r/ECE • u/throwaway8490247 • 3d ago
HOMEWORK (GOOD) Help with Thevenin equivalent circuits
In advance, I appreciate the help. I know this sub has a "do your own homework" attitude, but I've been staring at this problem for hours and can't figure out how to proceed. I understand TEC circuits pretty well, and had minimal issues with every other problem, but this specific one has stumped me. I've reread my chapter and watched TEC lessons on Youtube, but can't find anything that fits this specific example.
I've got half of this solved. Finding the Thevenin Resistance is simple enough, I shorted the Voltage Sources and opened the Current Source. 1200 +3300 + ((2200*5600)/(2200+5600)) is ~6080 ohms.
Where I'm running into an issue is finding the Thevenin Voltage. If the Load Resistor is opened, then it seems to me that the entire circuit is open. If the circuit is open, then the current source has nowhere to flow, and I can't find the solution. I would convert the current source to a voltage source if I could, but I don't think that can be done, because there is no parallel resistor.
The best solution I've come up with is to find the voltage at the junction between the two voltage sources and their resistors, while treating the current source as non existent because it doesn't have anywhere to flow to. However when testing this result (12.41 V) in a simulator it doesn't seem to work. What am I doing wrong?
Edit: Solved, Thank you everyone for your help. If anyone is having trouble with a problem like this themselves, I worked it out and posted the steps in the comments below.
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u/tombo12354 3d ago
To add to this, setting up two KCL loops, the voltage drop across the 3.3k resistor is apparent, as it's just the 5mA current source feeding it in that loop. But that's just the voltage drop across the resistor: to figure the voltage relative to ground, you need the voltage at the junction of the 2.2k, 3,3k, and 5.6k, which you can figure out from the other KCL loop.
Be careful with your signs (both voltage and current). I'd imagine that part of the intent of this problem, given two voltage sources of opposite polarities. While you only need to find the voltage once, you can check the voltage a the resistor node by calculating it off both voltage source. That should help with checking the polarities.