walters said:
I still don't get how the transistors,resistors,capacitors current ratings can tell me how much the power supply current should be outputing like 400ma or 1250ma how would i know this ?
I think this has to do with Power and Wattage formulas maybe to find out the power supply current
Well this is how I do it (other might not).
I design the cct to work (using say generic OPAMP and such).
Once I get past this say concept design I move onto detailed design where I start specifying actual components.
ie I need a Schmitt inverter so I us a 74HC14 from Philips, I need an OPAMP and due to certain requirements (temp, slew,...) I choose a TLE2021.
I get to the resistors and check what voltage/current they will be seeing and make sure their footprint is sized for their power.
Once a detailed design is done I start by going over all datasheets and summing up all the MAXIMUM current (in worst-cases ie high temp).
I then have a rough current-level for the design.
Vero-board the design up and attach to a bench power-supply to get a more realistic current-drawn.
Now I do have a current-level being drawn I design/spec the DC
C converter. Say I had a design that took 100mA from 28V on the bench. I could design it for 1A at 28V but the isolation trasformer and FET's would be bigger then they need to be and thus useless, so I spec and size everything for 150mA (or higher depending on the inrush but that is part of the detail testing)
That way my PSU is only as big as it needs to be
REmember a load will draw as much current as it needs. IF the PSU is capable of supplying more, doesn't mean the load will take it (it will in a fault condition, another reason for keeping PSU to their minimum current for protection)