RC time constants - advice please

[gjpollitt]

Hi all,

Teaching myself basic circuit theory with a few books and tutorials off the net etc

Question - With RC time constants if I want a time of 5 seconds with a 10K resistor using formula C=T/R gives me a capacitor value of 500microfarad. So does this mean that to reach 63% ish of my total value it would take 5 seconds using 10k and 500 microfarad?

Reason I am asking is that when using Livewire simulator with a 9V DC source and a scope connected across the capacitor, the value jumps straight to 9V across the capacitor. Is it not supposed to take around 5 seconds to reach 63% thus giving me a nice linear rise on the scope?

Thanks
Graham

 

[audioguru]

Yes, it should show that the cap reaches 63% of the supply voltage in 5 seconds, if the input impedance of the 'scope is very high.
Simulators screw-up if you don't enter all parameters correctly.

The voltage rising on a capacitor isn't "a nice linear rise", it is exponential. As the capacitor's voltage rises then its charging current decreases.
You will have a nice linear rise when you replace the resistor with a constant current source. :lol:

 

[checkmate]

The circuit seems right. But it looks like the initial state of the capacitor is fullly charged. Check if there are any settings to set the initial voltage to zero. Otherwise, use a switch to discharge the capacitor, then start charging it up.

 

[gjpollitt]

ok thanks guys

I have tried adding a switch to discharge the cap but when the switch is opened the voltage ramps back to 9v. As far as i am aware there are no options within Livewire simulator to start with fully discharged cap although I have not had probs with other basic cap circuits.

Thanks
Graham

 

[zachtheterrible]

i know that in switcherCAD III, there is an option to give you varying voltage in a square wave, so you can set it to be off first, then turn on, maybe there is something similiar that you can do w/ that signal generator looking picture.

 

[gjpollitt]

ok I tried again with the software, adding a switch works now, I have to start the circuit with the switch open then close it and hey presto.

Question now is as the voltage in the cap rises how would I connect an led to the cap so that the led gets brighter as the cap charges?

i tried putting an led in parallel with the cap but it dropped the max charging voltage to around one volt from nine ! Adding a resistor in series with the led enabled the cap to charge but the value of the resistor had an effect on the amount of voltage stored in the cap.
Either way the led would not light.

Thanks
Graham

 

[audioguru]

as the voltage in the cap rises how would I connect an led to the cap so that the led gets brighter as the cap charges?

1st, you need a voltage sensor with a high input resistance like a transistor emitter-follower.
2nd, you need to level-shift the cap's voltage to the voltage required by the LED.
3rd, you need a current-limiting resistor in series with The LED to prevent it from blowing-up.
All the above can be made with an opamp and driver transistor configured as a current sink.

Since your vision's response to brightness is logarithmic to allow you to see in moonlight and in sunlight, you probably won't see the brightness of the LED change very much as the cap charges. Half voltage and max voltage will look about the same.
Perhaps you should consider using an LM3914 bargraph LED driver IC as a voltmeter. The LM3914 has all the required circuitry that is mentioned above. :lol: