I have a 200W 5VDC power supply (Meanwell LRS 200-5). When I shut off the AC it takes about 60s for the supply to drain fully through the connected sensitive components (processors and storage drives).
When the supply is shut off I require either for it to drain within about 200ms, or to be "disconnected" from the load and it can drain as slowly as it likes. Basically, either 5VDC or nothing is supplied to the load. As a stopgap, I hold a power resistor to the PSU by hand each time I shut it off.
There are two loads in parallel so I can either introduce two protection circuits rated 100W each, or a single 200W solution.
Researched solutions:
- An UVLO sounds promising, but I need to design one for 5V at the high wattage which I don't know how to do.
- A Zener diode and resistor to drain the PSU, but I'd prefer to not introduce more resistance on the PSU as it's maxed already.
- A relay that shuts off on low voltage, but I've only been able to locate automotive 12V relays at the power required. Also, I'm worried about back EMF as the relay shuts off so then I'd have to introduce more circuitry.
What solutions are used in the wild? If I could go with the simple Zener+resistor solution on the regulated 5V PSU, what values of Zener and R would I be looking at?
Update: The solution I’m adopting is the relay solution proposed by @Transistor:
Best Answer
simulate this circuit – Schematic created using CircuitLab
Figure 1. AC monitoring and shut-off.
Monitor the AC instead. RLY1 will drop out when the AC is switched off. It's contact will open and disconnect the DC output.
simulate this circuit
Figure 2. PSU discharge circuit with no extra power loss during normal operation.