# Lost on RC time constant

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#### 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
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
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

#### 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

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