I'm trying to use off-the-shelf LED track lights (Amazon.com) and (Amazon.com) for a project that requires adjusting the brightness as a function of the time of day.
One of the products says its non-dimmable, and the other says it is dimmable but both will dim as a function of voltage (above a threshold of ~8V) and both will buzz if I try to apply a PWM signal.
I haven't yet sacrificed one and broken apart the casing, but I suspect there's a voltage regulator or similar circuitry preventing using a PWM dimming solution.
My question is two fold:
Is it safe to use voltage to adjust the brightness of these LEDs, or will I be damaging the components?
If it is safe, how can I use an arduino (and other components) to output an adjustable voltage between 8-12V (using a 12V power supply) -- the current will be in the 2A range.
Looks like the main chip is a PT4115, which is a LED driver/buck converter, so it's probably configured to drive the 5 LEDs in series at constant current.
Certainly would be. So it is specifically designed to compensate for any variation in input voltage and keep the brightness constant.
Note the quite large capacitor "Cin". (Interesting that the PCB is labelled even on a disposable part!) If this is indeed across the input, it will (apart from suppressing interference that the device causes,) resist the application of PWM. It presumably is across the input; its proximity to the output is purely in order to fit inside the tapered enclosure.
If you want to dim the lights you should investigate getting rid of the power supply that comes with them and design your own. You first need to know how much current they draw and what voltage they drop at that current. You then need a variable current source or a fixed current source that will withstand PWM.
Alternatively you might find that if you insert a MOSFET into the DC side of the power supply, between the power supply and the LEDs, you might be able to apply PWM there to chop the current to the lights. I have used this successful, but there is no guarantee that it will work with the particular lights you have.
Personally I would only consider doing this if I really wanted to use the specific lights and the case they are supplied in.
Once the luminaire contains a constant-current driver, you are stuck because it is designed to do just that. Feeding PWM into a large capacitor simply means that during the "on" cycle, the capacitor draws as much current as it needs to charge fully, you are merely stressing the system (with unnecessarily large current pulses) to no avail. Feeding a lower voltage is simply something the constant-current driver is specifically designed to compensate for until the voltage is so low the LEDs will not fully light.
"Dimmable" lights use a much smaller capacitor so that using AC phase dimming, the constant-current driver completely stops during the part of the AC cycle that the phase dimmer is switched off. It is effectively a much slower PWM (at 100 or 120 Hz). If you try to use a faster PWM, the smaller capacitor still charges fully during the "on" cycle.
Not entirely. What @Boffin points out (and I agree) is that many led drivers implement a dimming function, as does the PT4115 used here (click for datasheet). Pin 3 can be fed a logic level PWM signal which will PWM the led. I use a conceptually similar product and approach all the time.
The only point of attention is that an opto-isolator must be used if the LED bulb is powered from AC when driving the PT4115 with a PWM signal from an external source (like an Arduino). Otherwise, fireworks may occur. Powering them with 12V DC and tying the ground of both the Arduino/PWM source and the led bulb together would also work.
From a practical viewpoint, the drawback of course is that the led lamp housing and pcb need to be modified.
And in this case, quite a number of them - the two items described int the OP are a pack of 10 and a pack of 12.
Since they are intended to be driven from AC, they each contain a bridge rectifier so even when operating from DC, there is a diode in series with the negative supply (as well as the positive).
5 White LEDs in series would require at least 15 V. I count six LEDs in the second, presumably two parallel chains of 3.
Yeah, modification would be a series-production thing, I agree. I'm not worried about the bridge rectifier as it can easily be bypassed for running these off DC while maintaining the same ground reference. It's feasible, but if it's worthwhile, is a different matter.
