TI E2E Community
Other Analog & Touch
/etc... Analog Forum
How to calculate time constant of caps in following ckts
1. in fig 1, how to calculate time constant for C1 & C2. What are other parameters included in C2. & what are initial current in C1 & C2 when charging just starts. It is given by I= Vs-Vc/R (Let vc=0v initilally)
2. in fig 2, zener will keep 12 V across cap, assume R1=1M (in fig its 10k, ignore this value). While calculating initial current direct relation cannot be used. I=Vs-vc/R As current will be limited by R1 here. How to find that
3. in fig 3 lets say I have a initial cap value =14v & voltage source=12V. In this I don't think cap will discharge as there no path left.
Are your questions with respect to a particular Texas Instruments' IC? If so, can you provide us with the TI part number in question?
Audio Applications Engineering Manager
Dallas, TX USA
I m grad student doing industrial training as part of course. in company dey are working on msp430g2553 & bq78pl116. i m working on project for reading data from pl116 & send it to PC & interfacing 8 temp sensor ..
1. Now fig1 is related to msp430g225s internal adc. here the discussion related to msp. but i want to know in general how to calculate time for that charging c1 & c2.
I know how to calculate time for simple RC, RLC, LC. But here there are two caps.
2. Now fig 2, is relaled to bq78pl116 where mosfets are charged with this way. (not exactly dis ckt but part of it.)
cap is input cap of mosfet to charge.
3. Also if you can here:
1. Time const of C1 branch (ignoring the C2 branch it has to charge in turn) is R1* C1 = 1ms.
Time const of C2 branch (which is charged as C1 gets charged) is R2* C2 = 270ns.
=> C2 branch gets charged much faster than
The overall circuit's time constant is only marginally greater than 1ms.
Initial current in C1 = VBat / R1 = 1.2ma; in C2 = VC1/ R2 = 0.
2. Assuming VC1= 0 initially, initial current is VBat / R1 = 62ua. The RC circuit will charge as usual, till VC = Vz = 12V. After that, the current (VBat - Vzener )/ R1 will all drain through the zener path, leaving the capacitor charged at Vzener = 12V.
3. Well, the capacitor's discharge path will be its leakage resistance (parallel to the cap in its classical model) and some reverse leakage current through the diode into the battery! Of course it will retain voltage close to VBat due to the normal charging path from the battery.
Hope this helps.
Thanks Suresh for your reply. I have some questions :
1. Fig 1: Time constant for C1= 1ms & c2 is 270ns
Now C2 will be get charged upto voltage of C1 . But since C1 gets charged in 1ms, how come C2 can be charged upto that voltage in less time.
2. Fig 2: So here time constant will be R1*c1.& charging voltage upto 12V.
to calculate time for capacitor to charge
VC= V(1- exp(-t/RC))
So Vc=12V & V=62V, R&C are known . So time can be calculated. Is it the method?
1. You are right. The cascaded charging time constant for C2 is also nearly same as that of C1; just delayed by some ns due to R2, C2.
I was trying to simplify the load behaviour of the two branches, so that we can understand/ estimate the circuit behaviour. For the given part values, I expect the signals at C1 and C2 to be nearly same as that without the R2, C2 branch altogether!!!
2. You are right. This assumes that there is negligible leakage current through the zener, till its rated voltage.
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with respect to these materials. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.