All About Circuits Forum how to select the capacitance of a capacitor?
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 General Electronics Chat Discussion forum for general chat about anything electronics related, including asking questions about material in the All About Circuits E-book, Worksheets, and Videos.

#1
11-02-2009, 08:11 AM
 almotions Junior Member Join Date: Feb 2009 Posts: 46
how to select the capacitance of a capacitor?

Hi all,
assuming i want a constant voltage of 100V to be supplied from a capacitor,how should i choose the value of the capacitance? Example,in simulink, the capacitor parameters only take in the capacitance value and initial voltage,how do i specify that the capacitor actually supplies 100V?

Thanks.
#2
11-02-2009, 10:04 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,950

Quote:
 Originally Posted by almotions Hi all, assuming i want a constant voltage of 100V to be supplied from a capacitor,how should i choose the value of the capacitance? Example,in simulink, the capacitor parameters only take in the capacitance value and initial voltage,how do i specify that the capacitor actually supplies 100V? Thanks.
If you had an infinite capacitance charged to 100V you could use it as a constant source. Trouble is, all finite capacitances discharge as you draw current from them, so what you are asking is not possible - unless you are really talking about selecting a capacitive filter value for a rectifier circuit, for instance.
#3
11-02-2009, 01:41 PM
 John P Senior Member Join Date: Oct 2008 Location: Massachusetts Posts: 1,313

For most practical purposes, you have to decide how much current you need to draw from the capacitor for how much time, and how much voltage drop you can allow over that time interval. It's like drawing water out of a tank--the pressure will drop as it empties. And the bigger the tank, the longer it will hold up the pressure.

The equation is very simple:
I*T = V*C

So if you had a 1 farad capacitor charged to some voltage, and you let 1 amp flow out of it for 1 second, you'd reduce the voltage on it by 1 volt. It's that simple! But those aren't very likely numbers.

Your "supplying a constant voltage from a capacitor" isn't realistic. You have to think in terms of what variation in voltage you can tolerate.
#4
11-02-2009, 10:28 PM
 russ_hensel Senior Member Join Date: Jan 2009 Location: South Dartmouth Ma Posts: 814

100 v is a lot for a capacitor supply unless current is very low. Charge to 200v and add a regulator.
#5
11-05-2009, 12:59 AM
 almotions Junior Member Join Date: Feb 2009 Posts: 46

hi tnk,
Sry for not making this clear.i'm using the capacitor as a source of an inverter supplying a pure reactive load.it could be two or three level inverter,therefore the current is AC 90 degrees out of phase with the voltage,the charge will remain constant.However,i just have the problem fixing the DC voltage across the capacitor in SIMULINK. Unless my assumptions are wrong, any ideas?
#6
11-05-2009, 03:10 AM
 MikeML Senior Member Join Date: Oct 2009 Location: AZ27 Posts: 1,437

Are you asking how to set the initial conditions (starting voltage)? This is called .IC in Spice; dunno about SIMULINK?
#7
11-05-2009, 05:09 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,950

Quote:
 Originally Posted by almotions hi tnk, Sry for not making this clear.i'm using the capacitor as a source of an inverter supplying a pure reactive load.it could be two or three level inverter,therefore the current is AC 90 degrees out of phase with the voltage,the charge will remain constant.However,i just have the problem fixing the DC voltage across the capacitor in SIMULINK. Unless my assumptions are wrong, any ideas?
I see ...

I imagine Vcap won't remain a constant or fixed DC - rather it will probably cycle at the inverter output frequency about an average DC value.

As MikeML suggests you should be able to set the capacitor initial value. Like him I've not used SIMULINK - but it would seem absurd not to have this capability. Otherwise you might be able to set up an initial capacitor charging sequence through a switch + resistor network which "disconnects" after a preset time in the overall simulation sequence.
#8
11-05-2009, 04:40 PM
 Thav Member Join Date: Oct 2009 Location: Raleigh, NC Posts: 82

tnk is right here, the average capacitor voltage and current will be $V_{C0}$ and 0 respectively, but there will be ripple currents and voltage quite handily related. If you know the amplitude of the output current you're going for, you can solve this...
$\Delta V = \frac{1}{C} \int_0 ^{\frac{T}{2}} I_{M} \sin{\left(2 \pi f t\right)} \mathrm{d}t$
... which will give you the ripple voltage in each cycle, assuming the current is independent of the capacitor voltage. That assumption make this good as an estimation, but if your cap is large enough it is a good estimation.

Also, in any real inverter supplying a purely reactive load, your capacitors will still discharge because of losses in the cap, switches and wiring.

 Tags capacitance, capacitor, select

 Related Site Pages Section Title Video Lecture RC Time Constants (Part 1) - Pulse Circuit Response - Inductors,Capacitors,Transformers Video Lecture Capacitors (Part 1) - Inductors,Capacitors,Transformers Textbook Solving for unknown time : Rc And L/r Time Constants Worksheet AC power Worksheet Resonance Worksheet Time constant calculations Worksheet Time constant circuits Worksheet Capacitance Textbook Calculating power factor : Power Factor Textbook Voltage and current calculations : Rc And L/r Time Constants

 Similar Threads Thread Thread Starter Forum Replies Last Post shenhome General Electronics Chat 8 10-12-2009 07:57 PM b.shahvir Physics 28 05-13-2009 07:09 PM jpanhalt General Electronics Chat 2 02-11-2008 11:46 PM Xoy General Electronics Chat 1 10-10-2007 01:06 PM Nirvana Physics 15 05-07-2007 01:08 PM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 08:12 AM.