Hello evilheart and All,
tron here....
i am working on split power supply (5,12,-5,-12v) , that i am going to use for general purposes
i used :
12-0-12 about 1A transformer
2A bridge
7805
7912
2x2200u 16v
You have a number of issues at work here, one is the size of your ripple capacitor, its initial ramp-up time could be causing a damaging spike which in theory could cause a latchup you are worried about. Do you NEED that size capacitor?
Another question, is this power-supply to be used principally with digital or analog circuits? If digital, I would recommend using some small adjustable buck switchers, that would eliminate much of the heat generation, the big capacitors and you can use one bridge for all the voltages. If you are going to be using this power-supply for sensitive analog circuits then I would suggest that you buy a good quality adjustable current/voltage power-supply on eBay, I see them go all the time for around $50 to $150....
If you are set on making your general purpose power-supply....
From the center tapped transformer, the center tap goes to 0 ref (do not put this to chassis ground). You wanna keep your DC isolated from AC ground for both noise and safety.
The bridge for +- 12 Volts should be at least 50 VAC rated, personally I would select something 3 x the RMS to handle power-line spikes, although I have gotten away with much less.
The +- sides of the bridge I would move to 220 uF caps, rated at 50 Vdc (min) and low ESR, (besure to move the + side of the ripple cap to 0 reference for the minus supply, otherwise there will eventually be a big bang!).
Ok, other issues, the +- 12 Vdc regulator series (LM78xx) typically need a small amount of power to operate, and as result there is a input to output voltage differential of something like 2.5 Vdc as I recall, meaning that to get 12.0 Volts regulated output, the regulator will require 14.5 Vdc minimum input, ok? That means your transformer needs to output appx = 14.5 x 2 or 29 VAC center tapped to enable the regulator you spec'd to function properly.
Looking at the spec sheet from DataSheet Server (great databook server)
www . datasheetcatalog . com (remove spaces) & search on LM7805, LM7812, LM7905 and LM7912 data sheets.
Regarding Your +-5 Vdc regulators, your design, is differencing 12Vdc down to 5 Vdc, which means you are dropping the difference between the 12 Vdc and 5 Vdc = 7 Vdc as heat. First that simply wastes a lot of power, 2nd, it pulls much power from your transformer which would normally be used by the other parts of the power-supply system, 3rd, it generates a lot of internal heat, cooking all the components and will require some honkin big heat-sinks I would do something different here, like use a 2nd transformer, rated at around 7.5 Vdc (min...you wanna handle mains-primary line voltage sags and still be within the regulation point of the regulator).
With these above two issues in mind, the transformers should both be center-tapped,
the 12 Vdc power-supply transformer s/b around 29 VAC (14.5 VAC-CT) not too high to drop a lot of power, yet high enough to handle mains-primary line sags, and the +-5 Vdc power-supply transformer s/b 18 VAC (9 VAC-CT), again not too high but low enough to keep from cooking everything inside.
Another matter is regulator package configuration, whether to use LM78/79xxT or LM78/79xxK packages. The 'T' package is the familiar TI-220 plastic power-pkg, while the 'K' is the old style TO-3 power-pkg, well beleive it or not, if you use only one transformer and you still wanna get 1.5 Amps out of both +-12 & +-5, you will need the K package for the +-5 and two big finned heat sinks, else they will go into thermal runaway and shutdown. My recommendation is to use the 'T' pkgs & two transformers.
Regarding transformer current rating, +- 12 @ 1.5 Amps will require 2 x 1.5A (+ the efficiency of the transformer loss), I would rule of thumb select a 29 VAC @ 3.75A or 4 Amp transformer, and for the +-5 transformer, I would rule of thumb select a 18 VAC @ 3.75A.
Also, according to my old National Linear Handbook section on LM linear regulators, you should to add some small decoupling caps on both input/output to and from the linear regulators (later tune up any ringing with toroidal beads).
So, here's a quick BOM:
1 x LM7805T - 5Vdc @ 1.5A positive regulator
1 x LM7812T - 12Vdc @ 1.5A pos reg
1 x LM7905T - -5Vdc @ 1.5A negative regulator
1 x LM7912T - -12Vdc @ 1.5A neg reg
2 x 40 VAC @ 6 Amp bridge rectifiers (or bigger)
1 x 29VAC @ 3.75A, center tapped transformer
1 x 18VAC @ 3.75A, center tapped transformer,
2 x 220 uFarad, 50 Vdc electrolytic ripple caps (low ESR rating important for P/Ss)
4 x .1 uF, 75 Vdc tantalum decoupling caps, (orange or blue-drops are nice)
4 x different coloured LED voltage 'on' status indicators & appropriate resistors
Its really neat & useful to add a polarity reversal (DPDT) switch on these type power-supplies.
One last thing, there are many modern linear regulators these days which are known as LDOs or 'low-drop-out', meaning that instead of needing 2.5Vdc to operate, they work on a few miliVolts, so a 13.2VAC transformer can adequately supply and still be regulated +- 12Vdc. Its a matter of looking around and finding the parts, DigiKey or Mouser is premium, but finding these type parts is much harder at places like All Electronics or Electronics GoldMine etc.
Hope this helps...
tron