Hi there, we've just done an experiment at college based on proving Thévènin's theorem. However the practical results obtained did not match that of the theory. I've attached a picture of the circuit running in multisim and the results generated in this match the theory exactly. The voltage measured across the load practically was higher @ 10.45V and the current though the load was 32.36mA. The power supplies used to generate the voltage sources were Rapid PS3025 variable o/p bench power supplies. When the power supplies were swapped for Farnell E30/1 PSU's the voltage measured across the load came down to 10.2V and the current passing though came down to 31.34mA. The resistors measured 33.1Ω, 43.3Ω and 327Ω so weren't way off value. Why are results not matching the theory? Is it because the 8.4V PSU doesn't like having current passed the wrong way though it?
Any thoughts appreciated.

 

I would not be surprised if your voltmeter accuracy is at the bottom of this mystery. Consider the fact that your voltmeter is not ideal. You will need to check the user's manual for your meter to determine its accuracy.

hgmjr

 

Can we assume you set the voltages unloaded and the supply can handle the load when it is attached?

Can we assume you measured your resistors prior to inserting them?

Have you looked at your meter specifications?

After you put in the "measured" values of the resistors into the simulator ... did they match?

Sources of the error:

• test equipment calibration
• power supply(s) not capable of delivering a constant voltage at the desired load
• the resistors are not the stated value ... but within the tolerance stated
• the wiring characteristics
• the connections
• and anything else I can't think of right this minute ... related to the construction of your circuit

 

Voltmeter accuracy shouldn't be an issue as I used my own Fluke 117 DMM with a basic 0.5% DC accuracy and also used the DMM's avialble at college. The input impedence of the DMM is 10MΩ which I set to be the same in Multisim. I ran the simulation again using the actual mesured values of the resistors and it didn't sgnificantly effect the results. The voltage was set on the PSU's off load and the PSU's can easily handle the current demand placed on them by this circuit. As far as cable conections being an issue, I suppose it could be a problem as the circuit was constructed using breadboard. I would tend to belive that it was a PSU issue though. There were 5 groups running this experiement and all got voltages that were to high, however i cannot vouch for the accuracy of their setups. The only explination our instructor could think of was that Thévènin's theorem assumes a voltage source to be of a minute impedence ie s/c, perhaps our PSU's had a higher internal impedence?

 

Are you using the DMM to measure the voltage setting of the power supplies or are you relying on the power supply's output voltage display to set its voltage? If you have not done so, I would use the DMM to confirm that the power supply outputs are close to dead on.

That way you are using a common reference throughout the measurement sequence.

hgmjr

 

yes the PSU's were set using a DMM

 

Everyone's readings being high is very suspicious.

Do you have a table of your readings?

 

I only have my own results,

​

2 x Farnell Power supplies.​

Rapid/Manson power supply combination.​

VTH​

11.4 V​

11.4 V​

Voltage across load​

10.12 V​

10.45 V​

Current drawn by load​

31.34 mA​

32.36mA​

 

Are you taking into account the impedance characteristics of your PSUs?

 

how would you asertain the exact impedence values?

 

Hook the terminals of your DMM to the PSU terminals in a purely resistive circuit of known resistance.