experiment 3 Notes, Fall 2013

Experiment 3 Voltage Dividers and Thévenin’s Theorem  How do we find the bleeder current I1? We first connect the circuit of Fig A.1 maintaining a constant voltage of 10V at the input. With zero load current, (i.e. rheostat open), we measure the bleeder current I1 (in mA). We also measure and record the voltages VB to ground, and VA to ground.  We measure the bleeder current I1 with the use of the digital multimeter.  All currents in the circuit must pass through the ammeter. We connect them (components- especially the bleeder current I1 and the ammeter)) in series not in parallel because if we connect them in parallel the current would be divided.  An ammeter must have a low resistance.  To measure the current, the circuit must be broken to allow the ammeter to be connected in series. The connection of the ammeter How do we find the load resistance R for different load conditions? Definitions:  A rheostat is an adjustable resistor used in applications that require adjustments of current or the varying resistance in electric circuits.  The voltmeter= potentiometer is what measures unknown voltages by balancing it. We connect the rheostat in the circuit and adjust it to draw 2 mA by the digital multimeter of load current while maintaining V=10V, measured. We measure and record the bleeder current and the voltages VB and VA. We open the load resistor RL, with out varying the setting of the arm. We measure and record the resistance to which it was set to draw 2 mA of load current. We reconnect RL after this measurement (by the digital multimeter). In the circuit of figure A.1, how can we find a bleeder current of a certain value without changing the given resistances or using any new ones? Connect a rheostat across the bleeder resistors. Connect the ammeter across nodes B and C (in series with I1) and vary the arm of the rheostat until we get the required current (2mA for example) on the ammeter as a bleeder current. Remove the rheostat from the circuit and measure its resistance value by plugging the probes in the socket in the digital multimeter and press the ohm bottom. How do we get the required value of a certain resistor? If we didn’t have the required design-value resistor, we adjust a potentiometer connected as a rheostat to the desired value, or make up the resistor from a combination (series, parallel) of the other resistors. Or simply measure the value using a digital multimeter (according to the current value). How does the load current vary with the load resistance R? Explain why. The load current decreases as the load resistance increases. This is a logical observation since current and resistance are inversely proportional with respect to voltage (By Ohm's law). What is the effect on bleeder current I1 as the load current increases? Explain why. As the load current increases, the bleeder current I1 decreases. Even though the source current is not constant, we can see that the general sense of linear relationship between the three currents conserved: IS = I1 + IL. What is the effect on the voltages VA and VB at the divider taps as the load current increases? Explain why. Both VA and VB decrease as the load current increases. This is valid since these voltages are constant multiples of the bleeder current and the bleeder current decreases with the increase in load current. What are the methods for finding Vth and Rth? Method 1: Measure open circuit voltage (Vth) and put a short circuit in the place of the supply to measure Rth (across A and B) with a digital multimeter. So open circuit Vth and keep RL in place and measure Rth using multimeter across A and B. Method 2: Apply two different resistors on RL and measure the voltage across it. In the circuit below: We find th L th L L R R V R V    1 1 1( ) and th L th L L R R V R V    2 2 (2) Solve two equations with two unknowns and find Vth and Rth. Method 3- The Matched Impedance Method: We use a potentiometer as a variable resistance between the AB terminals of the circuit of the above figure. We vary the resistance until load voltage drops to half of the measured VTH (open-circuit voltage.) Then we disconnect the load resistance and measure its resistance with the multimeter. RL=Rth → VL=Vth/2 and thus Vth= Voc and RL=Rth Simplified Procedure:  open circuit V and measure Vth  vary the arm of the rheostat until VL= ½ Vth and this RL= Rth since Vth = VOC (from circuit C.4 and by voltage division)