Characterizing some LEDs

 Not every LED selling source can supply high quality datasheets. When the prices are too good to ignore, it's time to do your own characterization.

What is needed ...

• a voltage source - My voltage supply is a Ryobi 18v battery, with an adapter found on eBay for powering Hot Wheels go karts. The adapter has both an on/off switch and an inline fuse. The default fuse is 30A, but I've changed mine to 3A.

• a mini board that holds a regulator - These are little PCBs that will hold one adjustable regulator (AMS1117-ADJ or LM317LZ, your choice, they both have a 1.25v Vadj terminal. You will be using this device as a two-terminal constant current regulator circuit. This means that There is a Vin, a Vadj, and a Vout. You have to supply a resistor to program what current value the regulator is targeting. The regulator will then home in on the desired current, while dropping any excess voltage required to do this.

• a mini board with positions for 8 SMD resistors - I'm partial to the 0805 footprint, and load up the board with eight values between 91R1 and 200R. Some spread of values. You them use one of the eight resistors as the current sense for the regulator (mentioned above).

• a mini board with positions for 8 SMD LEDs - These do not have to be SMD LEDs, but using SMD is more compact. All eight LED devices should come from the same batch, cut tape, or reel. You then plug this board into your breadboard, and jumper so that the eight LEDs are arranged in series. Depending on the voltage required by each LED, you may have to dial back to 7 or 6 or 5 in the series string.

• A couple of DVMs - One is using to measure the current flowing the LED string, while the other monitors the voltage across the entire string. Divide the total voltage, by the number of LEDs in the string, to find the average Vfwd. These DVMs could be of any type, even the low cost red ones commonly seen at flea markets.

Rule #1 - Do not change jumpers with power applied. Turn off the power source, rearrange your circuit, power back up, and check your meter displays.

As you change the current control resistor choice, you can then build a spread sheet like this:


That shows the eight resistor values (across the top)

Each different group/color of LEDs is recorded in a block of data entries. After recording the data for one such entry, then we can link the values to a second sheet, and generate a chart showing the Voltage/Current captures and allowing you to determine at what current you want to run these devices.


Columns on left are the raw data points, vertical axis in the voltages, horizontal axis is the current point. The blue squares are the plot of voltage vs current. The last measured points show that the average device was being powered slightly below 2.12v and had a current flow (at that voltage) of ~14 mA. As the voltage is reduced, the current drops (or vice versa).

Hopefully this makes sense.

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