Amiga ATX power switch

[mikolas]

Hi,

Has anyone tried to implement the ATX power switching circuit here: http://www.ianstedman.co.uk/Amiga/amiga_hacks/Amiga_Power_supplies/body_amiga_power_supplies.html

I did it by the book but for some reason, the schematic simply did not work, ie. pushing the power button did nothing. After checking and double checking with multimeter, I accidentally turned on the PSU and found out that removing the 100k resistor and replacing it with a wire, the circuit started to work.

I'd just like to know if anyone has had success with the original schema, as I did not find any errors on my protoboard connections.

[mikolas]

Mugo wrote:

If your 100nF capacitor is defective, the circuit can reacts in a wrong way.

Also, note that you should keep the button pressed for some seconds (or some fraction) until the circuit reacts. This time is determined by the C1 and R4 values.

Try to reduce the R4 value to 47k and below, until you find a satisfatory timing.

The capacitor was not at fault as I used multiple different ones and no luck. I also thought the the resistor was at fault. However, one could also simplify the whole schematic if the switch is already debounced as I think ATX power switches are - I might be wrong though.

Mikko.

[mikolas]

Tried different resistor sizes, still no luck.

This makes me crazy as the schematic is really simple and I see no reason why it should not work. The debounce in this case actually should work so that C1 charges fast in order to generate a clock pulse (clk is posedge) and it really does not matter how fast it discharges as negative edge has no function at all. Of course R4 reduces ripple from the button as it reduces the discharge speed so you would not get posedge triggered by accident.

All in all, this still does not work unless R4 is removed altogether and I can assure you the protoboard is correctly wired. Maybe my 7474s are borked and sink too much current.

[mikolas]

@moto: I want to be able to use the original ATX power switch in order to make a perfect case mod :-)

[mikolas]

Hans_ wrote:
Looking at that circuit again, I don't think that tying all the +5V pins together is a good idea.

I intend not to tie the 5V lines together but instead use single 5V for the Amiga and power the rest of the system using molex connectors from the PSU. Only exception to this rule is feeding more power to the motherboard (for BPPC) using the floppy connector.

[mikolas]

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.

[mikolas]

@Hans_: Yes I meant you don't need a resistor for capacitor discharge, as it is also missing from Ian's proper Eagle file. A pull-up is of course necessary to keep the clock line high when the button is not pressed.