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We are currently seeing some issues with an LED driver circuit.

I am not sure about root cause because there are too many variables.

During operation the LEDs are dim or do not light at all (after 2-3 months in operation.)

Attached is the portion of the driver circuit for the LED.

Any ideas that would cause excessive electrical stress in the form of excessive current applied to the LED or the LED to be dimmed or damaged? What would cause excessive current, besides a short? We have too many LEDs being damaged or going dim.

enter image description here

enter image description here

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    \$\begingroup\$ What LED current do you measure with a new LED in this circuit? \$\endgroup\$ Dec 23, 2021 at 14:05
  • \$\begingroup\$ Have you considered that the current could be within specifications, but the cooling might be insufficient? \$\endgroup\$
    – Simon B
    Dec 23, 2021 at 14:40
  • \$\begingroup\$ Show the power rails for the LM324 please. \$\endgroup\$
    – Andy aka
    Dec 23, 2021 at 14:42
  • \$\begingroup\$ Have you tried to independently test the circuit that connects to the anode and the circuit that connects to the cathode? Does the anode circuit supply enough current at the desired voltage? Does the cathode circuit? What happens if you replace the LED with a resistor that has the same desired current and voltage drop? \$\endgroup\$ Dec 23, 2021 at 15:00
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    \$\begingroup\$ The datasheet you show says the diode voltage is 3.2V. Why does your schematic say 2.4V? \$\endgroup\$ Dec 23, 2021 at 15:02

1 Answer 1

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Your circuit can easily push more current through the LED than the LED is rated for.

Your circuit tries to set the voltage on R59 to match the DAC voltage.

To get 5V at the junction of R59 and the LED cathode, you have to have something like 40 mA through R59. That means 40mA through the LED as well.

Your LED is rated for 20mA in normal operation. It has an absolute maximum of 30mA.

When the DAC signal exceeds 3.63V, the LED current will exceed its absolute maximum. At 5V from the DAC, the LED will have to handle over 40mA.

To stay in a safe area, you need to limit the voltage across R59 to no more than 2.42V.

Put a voltage divider between DAC and the positive input of the op-amp.

Like this:

schematic

simulate this circuit – Schematic created using CircuitLab

R2 and R3 reduce the DAC voltage to a maximum of 2.4V. The LM324 will adjust its output to get the DAC voltage at the junction of R1 and D1. 2.4V across R1 gives you a smidge under 20mA. Lower DAC voltages result in lower current.

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  • \$\begingroup\$ Or increase R59 appropriately. (simpler, but may run out of headroom with a 7.5V supply) \$\endgroup\$ Dec 23, 2021 at 15:52

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