Author Topic: need help with Capacitor sizing  (Read 6022 times)

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Joseph Turrisi

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need help with Capacitor sizing
« on: February 16, 2009, 06:45:49 PM »
finding the right size capacitors. The power supply will be 120 volts ac step down to 12 volts with paralell 35 amp full wave bridge rectifier. I want to install two caps. to filter the output for 100% clean 12 volt dc power. Can someone tell me how to choose the right capacitors
« Last Edit: February 16, 2009, 06:45:49 PM by (unknown) »

martin1

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Re: need help with Capacitor sizing
« Reply #1 on: February 16, 2009, 01:08:34 PM »
You have to decide max ripple. You can never get 100% clean DC. Then the reservoir capacitor can be found using:


C = (t * I) / V

Where:

I = average load current drawn on DC side

V = ripple voltage peak-to-peak

t = duration between charging cycles and is equal to: 1/twice mains frequency.

In Europe the mains frequency is 50Hz, so: t = 1/100 = 0.01 seconds. In America the main frequency is 60Hz, so t = 1/120 = 0.0083 seconds

« Last Edit: February 16, 2009, 01:08:34 PM by (unknown) »

FishbonzWV

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Re: need help with Capacitor sizing
« Reply #2 on: February 16, 2009, 01:39:25 PM »
I have a 20v/5A power supply like you are talking about.

It is nothing more than a small xformer, cap, bridge and fuse.

It has a 28k uF, 40v cap (big!).  Instruction sheet says 285 MV RMS ripple.

Bonz
« Last Edit: February 16, 2009, 01:39:25 PM by (unknown) »
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Opera House

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Re: need help with Capacitor sizing
« Reply #3 on: February 16, 2009, 01:50:20 PM »
All caps are not created equal.  Every cap has internal resistance and basically any current drawn from the supply will be the same as in the caps.  You are generally better off using multiple capacitors in parallel to create a lower effective resistance.  Your power suply voltage will vary from the peak voltage of the sine wave to the RMS value at high loads.  For this reason, electronic regulation has taken the place of really large caps.
« Last Edit: February 16, 2009, 01:50:20 PM by (unknown) »

FishbonzWV

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Re: need help with Capacitor sizing
« Reply #4 on: February 16, 2009, 03:26:24 PM »
I have a medical grade 12v/10A power supply that I could part with.

It's an Advance MG12-10c.
« Last Edit: February 16, 2009, 03:26:24 PM by (unknown) »
"Put your brain in gear before you put your mouth in motion"
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Joseph Turrisi

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Re: need help with Capacitor sizing
« Reply #5 on: February 16, 2009, 07:00:54 PM »
I dont quite understand what is ment by max rippel or how to figure it out. I do understand that 100% DC is not attainable but would like to get as close as possible. Would you be kind enough to give me an example using your formula.
« Last Edit: February 16, 2009, 07:00:54 PM by (unknown) »

nick1234

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Re: need help with Capacitor sizing
« Reply #6 on: February 16, 2009, 07:38:07 PM »
if you need a very clean DC use a very large rechargable battery as the final capacitor on your power supply with a hi power zener to chop the ripple before the battery
« Last Edit: February 16, 2009, 07:38:07 PM by (unknown) »

imsmooth

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Re: need help with Capacitor sizing
« Reply #7 on: February 16, 2009, 08:23:04 PM »
« Last Edit: February 16, 2009, 08:23:04 PM by (unknown) »

wooferhound

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Re: need help with Capacitor sizing
« Reply #8 on: February 16, 2009, 10:49:22 PM »
You want too rectify a 12vac transformer to DC voltage . . .

Most 12v transformers are 12.6 volts, after you rectify it and filter it with a capacitor the voltage will be 1.4 times the AC voltage.

12.6vac times 1.4 equals 17.65 volts DC

So without regulation you will be building a 17.7 volt power supply.

« Last Edit: February 16, 2009, 10:49:22 PM by (unknown) »

Nothing40

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Re: need help with Capacitor sizing
« Reply #9 on: February 17, 2009, 12:56:17 AM »
Woof, you forgot the rectifier losses. :) subtract about 0.7V per diode,or for a bridge rectifier about 1.4V. So,for a bridge rectifier,you'd get about 16.3Vdc output (no load)..It will drop a little bit with a load.

A bit high for 12V battery charging,or running 12V appliances from.
« Last Edit: February 17, 2009, 12:56:17 AM by (unknown) »

martin1

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Re: need help with Capacitor sizing
« Reply #10 on: February 17, 2009, 04:46:40 AM »
Let say you choose a volage ripple of 0.5 V and your load is 2A.


C = (0.0083*2)/0.5


C= 33000 uF

« Last Edit: February 17, 2009, 04:46:40 AM by (unknown) »

Opera House

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But, you forget
« Reply #11 on: February 17, 2009, 07:43:09 AM »
The plot thickens.....Transformer voltages are specified at rated current. So no load voltages will be higher.  Ferroresonant transformers will have a more stable voltage.
« Last Edit: February 17, 2009, 07:43:09 AM by (unknown) »