Most spotlamps to build into a ceiling are ridiculously high, 8 cm or more. The reason is that most of them are still to fit halogen lamps that require a special mirror and produce a lot of heat. LED lamps should not have this problem because they spread light better and because they are so efficient as to not produce a lot of heat. But most lamps are still high, even if these contain LEDs, probably because companies need to get rid of parts/machines first before they renovate …

There are exceptions and these we bought, at good price too. These are 6 Watt lamps, require only 4 cm of height (even less if needed), and produce an enormous amount of light: 450 lumen. Halogen lamps would need at least a power of 15 Watt to produce the same light intensity: about 10 Watts wasted to heat! The set consisted of three lamps, but two turned out to be sufficient for our renovated bath room. In actual fact, after mounting them we found out that the light was still too intense to be comfortable but there was no way of dimming the light. (Well, later on we found that in Germany there was a power supply available that would allow electronic light level control, but alas.) Here the challenge for me, a former electronics engineer, popped up.

LED lamps are not connected in parallel as we are used to but in series, like a light string for the Christmas tree. To control the light intensity efficiently, one uses a Pulse Width Modulator (PWM): basically a controller that switches the lamps on and off at a rate that is too fast to be noticed by the human eye. The duty cycle, i.e. the part of time that the light is switched on in relation to the total time, determines the observed light intensity. Such a controller can be readily bought. The one I selected was an RCD-48-0.35 from RECOM, available from Conrad. It required 48 Volt power input and delivered pulse width  current with an amplitude of 350 mA as the LED lamps required. The duty cycle is controlled by an input voltage. A suitable power supply for 48 Volts and more than the required current, actually 630 mA, is also available at Conrad; a Mean Well AC/DC print power supply. We had good experience with “Klik aan Klik uit” (KaKu) remote light control units so we wished to install one here too. For LED lamps there is the special version ACM-LV10.

This choice turned out to be the source of some trouble. The KaKu controller needed 12-24 Volts supply and delivered 0-10 Volt control voltage, with 0 Volts for no light and about 10 Volts for full light intensity. The PWM unit required 0 Volts for full light intensity and 5 Volts for no light. It seemed ridiculous with the amount of excess power available in the power supply to use a separate power supply for the KaKu, so I built a simple DC/DC-converter myself (see below).

In principle, a simple 78L12 integrated power regulator should be enough to supply the required 12 Volt were it not that it cannot handle the 48 Volt input voltage. Current demand is estimated to be less than 30 mA for the KaKu device and some extra electronics circuitry. A pre-regulator was added based on a BD243C and a zener diode at 20 Volts. Admittedly, the BD243C is a bit of an overkill but it can easily handle both the voltage (Vceo = 80 V) and the power. A resistor of 3k3 is dimensioned such that the zener diode gets some 10 mA and the 1N4004 is added to protect against short circuiting.

To convert the control voltage from the KaKu device into something the PWM unit could handle an inverting level shifter was built. The input JFET is needed as input buffer because the KaKu device cannot deliver a lot of current. At the other end an emitter follower is added so that enough power was available to drive the PWM unit. The middle transistor serves as the inverting level shifter. The diode in the JFET source line causes the voltage across the 1k8 resistor, roughly one tenth of the output voltage of the KaKu device, to be about the same as through the emitter resistance of 390 ohm. A 10 times voltage drop will develop across the collector resistance of 4k7 so that at the collector the same voltage swing is avaible as output from the KaKu device except inverted and level shifted. The resistor divider to the emitter follower halves this swing to render it suitable for the PWM unit. The JFET and transistors are standard transistors with moderate current amplification.

The electronic circuitry nicely fits in a low height plastic box that can be put in the space over the ceiling. The only care that needs to be taken is the conncetion to the LED lamps. A special connector was acquired for that purpose.

The settings for minimal light intensity are controlled through the KaKu device, please, check their manual.