Current decreasing while measuring the current through a constant current source resistor

Question

My Circuit :

The above is my circuit.

The Forward drop of LEDs is typically 2.8V and a maximum of 3.15V. I designed the current through the string to be around 21mA, so the emitter resistor is 33ohms.

Now, while performing the actual measurement, I try to measure the current through the 33ohm resistor using the multimeter (Link attached below).

But what I am getting is that, initially, the current reading that I get from the meter is around 21mA, but if I let it measure for some more time, like for say,1-2 mins, the current reduces to 20mA.

This trend continues. The longer the time I measure the current through the resistor using the multimeter, the current reading keeps on decreasing. I kept the reading ON for like 5 mins and the reading turned out to be around 16mA. I didn't extend beyond that.

But I do not see the intensity of the LEDs decreasing. The Intensity and the brightness of the LEDs are the same (Like if the current through them was actually 21mA. No change in the LED brightness). If I measure the forward voltage of the LEDs, it is around 2.74V.

Any reason why this is happening?

Multimeter Link : 6 and half digital multimeter

Answer 1

All semiconductors (as well as some conductors) change forward voltage drop depending on temperature.

See which parts are getting warmer, using an infrared thermometer, and put a heat-sink on them (or just heat-sink likely culprits). Don't forget your LED string.

As to why you don't see a difference in brightness, visual response to light is approximately logarithmic. The stellar magnitude scale is based on that response. Simply put, you would barely notice a difference in brightness by a factor of ~1.4 of two lights, unless they were adjacent. A difference of 20 mA compared to 16 mA, ~20%, would not be apparent.