New Light in Our Old RV
My wife and I bought an old RV last year and as is usual with old vehicles, the interior lighting is not state-of-the-art. Our new gadget is now 29 years old and the installed technology is just as old.
Figure 1: Our new old RV. (Image source: Michael Marwell)
The old lamps produce a nice dim light, which is pleasant for a romantic dinner, but only just enough for reading or board games. With full lighting, a lot of energy is also needed to illuminate the RV accordingly.
If we are on the camping site with the RV and have a power line available, the electricity supply is not a big problem. However, we plan to drive this year with the camper to Romania and camp there one or more nights in the free nature and wilderness. In this case, the lamps must be operated by the battery. Therefore, I want to achieve the best possible light with the lowest possible power consumption. My solution is to convert the lighting to LEDs.
The simplest possibility would be to take a 12 V LED strip, install it, and the project would be finished.
However, it is not quite that simple if the LEDs will have to provide reliable light for a long time. LEDs typically heat up relatively strongly during use, so the LEDs should not be glued directly to wood, but to a metal profile to ensure sufficient cooling.
In the past I was not very lucky with LED strips because the glue was loosened by heat and the LED strips were standing away from the ground.
That's why I prefer LED modules on a solid carrier, like a circuit board. Digi-Key offers a variety of LED modules available from stock (LED lighting - COB, Engines, Modules).
Considering color temperature, voltage, power, and other parameters, I chose the SI-B8T06128CWW LED module from Samsung Semiconductor, Inc. With a color temperature of 4000K, the light is not too cold for me and not too warm for my wife.
Although the on-board voltage is 12 V, the actual voltage is typically higher than 12 V. The chargers and the alternator feed up to 14.7 V, depending on the battery technology used and the charging curve set. If I connect the module directly to the on-board power supply via a light switch, there is a danger that the LED module will no longer function reliably due to a too high voltage, become too warm, and eventually fail.
So I need a suitable LED driver. Since I am a fan of dimmable light sources, the LED driver should also be dimmable.
Even if I know a bit about LED dimming, there is certainly much more to consider than I can think of. The technical article How to Dim an LED Without Compromising Light Quality is just what I need to learn more about this.
Nevertheless, I decided not to build the circuit myself and to use a ready-made solution.
I chose the RBD-12-0.50/W from Recom Power based on the important criteria of dimmability, suitable current limitation, input voltage range, and saving a printed circuit board. This version has cables that I can connect to the LED module and the existing cables.
This LED driver offers me the possibility to adjust the brightness either with a simple potentiometer, or alternatively with a PWM control. Furthermore, the buck-boost driver doesn't limit the voltage of my battery.
I quickly connected the components to a laboratory power supply and did various tests.
The first test with the LED module directly on the power supply shows that by increasing the voltage from 10 V to 15.5 V, the current increases significantly and the LEDs also increase their brightness. Since the modules are not specified for 15.5 V, I did this test only briefly to confirm my assumption. I strongly advise against doing this test, as the LED module became relatively warm after a short time.
As a second attempt I connected the LED driver with the LED module, isolated the unused cable ends of the LED driver and tested the voltage from 10 V to 15.5 V at the power supply again. The result exactly met my expectations. According to my subjective view, the LEDs kept a constant brightness over the whole input voltage range.
Figure 2: Quick test setup. (Image source: Digi-Key Electronics)
The voltage measured at the LED module was stable at 12 V despite different input voltages. I did not measure the output current of the LED driver, but the input current was reduced from about 600 mA at 10 V to about 400 mA at 15 V in the expected range.
For the third attempt I switched a potentiometer between the reference voltage output of the LED driver and ground and connected the tap to the analog dimmer line of the LED driver. This worked right away to adjust the brightness of the lamps.
However, the brightness control only worked in a part of the turning range of the potentiometer. For about half the potentiometer turning range, the LED was at full brightness.
The reason can be found quickly in the data sheet. As early as 1.5 V at the analog input, the output current is regulated to 100%. Since the reference voltage output provides a voltage of 5 V, the potentiometer also regulates the voltage from 0 V to 5 V. As a solution I placed a resistor between the potentiometer and the reference voltage output, and already I could use the potentiometer over the whole range for brightness control. To avoid overloading the reference voltage output I used a 27 kOhm resistor and a 10 kOhm potentiometer.
Analog dimming is sufficient for my requirements, so I didn't use the PWM input at the moment. Maybe I will make my motorhome a little smarter later, and then dim the lamps via a PWM circuit. At least I have the possibility to do that in the future now without a complete redesign.