Electronic – how to calculate time for timer relay


In the below circuit of Timer Relay, how I can calculate the time for C1 to be fully charged?
please don't use regulator, transistor or any other device I just want to use Resistance and want to calculate how much resistance required to charge the C1 for how much time?
If ohms law(E=IR) is suitable than what is the relation of (I) with C1?

enter image description here

Best Answer

Capacitor resistor charge time: -

enter image description here

If the applied voltage is 12V the capacitor charges to 63.2% of 12V in a time equal to capacitance in farads x resistance in ohms. In twice this time it gets another 63.2% closer to 12V i.e. 86.5%. Each duration of time equal to CR it gets another 63.2% closer. See below: -

enter image description here

how I can calculate the time for C1 to be fully charged?

Theoretically a capacitor never gets to 12V but, for practical purposes it gets to over 99% charge after 5 x CR.

As for the circuit in the question, it appears to be using a 555 timer chip and the relevant point at which this activates the chip is not to be confused with "fully charged". Read the data on the 555 to determine what that point in time is.