Maybe it would work ok with just the LED and a resistor - Even on AC (The LED is a diode in itself)? OK, maybe the led would flash at 50Hz , but this shouldn't be much of a problem.
I thought about that, but I figured making a bridge rectifier should be easy enough and it was. I even added a capacitor to smooth out the pulse. The led looks rock solid when lite. BTW we have 60hz @ ~120V on this side of the pond.
EDIT: What's the relay switching? The 9VAC side or the mains side of the transformer? If im right in saying it’s a reed relay; they aren’t normally designed to switch much current. Maybe a 100mA or so at most, and not usually mains either.
It is indeed a reed relay and it is on the 9VAC side, not the main. I could have put it on the main side, it is rated for 1A at 125V. C128 brick power supplies are rated for 1A for 9VAC, so this can easily handle it.
The step down transformer I am using for 9VAC is freaking huge, only because it was the best/cheapest I could find for this project in a quick search. It is completely and entirely overkill for this application. It is a multi-tap transformer with only the 9VAC being used and it is rated at like 2A on the main side. It was cheap, it cost me about $6 + shipping. The main electronics houses here didn't have anything even close in stock.
EDIT2: Hey, what about having a high current relay, controlled from the ATX PSU, switching the mains side of the transformer? That way, you can save on the energy costs of having the transformer powered up all the time, and you can use a neon indicator (W. resistor of course) on the mains side rather than trying to use a LED.
I was originally thinking about putting this relay on the main side, but I decided I didn't want exposed 120V inside the case. Yeah I probably could have mounted this board in the PS itself, but I didn't feel comfortable doing that. The transformer will be energized all the time, but how much loss can it really have idle? It can't be much. I can always still switch off the ATX switch, which will kill it too. It picks up power after the ATX switch.
Thanks for the great comments and suggestions! :-)
