Originally Posted by yawg3d
Do I need some form of inrush current limiting for those caps? The simulation shows 7A of charging current during the first charging phase, which is way too much for my AC Adapter and the diodes of course. Are there any real-world effects which do not show up in the situation and help prevent this? If not, what should I do?
Add 1.53 ohms of series resistance in your 9Vac spice voltage source model (or just add a 1.53R resistor in series with your ideal voltage source). That is the secondary series resistance I just measured on a CUI EPA090100-P5-SZ 9Vac 1A wall wart that I have
Like 00940 says you will find that will dramatically lower the charging current in the input filter caps and through the transformer secondary (and diodes). It can still be significant however, and you need to make sure the ripple current rating of those capacitors from their data sheet is adequate. Remember to de-rate the capacitor's ripple current for frequency by the little table the cap data sheets give. At 50/60 Hz the rated ripple current (often given at 100kHz in the data sheet tables) is usually only 0.6 or so of the listed value. You may (probably) need to use two of the 2200 caps in parallel to split the ripple/charging current spike between them to stay within bounds, if the de-rated ripple current of one 2200uF is not enough. I would leave at least 20% overhead (extra) on that de-rated ripple number on each cap.
Also consider using a LT1963A (positive) and LT3015 (negative) adjustable regulator adjusted for +/-8V, respectively, instead of the LM fixed voltage regulators. Both of those LT low dropout voltage regulators only need about 0.5V across them at 250mA which would recover that 1.5V you are short on each rail. Both LT regulators also have significantly better noise ratings than the older LM parts. The LT regulators don't need those protection diodes across them either. The downside is more expensive and a few more parts on each (resistors and one small capacitor) to set the voltage.
And consider going with Schottky rectifier diodes, like the STPSL260
that would only have about 0.2Vdc of voltage drop at 250mA vs. the 0.7Vdc or so for the standard silicon rectifiers. That would recover another 0.5V or so that you are loosing on each rail. The Schottky's don't have quite as large of a current surge rating so that is why I've picked a 2A unit here. If you want to add a snubber to reduce any chance of secondary ringing caused by the rapid switching of the Schottky's, add a 0.01uF cap across the 9Vac line near the diodes, followed by a 0.068uF in series with a 110R resistor, also across the 9Vac line. The 0.01uF cap lowers the ringing frequency while the 110R resistor dissipates the energy. For the details see Hagerman's famous paper
Good luck and have fun!
Edited by agdr - 6/18/13 at 5:56pm