Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Lost on RC time constant

Status
Not open for further replies.

mstechca

New Member
I was ignoring the curves in my example. I like to follow the KISS method (keep it simple stupid method) LOL
 

audioguru

Well-Known Member
Most Helpful Member
Mstechca,
Your "0.69" calculation for the amount of time a capacitor would take to charge to a certain voltage is not correct at any point on the curve. It is not as simple as you think. :lol:
 

mstechca

New Member
seems that experimentation is the best way to learn electronics.
 

ljcox

Well-Known Member
As I said in a previous post the time for a capacitor to charge to 20% is

0.223 RC

It can't be much simpler than that.

Len
 

audioguru

Well-Known Member
Most Helpful Member
Looking at my graph of the percentage of charge vs RC time is simple and accurate too.
Experimenting with an oscillator having a repetitive RC charge will let you see my graph right on the screen of a oscilloscope. A simulation program would do the same thing.
 

ljcox

Well-Known Member
Here are graphs for charge (red curve) and discharge (blue curve)
The time axis is equal to RC.

Example 1
If R = 1k, C = 1 uF Time constant = RC = 1 millisecond.
Time to charge to 40% is about 0.58 ms

Example 2
If R = 1k, C = 1 uF Time constant = RC = 1 millisecond.
Time to charge to 80% is about 1.6 ms

Example 3
If R = 10k, C = 1 uF Time constant = RC = 10 millisecond.
Time to charge to 80% is about 16 ms

Len
 

Attachments

  • charge_discharge_315.png
    charge_discharge_315.png
    2.9 KB · Views: 492

audioguru

Well-Known Member
Most Helpful Member
Thanks, Len.
I stretched-out your curves to make it easier to see and marked them.
He, he. The curves in my book are more accurate than yours. :lol:
 

Attachments

  • charge-discharge_stretched.png
    charge-discharge_stretched.png
    12.2 KB · Views: 463

zachtheterrible

Active Member
ljcox said:
As I said in a previous post the time for a capacitor to charge to 20% is

0.223 RC

It can't be much simpler than that.

Len

i missed that for some reason len, its an even simpler way :lol:
 

ljcox

Well-Known Member
The formula I posted previously

t = RC ln {(Vdc/(Vdc - V)} is more general. You don't need to be a mathematician to use it.

ln = log natural and is on scientific calculators. It can also be found in the functions (maths & Trig) of the Excel Spreadsheet.

I used Excel to produce the graphs.

The discharge curve is t = RC ln {Vdc/V) where Vdc is the initial voltage and V is the voltage atany time t.

Len
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top