Hello, I am recently planning to build a power supply that will put out: 12v @1.5A (max) 5v @ 1.5A (max) 5v @ 500mA (max) The total wattage output, assuming all regulators are loaded, is 28 watts; the transformer I am thinking about buying can output 57.6 watts. The transformer is rated 24v @ 2.4A, without rectifier drops. My question is can I extract more current than the transformer can supply? I know that if your application demands more current than the transformer can supply, the voltage will drop, but is it okay for this - the voltage won't drop below 18v? Also, are there any side effects to doing besides the possibility of the tranformer getting hotter? Side questions: I've always used a PI filter before the voltage regulator, does it help if I add another PI filter after the voltage regulator? When current is cut, will the back EMF hurt the components that are connected to the PI filter, that is in front of the voltage regulator? I am going to add a feedback diode from the voltage regulator output to the voltage regulator input that will mitigate the stored current in the capacitors from discharging through the voltage regulator the wrong way, will this cover the back EMF? Or should I have a reverse biased zener diode that has a breakdown voltage higher than the output regulator to automatically short the voltage spike to ground whenever the power is cut? In the past, I've used varying induction values for PI filters but is there an equation to tell me how much inductance I need for my power supply? I know it has to do something with Q material and saturation but I'm not sure. Thank you.
Why are you thinking about buying a 24V transformer for a 12V supply? 15V would be more than ample. You can't base the wattage on final V*I figures, as you also have wattage losses in the regulators and rectifier. So you're pulling 3.5A max from a 2.4A transformer... ...accompanied by a smell of burning.
Thanks for the reply Cpemma, my reason for buying a higher voltage one is because I was thinking about sacraficing voltage for amperage but I wasn't too sure that would work - hence asking. I've sighted another tranformer, 16VCT @ 3.5A, would that be enough to power one 12v regulator and 2 5v regulators, assuming voltage drop via rectifier diodes?
That's a better voltage, but still a bit low on current if you expect to use all the max levels much of the time. There's a lot of wasted energy in a linear psu. The spec 16V should be at rated load, so anything less & you'll have a few more volts in hand. 16V RMS AC will give you 22.6V peak DC, allow 2V loss through the 2 rectifier diodes =20.6V and you can still afford a few volts of ripple after smoothing & before the regulators, which will allow smaller caps to be used than if it were unregulated. There's a very deep article here on psu design, but the summary gives most of the formulas needed.
The transformer is rated for over twice the wattage of your application circuit. It will work (with a lot of room for ineffciencies).
