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Sunrise LED

A project log for easy-alarm-clock

Improvements for the ubiquitous alarm clock

christoph-tackChristoph Tack 06/24/2018 at 16:120 Comments

Background

A gradually brighter light can mimic the sun rise.  It will allow for gentle wake-up in the morning.  

Experience with the Philips HF3463 showed me that you shouldn't count on it to get up on time.  I sometimes find myself waking up by the alarm sound and facing the alarm clock in full brightness.  When you're sleeping on your side and the light is behind you, it won't wake you either.

LED driver modules

The plan is to power the clock from a 5V USB power adapter.  The LEDs require about 12V/500mA.  So we need a boosting LED module, not a buck module.

If we decide to make the module ourselves, we need to select an IC:

Digikey selection (for 1A switch current): RT4533GJ6 and MP3202 (pin compatible), TP61165 (not pin compatible)

In the hindsight, the LM3410 would have been a better option, because it allows for larger drive currents.

Revision 1.0

Details about this circuit can be found on EasyEDA.

The circuitry above has some issues:

The RT4533 announces itself as boost converter for 10W LEDs 10 WLEDs.  It's important where you put the space.  Richtek means you can drive 10 white LEDs.  It's easy to misread it as : "RT4533 can drive 10W LEDs".

It may well be possible to drive LEDs, but not at 10W.  Let me explain:

  1. The maximum input voltage for the RT4533 is 5.5V
  2. The maximum switch current is 1.2A.  For a boost regulator, this is also the maximum current through the inductor.  The current through the inductor is essentially the input current to the circuit.
  3. So maximum input power is 5.5 * 1.2 = 6.6W
  4. Let's be optimistic and assume the output power is 80% of the input power : 5.3W, which is far from the advertized 10W.
Measurement setup: dual power supply (set to max. 1.96A. Voltage output was disabled when picture was taken), R&S Current probe, R&S differential voltage probe.

Measurement results

The output current drops linearly with the control voltage, as expected.

Efficiency is around 80%, as shown on the RT4533 datasheet.

The module already saw a second use as a replacement driver for lighting the christmas tree. The original driver was specified for 32V/100mA. Setting the two sense resistors to 3.9ohm and you're done.  Power draw on the 5V power supply was about 800mA.

Dimming the light using PWM

Earlier experiments with dimming LEDs showed me that 8bits is not enough to generate smooth transitions.  The step from 1/256 duty cycle to 2/256 is very noticeable.  Once you're at duty cycle 200/256, increasing the duty cycle doesn't make much difference.

A simple way to increase the PWM resolution is to use two PWM channels, a dual RC-filter and an opamp.  Hackaday already published an article about the subject.

Needed output power

A Philips HF3463 sunrise clock uses a Philps 100W 1200lm Softone lamp.  Replacing the bulb with a 60W 700lm still gives enough light.  So our light should also be able to output about 700lm.

Selection of the lighting source

Light source cooling

The above mentioned LED COB sources require that the aluminum substrate is not hotter than 70°C.  If we want to maximize light output, we'll have to add a heat sink to get rid of the excessive heat.

There's no reliable info about the thermal resistance of the LEDs.  Some tests will need to be done:

  1. Attach thermo couple to the LED strip
  2. Measure room temperature T1
  3. Leave LED panel hanging in free air.  Don't lie it flat on a table or other surface.
  4. Attach current source to the LED and regulate current (I) so that steady state temperature of Al-substrate of LED is 70°C (T2)
  5. Measure voltage over LED (U)
  6. Calculate thermal resistance of LED panel = (70-T1)/ (U * I)
  7. Attach heat sink to LED panel.
  8. Repeat step 1-5.

100x60x10mm : €1.55/pce heat sink will be used. 

This is not the cheapest heat sink around.  Care should be taken to the thermal resistance of the heat sink.  The LEDs can work up to 85°C.  Any hotter and their lifetime will decrease rapidly.  At a room temperature of max. 40°C, this allows for an extra 45°C temperature rise due to LED heating.  The total resistance of the thermal path should be smaller than 45°C / 10W = 4.5°C/W.  The thermal resistance of the LED is already 1.4°C/W.  This leaves about 3.1°C/W for the heat sink.  

AliExpress vendors don't publish the thermal resistances of their heat sinks.  We'll have to measure thermal resistance by ourselves.

Another way is to go with some simple rule of my old colleague : "If your heat sink is too hot to touch, then it's too small".

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