# Basic circuit question

1. May 7, 2005

### airkapp

I'm trying to determine the Thevenin equivalent circuit for the attached circuit. Also, the value of the load resistance R load for maximum power transfer. I'm not to sure how exactly the Thevenin theorem operates but I simply added my resistor in series and parallel (where appropriate). I suppose that gave me "R equivalence" which equaled 15 ohms. Not sure if that is correct after shorting out the voltages and open the currents (zeroing out). Can someone help figure the "load resistance for max. power transfer?" I'm attaching the diagram here.

thanks,
air

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2. May 7, 2005

### s_a

The Thevenin resistance is easy to find, you disable the current (turn it into an open circuit) and voltage sources (turn it into a short circuit), and then find the equivalent resistance across RLOAD (removing the resistor RLOAD itself of course). So you get Rt = R2 // R3 = (20^-1 + 45^-1)^-1 = 13.85 ohm.

To find the Thevenin voltage, one way is to find the open circuit voltage across where RLOAD (again RLOAD itself is removed), by using superposition. The voltage across RLOAD due to the current source only is 0 (the disabled voltage source provides a path directly to ground). The across RLOAD due to the voltage source is 20/65 * 6 V = 1.85V (by voltage division).

The value of RLOAD for maximum power transfer equal to the Thevenin resistance, which is 13.85 ohms.

3. May 7, 2005

### airkapp

okay, I understand what you did, but I do not understand why you ignored the R1 value? Also, do you know how to input RLOAD into multisim?

thanks,
Air

4. May 8, 2005

### SGT

When you open the current source, no current flows through R1, so it can be dismissed from the calculations of Rt and Vt.