Thevenin Voltage Value Homework

[mrowa196]

Homework Statement

I calculated theoretical value of resistance - I assumed that R4 and R5 are in series and parallel to R2 and R3 which are in series, because I've considered B as the starting point - this is the method I was taught, but I don't know any method of calculating the voltage with the diode. Experimental value of voltage was about 3.75 V.

 

[gneill]

There's not enough data to arrive at a numerical result. Were you not given component values?

 

[NascentOxygen]

I calculated theoretical value of resistance - I assumed that R4 and R5 are in series and parallel to R2 and R3 which are in series, because I've considered B as the starting point - this is the method I was taught, but I don't know any method of calculating the voltage with the diode. Experimental value of voltage was about 3.75 V.

What are the resistance ratios you were given?

 

[mrowa196]

Oh, I am sorry, I thought it was on the diagram:

R2 = 5.6 kΩ, R3 = 2.7 kΩ, R4 = 6.8 kΩ, R5 = 3.3 kΩ

R1 is apparently not necessary.

 

[NascentOxygen]

Suppose R3 and R4 (and, for good measure, point B) were all erased from the drawing, could you calculate the voltage V_A with respect to the lower end of the zener?

 

[cnh1995]

If I understand the problem correctly, it is asked to find V_AB i.e. the Thevenin voltage. Assuming the zener diode is in reverse breakdown region, voltage across it will be 6.2V, fixed. I believe the OP has assumed correct series-parallel combinations. But the voltage value I am getting is not 3.75V, which is obtained experimentally as said. Is there something wrong with the experimental value?

 

[mrowa196]

I've also measured the current and calculated experimental resistance and it was 4.64 Ohms, so very close to theoretical value of 4.60 Ohm. Hence, I think it was pretty accurate experiment.

 

[cnh1995]

I've also measured the current and calculated experimental resistance and it was 4.64 Ohms, so very close to theoretical value of 4.60 Ohm. Hence, I think it was pretty accurate experiment.

Was the zener diode in breakdown region? I mean was the voltage across it 6.2V? If yes, I think V_AB should be 2.14V.

 

[mrowa196]

The voltage applied was 12V.

Edit: I'm sorry. I measured the current and voltage for three applied voltages: 12, 15 and 18 V, but I need the value for 12V. I need to measure maximum theoretical power transfer.

 

[cnh1995]

The voltage applied was 12V.

Edit: I'm sorry. I measured the current and voltage for three applied voltages: 12, 15 and 18 V, but I need the value for 12V. I need to measure maximum theoretical power transfer.

Still if the zener voltage is maintained at 6.2V, V_AB will be same regardless of the supply voltage. Only the value of R1 will change.

 

[mrowa196]

Or maybe there is no enough information and I need to use the experimental value of voltage. After all what I need is to compare maximum theoretical power transfer and maximum experimental power transfer. Maybe I can only compare it by using theoretical value of resistance for one and experimental value of resistance for another while the voltage value would be experimental in both cases.

Edit: I read the problem again and it says specifically I need to use BOTH THEORETICAL values...

 

[mrowa196]

What if I ignored R3 and R4 part of the circuit and I would be left with:

Then I ignore R1 and 15 V source part and I would be just left with 6.2 "source" let's say and two resistors circuit. Then, using voltage dividor rule and the values of resistances of the resistors (5.6/(5.6+3.3))*6.2. That would give 3.9V at point A which is close enough to experimental value, but I just made this method up so I think it's bullshit.

 

[gneill]

Edit: I read the problem again and it says specifically I need to use BOTH THEORETICAL values...

Can you share the actual problem statement with us?

 

[mrowa196]

It's actually a lab report, so it's a whole document, but it's only one part of it - maximum power transfer.

 

[gneill]

The lab asks that you identify the values for resistors R2 and R4. What was involved in doing that?

 

[mrowa196]

Reading from the colour codes. That was checked - they are correct.

 

[gneill]

Did you also confirm that the other resistors (R3 and R5) actually had the values shown in the lab diagram?

I'm trying to understand how you could have measured 3.75 V across AB given this circuit arrangement. If the zener really is a 6.2 V one, then I'm not seeing a way to achieve this with the given component values.

 

[mrowa196]

Yes, I have. The resistances must be correct because theoretical and experimental values of resistances are almost identical.

 

[mrowa196]

I found the photo of the circuit.

 

[gneill]

Looking at the photo, I question whether the color code for R4 is Blue-Violet-Red (6.8 k) or Blue-Violet-Orange (68 k). If it's the latter it might explain your measured voltage. It would push it to around 3.8 V.

What short circuit current did you measure?

 

[mrowa196]

0.8 mA

 

[gneill]

0.8 mA

With what supply voltage?

 

[mrowa196]

12 V

 

[gneill]

Okay, with R4 at 68 kΩ I can see you measuring about 3.75 V and 0.8 mA. With those values your Thevenin resistance should be in the kilo-Ohms, not just a few Ohms. So something around 4.7 kΩ.

 

[mrowa196]

Yes, you're right, I've actually went through my notes and there is 68000 kΩ written but I've crossed one 0, because I could not get theoretical value of Thevenin resistance close the he experimental one otherwise... Did you get it?

Experimental data:

 

[gneill]

Yes, I can calculate about 4.7 kΩ for the Thevenin resistance, which corresponds nicely with your table values.

 

[mrowa196]

Could you please walk me through your calculations?

 

[NascentOxygen]

What if I ignored R3 and R4 part of the circuit and I would be left with:
View attachment 93993
Then I ignore R1 and 15 V source part and I would be just left with 6.2 "source" let's say and two resistors circuit. Then, using voltage dividor rule and the values of resistances of the resistors (5.6/(5.6+3.3))*6.2. That would give 3.9V at point A which is ...

That's half the working. Then follow a similar procedure to determine V_B. Using these results, you can calculate the value for V_AB.

 

[NascentOxygen]

Could you please walk me through your calculations?

The zener is a good voltage source, so replace it with a short circuit.

 

[mrowa196]

How about the resistance? I've had this:

But despite good result it must be wrong, because it shouldn't be 6.8 but 68 kOhms.