I own a really bright led lamp that I want to dim. It is powered by a constant current power supply that can supply 37mA between 11V to 17V. If my lamp is connected the voltage settles at 13.58V.
Please note, that there are more than four LEDs in unknown wiring in this lamp. I only drew four of them as illustration.
With a bench powersupply I determined, my desired brightness is reached at 1mA where the voltage drops to approximately 12.7V. To limit the current to 1mA while still using the constant current source I've added a parallel resistor like this:
For some reason this wont work. The voltage drops to 2.7V, the current on the branch with the resistor is 7.28mA, and the current on the branch with the led lamp is immeasurably small.
Is there something I'm missing? Or is there another possibility to dim the LEDs besides PWM? I thought abouth changing the sense resistor of the power supply but I don't really want to mess with the power supply.
Best Answer
Figure 1. LED I-V curves.
According to my Figure 1 a typical white LED could be expected to drop about 3.5 V at 37 mA. Four in series would drop 14 V which is in pretty close agreement with your 13.6 V measurement.
Accurate figures for low currents aren't often published but if my graph is any way accurate you should see a voltage drop of about 1.5 V at 1 mA or 6 V across the four in series. You're measuring 12.7 V so your LEDs don't match the graph. They appear to have a much more vertical I/V curve.
Your calculation looks pretty close as far as I can see so I don't know why it doesn't work.
A more efficient alternate solution would be to connect a series resistor to limit the current. The CC PSU will then rise to 17 V and you'll need to just find the series resistor value as you would with any constant voltage supply.