Amiga replacement PSU
From Grahams Wiki
Another relatively minor project was to replace my duff Amiga PSU with a slightly more modern and infinately more working one.
The original replacement done in 1999 was simply a AT style power supply, with the wires connected to the appropriate wires on an Amiga PSU lead.
When this died in 2009, i replaced it with a miniATX power supply. This involves a small circuit as it is electrically controlled. Tell you the truth, i could have got away with a switch between one of the common wires and the green pwron wire. However, thats no fun :-)
Contents |
[edit] How it works
The control circuit is quite simple. Its one half of a 7474 JK flip flop.
The circuit is powered from the +5vSB supply, which should be present whenever the power supply is plugged in. The only other lead from the board is the pwron which is active low.
When powered on, Whether the flopflop is on or off is determined by the Set/reset inputs. These inputs are pulled high to have no effect, however, depending on the state of the switch, one of them is connected to a 1uf Capacitor. This ensures one of the inputs goes high slightly after the other, which means the power supply can default to "ON" at power on or "OFF" at power on depending on the state of the switch. The exact values of the the capacitor and resistor are not really critical, however, if the capacitor is too low, there may be a delay before the 7474 can respond to the power button. 1 have found 1uf does just fine.
The power-on switch is debounced by another 1uf capacitor. This is pulled high by a 10k resistor. This provides sufficient debouncing. The only limitation on this is that resistor must be of a lower value than the resistors pulling up the S/R inputs, otherwise, the clock input may not be steady before the selected preset goes high, which means it may not power up in a consistant state.
The Bi-colour LED is of course optional, and only used cos I cant buy 1 of something, and have loads of the things. Its good to have some form of power indication though, a normal LED will do just fine.
[edit] pictures of the electronics
 The Circuit diagram |
 The MiniITX board. |
 The wiring loom connections. |
 The assembled control board |
 The assembled control board |
 The assembled control board with accessories |
[edit] The final box
Because i was bored, i made a wooden box for it. It isnt great, but it does the job. The box is in two sections, the bottom half containing the electronics and the top part containing the 12v power brick. I could have kept this seperate, but that would mean two boxes.
I have mostly wired everything together using terminal blocks. Cos its easier to mess around inside with them. There is also a terminal block mounted outside for the extra molex connectors required for things like CD drives. One end has a 3 inch fan and the other end has exhaust holes. At the bottom left, you can see the LED and the power on state switch.
The two sections are held together by wooden pins.
Note on the final pic, the power plug mounted on the side to provide somewere for the plug to plug into. Also note the terminal block next to it. Thats because there is only 1 +12v output from the power supply, and were also one 0v lead short.
[edit] pictures of the final box
 The box front |
 The box rear |
 Top and bottom. |
 Mounted Electronics |
 Control PCB location. |
 Power PSU. Note the power plug. |
