Current Regulated LED Tester

Step 4 How to use the tester

There are two ways to test an LED. First, you can plug it into the female header. Based on the 1st image, anode is the top hole and cathode is the bottom hole. Secondly, you can use the magnetic SMD adapter. Just place the LED terminals on the adapter and it will stick there. Similarly, anode is the top pad and cathode is the bottom pad. The magnetic SMD adapter, as the name suggest, is supposed to be used for testing SMD LEDs. I don't have any SMD LEDs at hand but the magnetic SMD adapter works as can be seen when I tested it with a regular diode. The pads are also great for quickly touching the leads of your LED onto to check for polarity, color, and brightness. You don't have to worry about shorting the pads as the current will be limited to a maximum of 32mA. No harm will be done to the circuit nor the battery.

This tester was designed for the convenience of measuring the voltage and current. You can either use the test pads or the test loops. The middle test pad/loop is common. The top test pad/loop (refer to 1st image) is for measuring voltage and the bottom test pad/loop is for measuring current. When measuring current, you will have to remove the shorting block. For intuitive purposes, the jumper was placed between the middle and bottom test pads/loops.

Assuming your LED doesn't come with any specifications, you'd want to know how much current and voltage to supply it to get the brightness you want. First, hook up you multimeter to measure the current and remove the shorting block. Place your LED on the tester and adjust the trimpot (you can make this simple tool to turn the knob) until you are satisfied with the brightness. If you are unsure of the maximum current that you can supply to your LED it is usually safe to assume an optimal working current of 20mA. Record down how much current is flowing through the LED (lets assume its 25mA). Next, replace the shorting block and measure the voltage. Record it down (lets assume its 1.8V). Now let's say you want to power this led from a 5V supply. You would then have to drop 3.2V from the 5V to reach the 1.8V needed to power your LED (5V - 1.8V = 3.2V). Since we know your LED consumes 25mA, we can therefore calculate the resistance needed to drop 3.2V from the equation V / I = R.

3.2V / 0.025A = 128 Ohms

You can now connect a 128 ohm resistor in series with your LED and power it with 5V to get the exact brightness that you want. Most of the time you will be unable to find a resistor with the exact value of resistance that you calculated. In that case, you may want to get the next highest resistance value just to be safe.

Happy testing!