back_ache Posted June 30, 2013 Share Posted June 30, 2013 I have had a simple idea and after googling around a bit havn't't found anyone else doing it and wondered if anyone else had. Basically I have seen lots of LED floods around which if were dimmable could be useful, I know I could hack each one by making a custom driver but wondered if there was an easier way, could you simply break in to the existing dc line inside the unit with a solid state relay and in turn feed that pwm? Obvious cavearts about mucking about with power but with an idea so obvious I can't help wondering whst I am missing. Tin foil hat on standby... Link to comment Share on other sites More sharing options...
Don Allen Posted June 30, 2013 Share Posted June 30, 2013 If you look at the driver circuit for a lot of leds, you may see an unused pwm input to the driver ic. This is where you insert your dimming component. To save costs, this pwm dimming function is not used as most users of budget floods do not ask for dimming. See page 10 LED Driver circuits. Link to comment Share on other sites More sharing options...
gyro_gearloose Posted June 30, 2013 Share Posted June 30, 2013 If the LED drivers don't have an accessible dimming function you could always use a Buck Puck driver. You could then use a simple potentiometer to dim the LED, or use a DMX PWM driver. I did post the circuit diagrams and firmware on here for one I made ages ago but I can't seem to find a link to it now. Link to comment Share on other sites More sharing options...
bigclive Posted June 30, 2013 Share Posted June 30, 2013 If you're talking about the generic LED floodlights on ebay that start at 10W and go upwards, then they tend to use minimalist universal voltage PSU modules with a restricted forward voltage and current range. I think you'd need to change the Whole PSU to get a dimming capability. On a plus note, I've got a couple of the 10W units and the construction is actually very good. They have a front LED section and a separate power supply section. The front unit is basically a large finned heatsink with the facility for a glass cover-plate. The whole LED thermal management thing is actually very good. Not sure how it scales up to the higher wattage LEDs though. It's worth noting that if you get a suitable LED from ebay with a roughly matching forward voltage and current, then it's easy to swap the LEDs out for other colours. The floodlights are also available with the other colours pre-fitted and an RGB version. The 10W LEDs themselves are interesting. They tend to be a square package with an array of nine 1W chips wired as either three parallel strings of three (10V 1A) or all in series (30V 350mA). I guess some may also use the multi-chip COB arrays now appearing that use a shaped aluminium cored PCB with the chips mounted in a suitable series/parallel array and then covered in a phosphor loaded or clear gel. These have incredibly good thermal transfer capability since the LEDs are within a millimetre of the main heatsink assembly. Link to comment Share on other sites More sharing options...
back_ache Posted July 5, 2013 Author Share Posted July 5, 2013 My idea was all LED floods, but initially the ones you get from UK wholesalers (Big Clives teardown on some of the stuff he found on ebay is an eye-opener!) stupid question though, if the device has an exist power supply that is giving the correct voltage and current, why can't you simply interupt its output to give pwm as you'd be cutting down the power not increasing it? Link to comment Share on other sites More sharing options...
timsabre Posted July 5, 2013 Share Posted July 5, 2013 Sometimes you can, but often the power supply will not behave well if you rapidly connect and disconnect the load by pwm'ming it. Link to comment Share on other sites More sharing options...
bigclive Posted July 5, 2013 Share Posted July 5, 2013 The power supplies have a rectifier and capacitor at the input to convert the mains to DC and then use a switching regulator to produce the required output over a wide input voltage range. You'd be trying to dim something that is designed to provide a constant output. This would result in a very odd dimming curve that basically went from full power to a sudden drop in a very small zone. The chopped mains waveform would also result in significant stress on the input circuitry of the light, possibly causing it to fail quite quickly. Link to comment Share on other sites More sharing options...
timsabre Posted July 5, 2013 Share Posted July 5, 2013 The power supplies have a rectifier and capacitor at the input to convert the mains to DC and then use a switching regulator to produce the required output over a wide input voltage range. You'd be trying to dim something that is designed to provide a constant output. This would result in a very odd dimming curve that basically went from full power to a sudden drop in a very small zone. The chopped mains waveform would also result in significant stress on the input circuitry of the light, possibly causing it to fail quite quickly. I think he was suggesting dimming the DC output of the power supply within the unit, not the input. Link to comment Share on other sites More sharing options...
bigclive Posted July 6, 2013 Share Posted July 6, 2013 I'm not sure you could even do that without causing issues, since the cheap PSU's might behave rather unpredictably when their load was disconnected. They also have rather minimalist smoothing on the output, so pulse width modulating the output might give odd results. Now replacing the entire PSU module would open up new options, but that could add significant time and material costs to a cheap product. Link to comment Share on other sites More sharing options...
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