Amiga ATX power switch

[Hans_]

@redrumloa

Why is this circuit excessive? It's a single chip with a few resistors and capacitors.

@motorollin

There isn't much in the circuit. A single d-latch (IC1 a) toggles the ATX power on/off pin whenever someone pushes the button. A capacitor and resistor (C1 & R4) ensure that you can't rapidly switch it on and off due to the switch bouncing.

The second d-latch (IC1 b) is unused, but you have to tie its inputs down, or the latch will switch on and off like crazy and waste power. You can't buy a single-latch chip.

The other capacitor is a decoupling capacitor to ensure a stable power supply to the chip (yes it's necessary) while the other resistors simply pull the input voltages to the chip up to 5V.

Hans

[Hans_]

Looking at that circuit again, I don't think that tying all the +5V pins together is a good idea. It's definitely not necessary for the power switching circuit, and your Amiga won't need that much current either. These pins could be connected to separate +5V outputs from within the power supply and are supposed to supply several amps each (IIRC). Any disparity in +5V from any of these will cause currents to flow between the multiple +5V outputs. At the bare minimum, it's a waste of power.

Hans

[Hans_]

mikolas wrote:
Well well well. I finally decided to check out the actual board file with Eagle. It seems that the PNG is outdated and the Eagle file has pretty much the same circuit (apart from one 4K7 resistor instead of 10K) that I made. My hunch was that the resistor was not needed as 74LS74 clock is Schmitt-triggered.

The resistor on the clock line is needed to pull the input up to 5V when the switch isn't pressed, so it is necessary. It is also part of a low-pass RC filter (R2 and C1) that prevents the switch bouncing from triggering the clock input multiple times for a single press. All switches bounce a little, that is, the contact will bounce up and down for a few milliseconds, causing the signal to fluctuate.

The value of R2 is pretty flexible though, so 10K should still work fine.

Hans