AC to DC for power supply (about transformer)

[filip_cro]

I'm designing my power supply. It will have 2 channels  5.12 V and 10.24 V, both 1.024 A. Lower will have step down first. Both will be driven by MOSFETs and Op Amps.

Power supply will be driven from mains.

So:

I'm thinking about using this transformer: http://export.farnell.com/pro-power/ctfcs50-6/transformer-50va-2-x-6v/dp/1780889?Ntt=ctfcs50-6
50VA, I will connect 2x6 in 1x12

After transformer rectifier: http://export.farnell.com/multicomp/gsib1504/bridge-rectifier-15a-400v-gsib/dp/1861524
With 1.1V drop at 7A.

Voltage shouldn't fail bellow 13V for 12V LDO: http://export.farnell.com/texas-instruments/lm2940cs-12-nopb/ic-v-reg-linear-12v/dp/1469072

After rectifier I'm thinking about using 5 x 10mF in parallel: http://export.farnell.com/nichicon/uvr1e103mrd/capacitor-alum-elec-10000uf-25v/dp/1899589

Now questions:
Will transformer survive turning on?
Before and after transformer which filter should I use?
Between mains and transformer 250mA fuse will be ok?

[filip_cro]

BUMP  ?

[mariush]

12v AC 50 VA transformer

full wave rectifier => 12v x 1.414 = 17v DC peak voltage. 

Current = ~ 0.62 x 50/12 =  2.58A  or easier ... 50va / 17 which gives more or less the same result. Let's say 2.5 amps.

Bridge rectifier forward voltage max = 1.1v , let's say 1v. So peak dc after bridge rectifier = 17v - 2x1v = 15v.

50.000 uF is A LOT.   A basic formula for capacitance is  Capacitance  = Current / 2 x ac x Vripple.  Your transformer can do maximum 2.5 amps , the ldo only needs 0.5v above 12v to work but use 1v for safety so do the math for a minimum of 13v at input, so you can afford 2v ripple.

Capacitance = 2.5 / 2x 50 x 2 = 2.5 / 200 = 0.0125 F or 12500 uF 

That's for about 2.5 amps.  If you think you're not going to use BOTH 5v and 12v at 1A at same time, then you can use lower capacitance. You're not going to exceed 2A if you use two LM2940.  So 50000 uF is kinda pointless, excessive.
I would use 2 6800uF 35v capacitors and be done with it.
Use 35v rated capacitors or even higher, they'll last longer and handle currents better.

Also always worth keeping in mind that at low loads transformers often output higher voltage than the label ... check the transformer's Reg % column .. for the 50% it's 9%... so at low loads the output of the transformer could be 13.08v AC or about 19 volts DC. Still low enough to use 25v rated capacitors but 35v rated capacitors are not much bigger.

As for fuse ... 50 va / 230 = 0.217 ... I would probably use a 250-300mA slow blow fuse.

later edit : I would use the TO-220 version of that linear regulator, as it makes easier to put a heatsink. At 1A, you'd need a heatsink even for the 13-14v in , 12v out regulator.

[filip_cro]

Thank, you.
LDO will only drive 4.096V ref. and three op amps.

So first from mains is switch then 2x 300mA slow blow fuse ( http://export.farnell.com/littelfuse/0239-300hxp/fuse-cartridge-300ma-5x20mm-slo/dp/1184906 ), then filter then transformer.

What about filter? Will 1 capacitor be enough or should I use inductor also? Do you have some formulas for that? I sean in one product which I fix capacitor after transformer. Should i use it also?