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
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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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