Electronics Teach-In Part 5:
Power Supplies by M.A. Stracey. 240V ±10% (with EU 230V range) 50Hz A.C. (Alternating Current) -> 6V D.C. (Direct Current). Has diagrams of:
Transformer which steps a Voltage down, at a ratio of 40:1 in this case, depending on how many wire turns each side has, from 'primary' side to 'secondary' side. Current goes down from 1A -> 1/40A or 25mA in an 'ideal' 100% efficient circuit. In actual fact 20% of current is wasted as heat in the conversion process.
Bridge Rectifier Circuit which converts A.C. -> D.C. using a network of 4 diodes giving a 'ripple current'. Obiwan posts to EPE ChatZone on Thursday, 15 June:
"Diodes have ratings, I select a diode for it's application if it isn't already in my box of 'goodies'. A 1N4001 that I might use in a simple power supply, say 100 mA, I will not use in one that delivers 10 Amps."
This has to be 'smoothed' using a Electrolytic Smoothing Capacitor which is connected from Vcc output to Gnd. Obiwan continues: "The value will depend on the current rating you are dealing with. A 5V power supply that supplies 10 mA will have a MUCH smaller capacitor than a supply that supplies 5V @100A."
A Regulator 3-pin IC is used to give a steady output voltage.
Typically: LM7805 -> 5V; LM7812 -> 12V ; LM317 ...
A Neon light or LED can be used to show the output is live.
Adjustable Power Supply by John Wilkinson.
"This power unit will deliver a current from zero to about 4.25A at a voltage from zero to about 23V." Used for repairing CB radios.
Doorbell for the Deaf by S. R. Wooding. Built using x4 2-input NOR Gates, a 10-line decade counter, and an opto-Triac which isolates the low voltage logic from the mains, while allowing it to trigger a 25W mains driven lamp. Plus some discrete components. With circuit diagram.