1
\$\begingroup\$

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.

UVLO

  • 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.

zener

  • 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.

relay

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:

relay

\$\endgroup\$
3
\$\begingroup\$

schematic

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.

schematic

simulate this circuit

Figure 2. PSU discharge circuit with no extra power loss during normal operation.

\$\endgroup\$
3
  • \$\begingroup\$ That's pretty clever. I never thought of that. \$\endgroup\$
    – Drakes
    Jul 27 '19 at 17:28
  • \$\begingroup\$ RLY1 is looking to be a pretty exotic solution to source. I like the idea, but an AC-powered relay here is looking to be too exotic and large. \$\endgroup\$
    – Drakes
    Jul 27 '19 at 17:55
  • 1
    \$\begingroup\$ They're not exotic at all. AC relays are commonly used in industrial applications. Look for "ice-cube" or "8-pin" relays for examples. \$\endgroup\$
    – Transistor
    Jul 27 '19 at 17:59
1
\$\begingroup\$

I'm appreciative of the solution by @Transistor which does work. I put this solution into practice and found that under no load the relay gets over 50 degrees C and emits a lot of EMI. I tried several relays to make sure it isn't a manufacturing defect. Darn.

High heat and EMI

I replaced the relay with a DPDT switch instead.

If this helps anyone else, it turns out that it is faster to bleed the load electronics than it is to bleed the PSU, so I moved the bleed resistor to the load electronics (see below), and also to eliminate a potential PSU short-circuit if the relay armature fails.

DPDT circuits gif

Here are the before-and-after voltage decay waveforms using the above solution:

Before and after power waveform

Problem solved. I made a detailed write-up for this here.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.