Created by
Created March 09, 2016
Last modified March 11, 2016
Tags constant-current-source   led  


LED yard lighting. Used to illuminate back yard with enough light to navigate paths safely at night.


This circuit uses three loops of 14 LEDs each. The LEDs are Everlight warm white high output. the system consumes about 3.3 W running and 0.3 W on standby. The protection circuit was added because the LED assemblies took quite a bit of work to fabricate and I did not want to destroy an entire loop of 14 if the system faulted, The fault shown in the diagram is a worst-case shorted voltage regulator.

Circuit notes:

The LM317 voltage regulator and 60.4 ohm resistor form a 20 mA current source in each of the LED loops, so in normal operation the total current through R4 is 60 mA and the voltage at the OA1- input is 60 mV.

If the current goes 10 mA above normal the voltage at OA1- will exceed the 70 mV reference at OA1+ and OA1 will turn off.

When OA1 turns off it pulls OA2+ below OA2-, turning off OA2. At that point OA2 is latched off through D2.

OA2 is reset with a pushbutton across R8. 'Fault Reset Switch' simulates this.

On startup, C1 keeps OA2- momentarily low long enough to turn on OA2.

In the Time Domain simulation: the fault occurs at 60 uS, it clears at 70 uS, and the reset pushbutton is pressed at 80 uS.

Not shown here: the ambient light sensor/timer that controls the LEDs.


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