need help

[dane8]

i got this school project to build and i need some assistance. Am suppose to build a 12 volt dc power supply from an ac source suppling 1 watt, 1a and ripple factor of <10%. I need to know how to calculate the capacitance needed in circuit using formulas. please help.

Thank you!

 

[blueroomelectronics]

Check the datasheet for LM7812

 

[dane8]

that site is no help

 

[flat5]

7812 datasheet
http://www.datasheetcatalog.org/datasheets2/52/529144_1.pdf

That aside, if the power source is 1watt and 1 amp then the rms voltage
of the source is 12vac rms. Am I correct so far?
The peak voltage available is 12x1.414=16.968vac, yes?
If using a 7812, he needs a capacitor that can hold the input to the
regulator above 15 vdc at a load of 1amp.
Best to use a full wave bridge.
Considering bridge voltage drop of 0.7v there is now a 16.268 peak ac voltage @60hz
going to the cap.
What is the formula to select the capacitor to keep the voltage above 15vdc at a
current of 1 amp?

I found a site that said "current is roughly Vripple*2*pi*F*C"

Our acceptable ripple is 16.268v - 15v = 1.268v

1 = 1.268 * 2 * 3.14 * 60 * C

C = 477.7824 farads can't be right.

If C is in microfarads then this makes more sense.

Am I completely lost?

I also found this link to a post by Mr Al
https://www.electro-tech-online.com/threads/power-supply-design.86669/

"What helps a bit is to estimate the peak to peak ripple voltage,
which is approximately:

Vpp=6000*I/C

where

Vpp is the peak to peak ripple voltage in volts
I is the load current in amps
C is the capacitance in microfarads (uf).
The frequency is 60 Hz and it is full wave rectified.

For example, a 10v rms 60Hz input to a full wave bridge when filtered
with a 6000uf cap and 1 amp load will mean there will be about 1v peak
to peak ripple. You can compute ripple factor from that if you wish."

He has more to say in the above link.

 

[dane8]

sorry about that wrong specifications. it is a power supply with an ac input source 120v, 50 hertz, which should give an output of 12volt dc, 1Amps, ripple factor of <1% and a power of 12 watt. i live in jamaica. i wasnt given any capacitor nor any other component values. i need the formulas please.

 

[blueroomelectronics]

Just build it as specified in the datasheet. Why is the ripple so important, what are you building?

 

[dane8]

am building a 12volt dc power supply from an ac source.

 

[dane8]

data sheet not helping

 

[blueroomelectronics]

Then you need to rectify the AC into DC first.

 

[dane8]

yes i know that already but i dont know how to calculate the capacitance needed nor the values for bridge rectifier and the value for the transformer. please help

 

[tajb]

hey im doing the same project and i need to know the formulas used in order to calculate the values of the components such as the capacitor, transformer, diodes.....we need the formulas help

 

[blueroomelectronics]

Here's a typical one with all the values, there are plenty just Google. Although I'd personally use a 12.6VAC transformer just so the LM7812 runs a little cooler.
**broken link removed**

 

[tajb]

thats thats help alot but i need the formulas on how to calculate the capacitors, transformer etc...how did u know to use that type of capacitor ..i need to calculate them tnaks help