Constant Current LED Tester

Posted on Feb 7, 2014

Most of my circuits still use 5 V regulated power for the chips, because 8-bit microcontrollers run faster and some sensors require 5 V. Since 5 V is more than enough for even a white LED, I`m not concerned with the voltage needed by a particular LED in use. However, since my robots are battery powered, I am concerned with the current usage, since that determines

Constant Current LED Tester
Click here to download the full size of the above Circuit.

how quickly the battery is exhausted. Furthermore, an LED can be destroyed by too much current (usually anything beyond 30 mA). Therefore, I decided to make the LED tester based on current, not voltage. As described previously, the LED tester can be set to supply a specific current from 2 mA to 26 mA. The circuit will automatically vary the voltage from nearly 0 V to about 7. 5 volts in order to supply the current. I don`t need to adjust the voltage, and I don`t need to care about the current after I`ve set it. Many people are accustomed to voltage regulators, like the standard 7805. But, surprisingly, most voltage regulators can be set up to regulate current instead. For this circuit, I chose the National Semiconductor LM317L that I purchased from DigiKey or Mouser Electronics (or LM317LZ from Electronic Goldmine ). The LM317L is cheap, small, and includes instructions in the datasheet for using it as a current regulator. The semiconductor circuitry inside of the LM317L uses a simple feedback loop to automatically vary the output voltage to meet the required current. SW1: (optional) The power switch is optional in this circuit because current can only pass through a test LED and the capacitor (C1). Low value ceramic capacitors at low voltages leak very little current (less than 1/10 of a nanoamp as far as I could measure). And, there`s no point of connecting a test LED with the power off. Since no power will be used when...

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