Now then, those huge caps, could you explain this to me Tim cos I'm lost there? I would prob in reality have tried two 330uf caps cos I have hundreds of them.
( my power supplies are usually in the order of 300 - 1,500VDC at around 60 -200 ma for valve HT supplies with about 20uf max and two chokes around 10-20H each)
The reason for the bigger caps is because of the greater current draw, from what I read, Eric was wanting a PSU for a stepper motor driver, perhaps CNC. If his steppers are rated for 3amp and he has at least 2 of the ( x & y axis) there will be times when both are operating at the same time. Now even if you have a (say for arguments sake) 10A transformer, with a instant current demand there could be a slight delay (1/100th sec if using a bridge rectifier, 1/50th sec if just normal rectifier) in the supply of said current demand, this could lead to lost steps and innaccuracies in the CNC. Hence the need for such big caps, they just hold more current that is instantly available for use. In fact I would be tempted to put 2 10,000uF caps instead of the 2nd single cap in case of greater current demand in the future (ie a z axis and even a 4th stepper for a rotab), and I would get 63volt rated caps in case he wanted to up the transformer voltage to gain a higher top end speed. ( I know steppers rotate in fixed steps, but the speed that they can "Step" from one position to the next is limited by the voltage they are driven at)
The reason you will have used lower values of Cap on your valve amps is that they draw much less current, although at heart-stoppingly high voltages. While the wattage may work out similar for Eric's psu needs, when dealing with lower voltage and higher amperage the uf of the caps need to be much higher for them to work effectively as buffers/smoothers of the DC.
In low voltage, high amperage you can not have too much uF, the only thing you have to be careful of is to limit the inrush of current to charge the caps at switch-on, and that is achieved by the 2 0.5ohm resistors, if you dont have those you will find yourself going through a lot of fuses and in extreme cases possibly damaging the transformer. (but that would be in a very extreme case)
But take all that with a healthy dose of salt,(or beer
) I am not qualified by any secular organisation, all this comes from just reading a lot, experimenting a lot, and also a miss-spent youth
messing around building various electronic projects like power mosfet amps, psu's, my dad's LGB railway layout controllers and other stuff.
Hope this explains my thinking,
Tim
