Circuit Theory/Lab4.5.1
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Example25a.png/220px-Example25a.png)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/6/60/Example25b.png/220px-Example25b.png)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Example25h.png/220px-Example25h.png)
![](http://upload.wikimedia.org/wikipedia/commons/8/83/Example25i.png)
Example, find the thevenin equivalent of this circuit, treating R7 as the load.
- Simulate the circuit, displaying load voltage and current as the load is swept through a range of resistance values
- Simulate the thevenin equivalent circuit and again sweep the load voltage and current through a range of resistance values
Finding Thevenin Voltage
[edit | edit source]Open the load (resistor R7), and find the voltage across it's terminals.
R5 and R6 are dangling and can be removed.
Vth = VA - VB
VA = 2-VR1 = 2 - (2-5)*2.2/(2.2+4.7) .. voltage divider
VA = 2.9565
VB = 5-VR3 = 5 - 5*6.8/(6.8+6.8) .. voltage divider
VB = 2.5 volts
Vth = 2.9565 - 2.5 = 0.4565 volts
Can check with this simulation.
Finding Thevenin Resistance
[edit | edit source]Remove the load, zero the sources.
Redraw up and down so the parallel/serial relationships between the resistors are obvious.
Finding Norton Current
[edit | edit source]IN = Vth/Rth = 0.4565/7.3986 = 0.0617 amp
Simulating the original circuit
[edit | edit source]In the simulation, can see the computed Norton's current when the load is 0 ohms.
Can see the computed Thevenin voltage when the load is around 20 ohms which approximates an open.
Comparing with the Thevenin Equivalent
[edit | edit source]In this simulation, can see the same values, except this time the load voltage is relative to ground, so don't have to look at a drop or differences between two voltages as with the original circuit simulation.
-
Sweep simulation of original circuit for R7 values of 47 ranging from 0 to 20
-
The thevenin equivalent with the R7 load in place
-
Sweep simulation of the thevenin equivalent circuit for R7 values ranging from 0 to 20 ohms