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I have a project where I made a series of high power Osram OSLON blue leds using custom boards on aluminium PCB. I just put 16 LEDs in series and joined them to an appropriate constant current supply. They all light up well and the currrent/voltages all seems to what I expected when measured. However, when turned off the LEDs have a significant glow and the supply has a significant voltage across the circuit.

LEDS: https://uk.rs-online.com/web/p/visible-leds/1751980/

CC Supply: https://uk.rs-online.com/web/p/products/7911118

Do I need to improve my circuit so that it safely discharges? I take it that this would mean putting a resistor somewhere?

Thanks for your help!

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    \$\begingroup\$ "When turned off" - how turned off? \$\endgroup\$ – Andy aka Jul 19 '18 at 17:18
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    \$\begingroup\$ And, to get the 10% LED current thing, what do you do? \$\endgroup\$ – Andy aka Jul 19 '18 at 17:23
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    \$\begingroup\$ In terms of what would be acceptable, I don't know TBH! I'd be happy for any of your ideas :) I actually don't know what happens when the power goes off? I assume that the current drops to zero leaving the same voltage that was used when the current was 700mA discharging from some massive capacitors. \$\endgroup\$ – Alistair Boyer Jul 19 '18 at 17:30
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    \$\begingroup\$ @Andyaka, good question. LEDs usually don't care. But the drawings show some additional die on the substrate. The datasheet also talks about external ESD protection circuit on the side. Where is our power LED expert "misunderstood" when we need him? \$\endgroup\$ – Ale..chenski Jul 19 '18 at 17:37
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    \$\begingroup\$ Low Current operation of Indium-based LEDs (Blue,White) causes rapid defect growth due to uneven charge flow so dim intensity for current useage <10% dimishes rapidly. Fortunately cranking of the current burns off these defects so normal high current restores expected brightness \$\endgroup\$ – Sunnyskyguy EE75 Jul 19 '18 at 17:54
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@TonyEErocketscientist has the credit for this answer!!! Adding the bleeder resistor worked nicely! 5k Ohm, 5 W in parallel with series LED chain results in no discernible dimming of the LEDs and the voltage drops smoothly to <5V in 20 secs!!

Thanks for your help!

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The switch is not an on off switch. It toggles between 100%, 40%, and 10% of the 700 mA output.

You have to add your own switch if you want a power on off switch.

The current Mean Well PLM-40 series do not have the dim switch. The link is to a manual from 2013.

Manual PLM 40 Series



enter image description here

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