Versatile LED Tester with Eagle Project Files
Eagle project files for the Muffsy Constant Current LED Tester. This is the preferred version, as it has a power on LED that acts as a load for the LM317 so that it can deliver lower currents.
x-zip-compressed - 32.30 kB - 11/29/2016 at 22:06
Eagle project files for the Muffsy Constant Current LED Tester. It's not recommended to use this project, since it will only give a minimum of ~2-3 mA.
x-zip-compressed - 22.32 kB - 11/28/2016 at 12:17
A voltmeter and ammeter should give all the necessary information about my "LUT" (LED Under Test):
Then, knowing the voltage of your circuit (Vc) will give you the following formula for deciding your resistor value (R):
R = (Vc - Vf) / If
Just a short recap on how an LED works before I continue. LED brightness is not controlled by how much voltage you feed it with, rather by how much current is running through it. To get the brightness you want, you adjust the current with a current limiting resistor according to Ohm's law (R = U / I)
The voltage (U) is the difference between your circuit voltage and the voltage drop in the LED. The current (I) is the current needed for the brightness you want. The voltage drop is (almost) always stated as a single voltage. Knowing the current, your circuit voltage and the voltage drop, you can use a calculator to find the resistor value.
So, know what voltage your circuit runs on, know the brightness you want, and find the LED's voltage drop and stuff it in a calculator (or use the formula above). Not too much work, right? Turns out I've been wrong for all these years.
I noticed that the forward voltage drop decreased at lower currents, so I got to read my very first LED datasheet. Here's a graph from a random LED datasheet that confirms what I saw:
On one of my cheap Chinese LED's had a variation in forward voltage drop from 1.6 to 2.6 volts. The result at lower brightness (i.e. lower current) is:
There's the explanation for why I always need higher value resistors than I expect at lower brightness.
This really made this LED Tester more useful, as it turns out that the formula for selecting the correct resistor value is really more of a guesstimate. (You can of course get it right by buying more expensive LEDs and getting the correct datasheet. Still, there's no substitution for actually seeing the brightness.)
Blogger theslowdiyer in Denmark has made a deluxe version of this LED Tester, complete with drawings:
(Picture courtesy of https://theslowdiyer.wordpress.com)
The discussion in the previous log showed that I'll never get less than 2-3 mA from an LM317, as that is the minimum current it can provide while still regulating. (Discussion here: https://hackaday.io/project/18624-muffsy-constant-current-led-tester/log/49640-testing-my-leds#discussion-list)
I looked for a way to make sure there was a current draw, while still allowing me to adjust the current to the "LUT" (LED Under Test), and found this: http://electronics.stackexchange.com/questions/211249/lm317-µa-constant-current-source-possibility
An LED from LM317's power output to ground acts as a load and draws current from the voltage regulator. Some 4 mA would be sufficient.
Here's the updated schematic:
Here's a PCB with the new circuit, the Eagle files have been replaced with this version:
The pictures didn't come out particularly well, as the highest brightness gave a lot of glare and the lowest brightness seems brighter than it really is.
Lowest brightness (it's barely visible in real life):
Highest brightness (pretty darn bright):
LEDs keep accumulating, and they are spread around in my work space. Most of them are clear, see-through LEDs. Some are dead (either by abuse, or because they are cheap Chinese diodes).
Most of them are high intensity LEDs, and they often need below 1 mA so they won't blind me.
In other words:
I need a way to test my LEDs to see if they work, which color they are, and find a suitable brightness.
After having used the LM317 in various projects, I had noticed that it could be used as a constant current source. This got me thinking about using it with LEDs.
Here's the circuit I came up with:
Forget for a moment that the pinout on the regulator is wrong (the attached Eagle project files are correct). Here's how it works:
I built it, and it works.
The linear pot that I used makes the LED go darker relatively fast, a logarithmic one is better.