Help me! Charging time of capacitor circuit?

[govindaAF]

I need to develop a circuit to measure charging time of capacitor
I am kind of less in practical knowledge (this is my first project)
So far I have thought of giving a reference input and capacitor voltage to comparator
then there is counter that will count the number of time it will go high
then the time divided by the no. of times it will go high will give me the charging time that will be displayed on LCD
Is this method Okay? Will it work?

Can any one please draw me a detailed circuit daigram
for the same PLEASE!!!

 

[PA4TIM]

You need to develop ? You mean like for school ?

So far I have thought of giving a reference input and capacitor voltage to comparator
then there is counter that will count the number of time it will go high
then the time divided by the no. of times it will go high will give me the charging time

Why you want to measure that time ? ( for instance if it is for school it is just the principle, if this is to measure the capacitance of a capacitor the circuit must be made more carefull. If it is some sample/hold thing you need a very low leakage capacitor )
Your idea can work. You need a resistor to limit the current and this depends on the capacitor. A 10F supercap charging through 10M can take a while and 10pF with 10 Ohm will go way to fast. The resistor value gives you resolution (in the analog part of the schematic)

One thing, I think you just wrote it a bit strange because now it looks like you want to count the number of times the comparator gets high and that is just one time. If the comparator inputs are equal the comparator fires. And that is when the charge voltage is equal to the reference voltage.

So you monitor the comparator while the microprocessor counts its clock (or so, I'm not into digital). Then you must calculate how mucgh time one cycle takes and multiply that number times the number of counts.

 

[chuckey]

I don't really understand your question. My thoughts are build a pulse generator and feed it into your test capacitor via a diode and a current limiting resistor. So every pulse that come along will "pump" up the capacitor a little. So by setting a comparator to stop the oscillator at a certain voltage and counting the number of pulses, the unit can be calibrated in terms of capacitance/pulse.
Frank

 

[goldsmith]

I need to develop a circuit to measure charging time of capacitor
I am kind of less in practical knowledge (this is my first project)
So far I have thought of giving a reference input and capacitor voltage to comparator
then there is counter that will count the number of time it will go high
then the time divided by the no. of times it will go high will give me the charging time that will be displayed on LCD
Is this method Okay? Will it work?

Can any one please draw me a detailed circuit daigram
for the same PLEASE!!!

Hi govindaAF
simply use a pulse source with low frequency and then use an ADC of an MCU and easily measure it ! that's all .
If you want theoretical methods you can simply use the main equation of a capacitor .
Or perhaps using a software like Pspice ?

Best Wishes
Goldsmith