Kemo LED tester

Here is a look at old KEMO M087 LED tester module (the original M087 is discontinued product but there is a newer different looking M087M product. With the aid of this test module it is made possible to carry out tests with all kinds of light emitting diodes in order to check function, brightness, colour and polarity. This is a quite neat little device to use to do LED tests.

The LED’s could be tested optionally with following currents: ca. 1 mA, 2,5 mA, 5 mA, 10 mA, 20 mA and 50 mA. I have tape over the 50 mA pins for safety (not to fry small LEDs accidentally with over current for them).

The test contacts are designed to accept brand-new LED’s as well as soldered out LED’s with residual soldering tin at the connecting pins. The connectors work acceptably for this application, but they do not always make the most reliable connection to LEDs.

In order to facilitate selection of LED’s of equal brightness, there have been placed two test sockets with the same currents (10mA) side by side. So you can quickly get comparable brightness LEDs before soldering them in your projects.

Let’s take a look at what is inside the box.

As you can see there are there are only connectors, resistors and battery in it. The 9 V/DC battery provides power and the resistors limit the current to the LEDs. Here is a closer look at the circuit board.

With 9V battery a resistor makes a decent current source only for LEDs with forward biases in a narrow range. The actual current will vary by about 10% for FB in the range of 1.6-2.2 V for most LEDs. The voltage drop over blue and white LED may be in range 3 to 3.5 volts, which causes the current through LED to be 20-25% lower than the shown current. The reason for that is that with traditional LEDs the voltage drop on current limiter resistors is around 6.8-7.4V. With blue/white the voltage drop will be only around 5.5-6V. The current through resistor is directly proportional to the voltage over it.

Here is a teardown of another cheap LED tester:
https://www.epanorama.net/newepa/2018/05/01/cheap-ebay-led-tester-fixed/