Lab 2 of "The Art Of Learning Electronics"

George Orwell 1

Joined Apr 14, 2018
7
I am teaching myself electronics using Howizter's book "Learning The Art Of Electronics" lab book and I am stuck on lab 2. It's asking me to meaure the time constant by determining the time for the output to drop to 37%(=1/e). Here's a pic of the question:

How do I do this with the oscilloscope? How do I know where 63% is?

ericgibbs

Joined Jan 29, 2010
11,643
hi George,
How many Graticule lines/boxes are on the Y axis of your Scope.?
E

George Orwell 1

Joined Apr 14, 2018
7
Here's a pic of my lab and oscilloscope screen:

It looks like it's taking up 31/2 graticule lines for the capacitor discharge path.

ericgibbs

Joined Jan 29, 2010
11,643
Hi G,
looking at that scope shot, your Yaxis has 10 lines, adjust the Y Gain so that the discharge trace starts at the top ie 10 and the end of discharge is at 0.
The 6.3 point should be easy to read on the X and Y axis.
E

George Orwell 1

Joined Apr 14, 2018
7
You mean like this?

I cannot match the top of the saw wave to the top as the next step from 200mV is 500mV and goes past the top. Not sure where I would be able to find the 63% point from the readings on the screen either.

crutschow

Joined Mar 14, 2008
26,058
You need to read the oscilloscope manual to get more familiar with what the oscilloscope can do.

You can use the vertical variable gain setting to adjust the output to fill the screen.

You can also use the cursors to determine the 63% point.

ericgibbs

Joined Jan 29, 2010
11,643
HI G,
I would say this marked up image is close enough, use the vertical dashes ie: 13 from top to bottom to measure the height.

An easier way would be to tweak the Variable Y gain pot.
E

BobaMosfet

Joined Jul 1, 2009
1,299
That book is a fantastic book, but an easier initial approach is this, and will get you farther faster and really help you dig into AoE, IMHO.

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

Don't rely on just one source, read several, each will provide a slightly different perspective, and one of those may be better suited for your mind's ability to absorb.

SamR

Joined Mar 19, 2019
2,930
On the Siglent there is a Measure button that does it all for you. Learn it and it will be your best friend.

crutschow

Joined Mar 14, 2008
26,058
On the Siglent there is a Measure button that does it all for you. Learn it and it will be your best friend.
That's sort of cheating for the lab exercise.

SamR

Joined Mar 19, 2019
2,930
Well I don't use pencil and paper for long division either

crutschow

Joined Mar 14, 2008
26,058
Well I don't use pencil and paper for long division either
Of course.
That's what slide rules are for.

SamR

Joined Mar 19, 2019
2,930
That's what slide rules are for.
Only after the batteries die in the calculator.

George Orwell 1

Joined Apr 14, 2018
7
On the Siglent there is a Measure button that does it all for you. Learn it and it will be your best friend.
I see the measurement button but how do I know where *precisely* 37% on the discharge curve is? I would guess the measurements would need to be precise for the results to make sense.

SamR

Joined Mar 19, 2019
2,930
I addition to the Meas button there is also the Cursors button that allows you to "manually" measure on the screen. Both are distinct advantages of the digital over the analog scopes. Digital scopes take the "guesstimation" out of finding values displayed by the trace by visually estimating the reading of the graticule on the screen. It's like comparing an Abacus to an Engineering Calculator. Does that model have the Help Button? It is useful for exploring the buttons on the scope along with the manual. RTFM https://siglentna.com/USA_website_2...al/SDS1000X&Xplus_UserManual_UM0101X-E02A.pdf

RBR1317

Joined Nov 13, 2010
595
...how do I know where *precisely* 37% on the discharge curve is?
In order to know where 37% is on the discharge curve you first need to know where the zero level is, i.e. the part of the discharge curve that is horizontal. But your discharge curve has no horizontal portion because the signal frequency is set too high. By the look of things on the scope I would guess you need to reduce the signal frequency by a factor of 5 to 8 times.

George Orwell 1

Joined Apr 14, 2018
7
I changed the cursor to measure the y-axis and found the delta-y voltage to be 1.33V and calculated that 63% is 0.8379. Now I placed the cursors at this delta and this is what it shows on display.

For some reason I cannot put both the X-cursor(to measure time) and the y-cursor on screen at once.

RBR1317

Joined Nov 13, 2010
595
Perhaps it would be helpful to review just where that 37% figure comes from. Looking at the attached first-order transient discharge curve, it will be noted that if the discharge curve would maintain its initial slope, then it would reach the zero level in a single time constant. However, since this is an exponential curve and not a straight line, the voltage only reaches 37% above the zero level in a single time constant. So in order to use the point that is 37% above the zero level, one needs to know where the zero level is. The simplest and most reliable method to establish the zero level is to follow the discharge curve out to about seven time constants. Otherwise, there is circuit analysis to establish the discharge zero level (it is not always zero volts) and somehow indicating where 37% above zero level is on the scope display.

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Zeeus

Joined Apr 17, 2019
608
I changed the cursor to measure the y-axis and found the delta-y voltage to be 1.33V and calculated that 63% is 0.8379. Now I placed the cursors at this delta and this is what it shows on display.
View attachment 192278

For some reason I cannot put both the X-cursor(to measure time) and the y-cursor on screen at once.