# Homework Help: Decay time constant

1. Dec 16, 2013

### MissP.25_5

What is the decay time constant of the waveform? How to determine it from this oscilloscope?
The waveforms are in the pictures below:

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2. Dec 16, 2013

### rude man

Look at the 100 ns/div. oscillograph (the orhers are useless).

How long would you estimate it takes the bottom trace to get to 63% of its way to the top?

(This would be just an estimate since there may be nonlinearities involved, or two or more time constants. But a pretty good estimate anyway).

3. Dec 16, 2013

### MissP.25_5

4. Dec 16, 2013

### rude man

Your trace begins at the bottom and ends at the top. How much time along the time axis to reach 63% of the distance from the bottom to the top? Use the 100 ns/div. trace.

5. Dec 17, 2013

### MissP.25_5

Should 8~10ns be correct?

6. Dec 17, 2013

### rude man

Yes.
To get a better estimate you should have run 10 ns/division instead of just 100 ns.

7. Dec 17, 2013

### MissP.25_5

Actually, 100ns is the sweep time.

8. Dec 17, 2013

### rude man

Actually, I know.

You should have chosen 10 ns/div. sweep time

9. Dec 17, 2013

### MissP.25_5

Well, I just followed the procedure in the textbook. It says to set the sweep time to 2.5mircrosec, 250nsec and finally 100nsec.
I tried calculated the decay time and taking the best point that I could and I got 9.2ns. Is this ok? By the way, thanks for helping me out.

Last edited: Dec 17, 2013
10. Dec 17, 2013

### rude man

Maybe your oscilloscope didn't have a 10 ns.etting.

Anyway, I think your number is fine.