# Help with a simple cap and resistor circuit...

Discussion in 'General Electronics Chat' started by thakid87, Jul 4, 2010.

1. ### thakid87 Thread Starter Active Member

May 23, 2009
122
0
In the circuit in the picture attached, why is there between nodes 1 and 2, i.e. the resistor?

But there is a voltage between 1 and 3.

I'm sure this is so simple that I am looking foolish here...

File size:
97 KB
Views:
56
2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
We do not know why the resistor is included in the circuit. That is not material to the existence of a voltage between node 1 and 3. 1 is the positive voltage from the source, the other (3) is ground. The voltage source's negative terminal is common to ground so the full source voltage has to appear between nodes 1 & 3.

3. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
The circuit may be to demonstrate the R/C time concept.

4. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92

It is the conditions of your transient response that are causing the confusion I think.
You needed to change your startup conditions to starting voltages at 0.

You also wanted to reduce your timebase so that you see the interesting bits easier.

thakid87 likes this.
5. ### thakid87 Thread Starter Active Member

May 23, 2009
122
0
That's perfect. That helps me get a better understanding of this.

Thanks very much.

6. ### dsp_redux Active Member

Apr 11, 2009
182
5
To add a little something, capacitors act like open circuits in DC. With that in mind, you now see that there is no voltage drop on the resistor since there are no return path to ground (open circuit). Your node 3 is the same as ground, so voltage between 1 and 3 is the same as 1 and 0 hence 10V.

7. ### thakid87 Thread Starter Active Member

May 23, 2009
122
0
Is there a way to get LTSpice to graph the charging/discharge time?

8. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92
Same circuit but change the Voltage Source Advanced Settings.

Send it pulsing DC Square Wave (0 rise and Fall Times) and do a transient response.

You should also see try a Constant Current Source for Comparison to the Voltage Source.

That will give you a triangle wave.

9. ### thakid87 Thread Starter Active Member

May 23, 2009
122
0
Potato Pudding,

I've tried to get this simulation but I don't think I am getting it right. I am getting the same graphs as before.

10. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
1,097
Try the attached circuit

File size:
1.1 KB
Views:
10
11. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92
Might be easier to see if using -5 and +5 for the values of current or and -250and +250 voltage. If you are only swinging to ground you see get a stairway increase.

If you widen the timebase on the pulses you can see the charging and discharging more clearly. With a narrow pulse the voltage and current seem pretty similar but they are very different.

And is there a way to upload files to the site the way pictures are?

I checked the link for your LTspice attachment and saw that it was located on site.

I wanted to upload my changed version and saw that it says I can add attachments in my profile but I've missed the how or where.

12. ### Potato Pudding Well-Known Member

Jun 11, 2010
684
92
Never mind. Post reply vs quick reply is all the difference I needed in order to see how to upload an attachment.

• ###### RC_34 Extended.asc
File size:
1.2 KB
Views:
13
thakid87 likes this.