- #1

verd

- 146

- 0

I performed a lab this past week that went over charging/discharing time constants, and I was asked the following about the circuit below:

http://synthdriven.com/images/deletable/EEN204-Lab3.jpg [Broken]

Calculate the charing time constant, (Tau_c), and the discharging time constant, (Tau_d). I was asked to them compare these results with those that I measured in the lab, for three instances. (C=0.1uF)

R1=470

R2=470

I measured:

Tau_c = 52.76us (<-us=microseconds)

Tau_d = 16 us

R1=1k

R2=470

I measured:

Tau_c = 35.18us

Tau_d = 35.18us

R1=470

R2=1k

I measured:

Tau_c = 24us

Tau_d = 48us

As far as the both the time constants went, I came up with the following formulas:

Tau_c=(Rs+R1)C

Tau_d=R2C

And these are my calculations:

Instance 1:

Tau_c = 52us

Tau_d = 47us

Instance 2:

Tau_c = 105us

Tau_d = 47us

Instance 3:

Tau_c = 52us

Tau_d = 100us

These results from the calculations are very different from what I measured in the lab. So something must be incorrect. We measured the time constants by setting up the circuit as shown (I'm 99% certain that the circuit we set up was correct), and measuring the voltage via an oscilloscope hooked up to a computer... That way, we were able to freeze the curve and perform precise calculations with the cursor on the oscilloscope. We were asked to reproduce the curves by hand to turn in with our reports. And by looking at those graphs now, the values the computer gave us for the time constants look to be correct... By the behavior of the curve on the graphs anyway.

So I'm writing because I figure that the error is in the formula I put together about the charging/discharging paths. I must be calculating the wrong thing because I probably set the equation up incorrectly.

Could someone tell me if the equations I derived for Tau_c and Tau_d are correct according to the figure?

Thanks!

Heather

http://synthdriven.com/images/deletable/EEN204-Lab3.jpg [Broken]

Calculate the charing time constant, (Tau_c), and the discharging time constant, (Tau_d). I was asked to them compare these results with those that I measured in the lab, for three instances. (C=0.1uF)

**Instance #1)**R1=470

R2=470

I measured:

Tau_c = 52.76us (<-us=microseconds)

Tau_d = 16 us

**Instance #2)**R1=1k

R2=470

I measured:

Tau_c = 35.18us

Tau_d = 35.18us

**Instance #3)**R1=470

R2=1k

I measured:

Tau_c = 24us

Tau_d = 48us

As far as the both the time constants went, I came up with the following formulas:

Tau_c=(Rs+R1)C

Tau_d=R2C

And these are my calculations:

Instance 1:

Tau_c = 52us

Tau_d = 47us

Instance 2:

Tau_c = 105us

Tau_d = 47us

Instance 3:

Tau_c = 52us

Tau_d = 100us

These results from the calculations are very different from what I measured in the lab. So something must be incorrect. We measured the time constants by setting up the circuit as shown (I'm 99% certain that the circuit we set up was correct), and measuring the voltage via an oscilloscope hooked up to a computer... That way, we were able to freeze the curve and perform precise calculations with the cursor on the oscilloscope. We were asked to reproduce the curves by hand to turn in with our reports. And by looking at those graphs now, the values the computer gave us for the time constants look to be correct... By the behavior of the curve on the graphs anyway.

So I'm writing because I figure that the error is in the formula I put together about the charging/discharging paths. I must be calculating the wrong thing because I probably set the equation up incorrectly.

Could someone tell me if the equations I derived for Tau_c and Tau_d are correct according to the figure?

Thanks!

Heather

Last edited by a moderator: