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LED slightly ON using constant current driver

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kuon:
Hello.

I use the following schematic to control a LED with a low current 1.2V control line from a CPU.



The LED is VLDB1232R-08‎, the driver is NCR402TR and the transistor is PDTC124XT,215.

My problem is very simple, when the control line LED_BLUE is at 0V, the LED is not 100% off, it is very slightly ON. And if I measure the LED anode, it's at about 2V.

This leads to the following questions:

- Why is it happening?
- Is it "normal" that it's happening, or is my design flawed?
- If it's "normal", should I add high value resistors in parallel with the LED to workaround it?

KT88:
The DS doesn't mention leakage current but it is likely a few uA. That already is sufficient to produce a slight glowing of the LED. Something in the ballpark of a MOhm should solve the issue.

Cerebus:
More than slight glowing if it's several uA. I use a single white LED as a night light in my bedroom. Supplied with a mere 1uA it chucks out enough light to illuminate the room sufficiently that you can get up in the middle of the night and see well enough to not bang into the furniture. It's just a 9V alkaline battery, a current limiting resistor and an LED - basically it pulls no more current than the battery's self discharge would, it lasts years of 24 hour operation on one battery. Modern LEDs are (1) very efficient, (2) produce light over about 5-6 decades of drive current.

kuon:
A 1MOhm resistor did dim the LED to something even lighter, but to get it completely off, I had to use 100kOhm. At 2V that's 20uA, that's not much "in absolute", but that's more than I thought.

Zero999:
Connect D2's cathode to Q4's collector.

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