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I am using a 24VDC output MeanWell LRS power supply. I convert it to 5VDC on my PCB using a buck converter. A green LED (power indicator of the system) is connected directly to the 5VDC of the PCB through a connector, with a 150 Ohms resistor in series. When I turn off the AC hot wire of the power supply, the LED only turns off after a couple of seconds, probably due to residual charge inside various capacitors. How can I make the switch off of the LED more immediate? Would it turn off faster if I powered it through a transistor controlled by a microprocessor?

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  • \$\begingroup\$ How would you connect such a transistor and microprocessor, and how would it drain the mains-side caps of the supply faster by any perceivable amount? \$\endgroup\$
    – Justme
    Commented Apr 1 at 21:40
  • \$\begingroup\$ I would simply power the microprocessor off the 5V as well and have one of its pins make the channel of the transistor conductive or not, but I'm not sure it would help much, I'm hoping someone has an explanation why it would, or a better and more conventional idea \$\endgroup\$
    – Tommy95
    Commented Apr 2 at 0:12
  • \$\begingroup\$ What is you concern about? About voltage not going away fast, or about your LED light following it? Do you want to have something like an early warning about power failure, or what? \$\endgroup\$ Commented Apr 2 at 2:12
  • \$\begingroup\$ I would like to have the LED turn OFF as soon as I switch OFF the AC -> DC power supply, despite its capacitors still holding some charge \$\endgroup\$
    – Tommy95
    Commented Apr 3 at 11:18

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Green LED's require a specific minimum voltage to glow at all, about 3.5 VDC for InGaN (though the voltage differs with chemical composition).

The easiest way to make the LED glow cut off sharply, given your circuit, is to tie the LED cathode not to ground, but to a voltage divider providing about 1.2 or 1.3 V above ground. And the simplest way to make such a divider is to put a resistor across the LED -- experiment with about 470 Ω as a starting point (exact value depends on the chemistry of the particular type of green diode, desired brightness, and desired shutoff delay).

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  • \$\begingroup\$ Thanks for your answer. I set a voltage divider at +1.2V. I connected the LED between it and the +5V. Unfortunately there was no time shut off improvement. I tried 150Ohms to 1kOhms resistors as the R1 of the voltage divider \$\endgroup\$
    – Tommy95
    Commented Apr 3 at 11:39

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