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Capacitor

09/06/2009 11:28 PM

I have made one circuit which gives pulsating DC after connecting to bridge rectifier to 12v step down transformer. So tell me how & how much amount of capacitor filter i have to use.&also give the formula for designing capacitor in dc circuit when used as filter.

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#1

Re: Capacitor

09/06/2009 11:58 PM

Sorry but your initial question is so vague and with very little pertinent information, I cannot directly help you. But here are a few pertinent equations that should permit you to understand some of what you seek:

  1. First a few definitions
    1. q≡charge in Coulombs
    2. C≡capacitor value in Farads
    3. t≡time in seconds
    4. V≡voltage in volts
    5. I≡current in amperes
    6. Δ≡change of a value
  2. Now a few of the equations
    1. Vcap=q/C
    2. I=(Δq)/(Δt)
    3. (ΔVcap)/(Δt)=I/C

This is as simple as I can get and still be reasonably accurate. If you prefer there's Wikipedia capacitor link. With some careful thought, you may realize that filtering alone will never fully remove any oscillation, just reduce the magnitude.

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#2

Re: Capacitor

09/07/2009 3:38 AM

It depends on the load being drawn.
The simplest way is to put on 1000uF, if that works fine (at full load), try something smaller say 470uF until the ripple voltage is more than you want.
Then put on a value at leas twice as big as you need to just do the job (These Caps are often +/- 20% tollerance.
Del

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#4
In reply to #2

Re: Capacitor

09/07/2009 11:53 PM

I agree with Del. You need to know the worst case amount of current first. This can be calculated by the amount your circuit draws or hook it up to a 12V power supply and measure Max I on your multimeter. So then I pick a large number maybe 4700uf at 16V from experience and I overkill the amount of capacitance because they are smaller and cheaper than years ago. I just designed a 12.6V filament DC supply for preamp tubes and you do not even want 1-5mV of 120Hz ripple because it will be amplified by a factor of 100 with a 12AX7. So I decided to use (2) 4700uf caps followed by a regulator like a LM317T. I do not think you considered using a regulator because you can adjust the voltage to were you want say 12.0V and regulators will also help filter out 120VAC ripple. So now you have the caps filtering followed by the regulator. And a another way to make your power supply clean is transient and High frequency noise may occur due to say digital switching from your circuit so use a .01uf & 22uf Tantalum in parallel to ground. The ABC's of PS design.

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#6
In reply to #2

Re: Capacitor

09/08/2009 1:59 AM

Ah ha, we missed all the caveats.

We should warn him about polarized electrolitics, and they have a working voltage rating.

Any and all information may or may not be usable or correct.

Use at your own risk, otherwise things may explode and then we would have a cat a tonic participant.

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#7
In reply to #2

Re: Capacitor

09/08/2009 3:04 AM

Seat-of-the-pants design - I was wondering if anyone still uses the trial-and-error approach. WTG Del.

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#10
In reply to #7

Re: Capacitor

09/08/2009 9:03 AM

Getting your PS quiet is sometimes seat of the pants. Many analog circuits such as a preamp can not handle 100mv ripple. In tube amps they use chokes with the capacitors to choke out the 120hZ ripple. Also designing by the seat of your pants boils down to years of experience in designing. Your PS can have 1mv of ripple but a large TTL circuit I designed in the 70's had TTL switching noise everywhere and one signal was spiking which resulted in a DAC not working. So I wrapped the signal with a ferrite bead or "magic bead" and no more noise. And decoupling your noisy circuit from ground bounce to HF noise is seat of your pants it is done not only with theory but experience. The 100mv ripple calculation I saw somewhere the capacitor theory does not work in the case of analog like preamps which will just amplify it. The whole SYSTEM has to be considered when designing a PS!

Designing = Theory + experience

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#3

Re: Capacitor

09/07/2009 10:41 PM

An approximation formula is thus: .006 x DC load current / Δ V, Where Δ V is the peak-to-peak ripple voltage at the capacitor. This works out to 6,000μF per amp of load for a 1 volt peak-to-peak ripple voltage for example.

(.006 x 1.0) / 1.0 = .006F

(.006 x .250) / 0.10 = 15,000μF (0.25 amp DC load and 0.1 volts ripple)

This formula works quite well and simplifies calculations

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#5

Re: Capacitor (How To Connect & Calculate)

09/08/2009 12:20 AM

Hola

Here Is a Link Where Explain How To Calculate The Power Supply Capacitor.

http://www.kpsec.freeuk.com/powersup.htm

the capacitor should be connected to the bridge rectifier according to the "+" and "-" the bridge and the capacitor

Can be calculated arbitrarily saying they are 1000uF. per ampere. But See The Link.

Greetings

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#8
In reply to #5

Re: Capacitor (How To Connect & Calculate)

09/08/2009 3:09 AM

GA, and, good site.

All depends on whether the OP (Original Poster) wants the theoretical answer (redfred and wiz) or the practical answer (Del The Cat et al). The linked site covers both.

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#9

Re: Capacitor

09/08/2009 3:34 AM

Rule of thumb: 1000uF for every 1A load

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#11

Re: Capacitor

09/08/2009 9:18 AM

I would also take into consideration what the supply will be used for, some circuits are more/less tolerant to noise and ripple as pointed out by the Tube Amplifier post. I liked all the answers and the site, glad I found it. Good Luck with your project

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#12

Re: Capacitor

09/08/2009 12:29 PM

1. Let Vo = desired output dc voltage

2. Let Vo / minimum value of load resistance = Io = max output current

3. let the maximum permissible ripple voltage = Vr = peak to peak ripple voltage/2

4. Use the equation CV = Q = Io * t where t = 10 millisecond ( assuming fullwave/bridge rectification)

5. C = Io * t/ Vr Farad

6. Let Io = 0.2A, t = 10 msec, Vr = 0.4V, for example

5. Then C = 0.2 A * 10msec/ 0.4V = 5000 microfarad

It is the load current and the ripple voltage that determine the capacitor value. Working voltage of the capacitor to be used should have a minimum dc working voltage of 1.5*Vo. Of course the transformer should be designed for the desired current.

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#13

Re: Capacitor

09/08/2009 2:08 PM

I agree that the OP's post was vague, probably because the OP is not experienced in PS design. Power supply design is complex by its very nature but, on occasion, a simple filter will suffice depending on the load's requirements. In power supply design, you must make estimates of your requirements before you can go anywhere. It requires more than just a output voltage to begin with. For instance, push-pull output circuits are relatively immune to ripple, a few volts peak-to-peak are of no consequence but to a class A amplifier, such as a preamp, which has a very poor power supply rejection ration (PSRR), ripple reduction is very important (even more so is high frequency rejection).

The OP asked about how much capacitance to use, unfortunately without detail, RedFred gave basic quantities involved in power supply design, I gave a rule-of-thumb formula which generally works very well in most instances. That is about as far as one could go given the lack of data. The other replies, basically gave varying degrees of design by brute force. These, of course, work because they simply overwhelm the requirements. Del the Cat's method is the least heavy-handed of the group and is quite acceptable for the trial-and-error route. There is nothing particularly wrong with this approach but it can put an unnecessarily high demand on the rectifiers and transformer due to exceedingly high peak currents, not to mention causing lousy power factors. This is not good practice nor good engineering.

For those of you who are not well versed in power supply design, there are rules-of-thumb which are applicable in many cases, such as the one I gave. In cases where the design is more complex, rules-of-thumb are only a beginning point at best. Ripple is only one component of power supply design and many other points are involved in designing a stable, clean power supply with good high frequency rejection and transient response.

For those beginning, trial and error can be a way of learning about power supplies but it cannot substitute for good engineering knowledge either. I do recommend careful trial and error but buy yourself a good basics of electronics book to use as a guide. There are many basic tutorial books available to help and a lot of material on line as well to guide you. Always ask questions when in doubt!

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#14

Re: Capacitor

09/08/2009 6:02 PM

I, with qualifiers, disagree with using only one cap. I (from experience) think you should have one small cap(.1Uf) in parallel with the larger value cap(i.e. 1000Uf). The small one is not a necessity but it will help. This applies for powersupplies that will be used on circuits containing chips that will be switching states in normal usage.

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#15
In reply to #14

Re: Capacitor

09/09/2009 12:35 PM

In fact, it is better to have several small caps in parallel to add up to the overall capacitance required than it is to have one big cap. This will lower the Equivalent Series Resistance (ESR) of the equivalent capacitor below the ESR of a single cap. This will allow the capacitors to run cooler and last longer. Remember that Electrolytic caps have a liquid electrolyte and they will dry out over time. The hotter the cap runs the faster it will dry out.

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#16

Re: Capacitor

09/10/2009 12:15 PM

The small capacitor being referred to is a non-polarised (ceramic or plastic film) capacitor connected parallel to the main electrolytic filter capacitor. Since the effectiveness of electrolytic capacitors to filter frequencies above around 400Hz is very low, the added non-polarized capacitor will filter out higher frequencies. A part of these high frequencies may have their origin in harmonics created by the rectifiers. The value of the non-polarized capacitor can be about 100 nF.

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#17
In reply to #16

Re: Capacitor

09/10/2009 3:01 PM

That agrees completely with my (limited) knowledge....Why did you mark yourself Off Topic, to me that was spot on......

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