As many electronic hobbyists, I have always been fascinated by LEDs, especially the really bright ones. I could now ramble on about 50W or 100W LEDs, but to connect to the story of my previous post, I will talk a little about driving 1 Watt LEDs, the ones that come on a star shaped heatsink (e.g. the first Luxeon generation). While you could connect those with a current limiting resistor as you do with the small 20mA LEDs, it is not a really wise thing to do as you would need large resistors, withstanding an unhealthy amount of heat. So the device of choice is a constant current source – in this case one that provides 350mA constant current. I have three different ones and took some time to evaluate them from a users perspective (meaning ease of use, etc. – I will not talk about output ripple, stability, surges, etc.).
The good, the bad and the ugly?
- Meanwell LDD-350L
- ANVILEX CCS2-350 (based on the ZETEX ZXLD1366 chip)
- No-Name from ebay (based on the XLSemi XL4001E chip)
They all work pretty straight forward, you supply them with a voltage between 9V and 30V (+/-) and they output 350mA of current with which you can drive one or more 1Watt LEDs in series (provided the combined forward voltage doesn’t exceed your input voltage). No suprise here, they all do that and work just fine. But what about dimming the LEDs? Each of the drivers has a dedicated pin for that. And here it get’s complicated…
This one is the most uncomplicated of the bunch. To use the dimming pin, simply apply a PWM signal (in my case from an Arduino), and the driver will dim the LED according to your PWM duty cycle. 10% duty cycle = 10% current (which is not neccesarily 10% brightness as the relationship between brightness and current is not linear with these LEDs), 50% duty cycle = 50% current, 100% duty cycle = 100% current. You catch my drift. The driver will align the output to the duty cycle (and even the PWM frequency) of the DIM pin.
Since pins 3/9/10/11 and 5/6 (on the Arduino UNO compatibles) use two different PWM frequencies, this is reflected in the above wave forms.
As mentioned in the previous post, the ZETEX chip an this current source is a little odd with regard to PWM control. In my tests, I found out that a duty cycle between 0% and about 45% gives you current control from 0% to 100% (using a 5V PWM signal). If you raise the duty cycle to more than 45%, the driver will exponentially increase the output current, thus overdriving the LED. A duty cycle of 55% will yield more than 520mA output current, guaranteeing premature ageing and death of your LED. So be careful. This driver also copies the PWM frequency from the input source, which is exactly what the datasheet said: Depending upon the control frequency, this will provide either a continuous (dimmed) or a gated output current. Since the PWM frequency is above 300Hz, we see a gated output waveform on the oscilloscope:
To work with this driver, you have to make sure to limit the PWM duty cycle to a maximum less than 50%, otherwise you might fry your LED (and maybe more…). This driver also produces hissing noises varying on the PWM duty cycle which may annoy you depending on your use case.
No-Name (with XLSemi XL4001 chip)
This is one of the cheap LED drivers you can get on ebay for just a couple of Euros. It has almost the same components as the CCS2, namely input/output smoothing caps, schottky diode, an inductor and some resistors to configure the chip. The only real difference is the XLSemi XL4001 chip that is used on this module. It it not per se a LED driver, it is a multipurpose DC/DC buck converter, but it features a current control loop, so that it can be used as a constant current source too. This driver can be dimmed using PWM as well, but the output waveform has nothing in common with the input waveform. In fact, it looks a little odd:
Dimming this driver offers some pitfalls: the PWM pin is the ENABLE pin of the chip, which is inverted. This means a PWM duty cycle of 0% will cause the driver to output a current level of 100% and a duty cycle of 100% will shut the driver down. Just keep that in mind when designing your sketch or whatever. The other thing: although the datsheet says, that the default state of the ENABLE pin is low, it will not return to that state if the chip already received a PWM signal and the signal wire is disconnected afterwards. Instead, it will spiral upwards, causing the output off excessive amounts of current to the LED, slowly killing it. So when experimenting with it, be sure not to pull out any wires while the circuit is still live.
There is no winner to be named here, these drivers all do what they were made to do. If I had to pick a favorite, it would be the Meanwell LDD-350L because it is *so* uncomplicated to work with and there a no suprises. You can work with the others as well, but be sure to read up on the special traits of each driver/chip so you can avoid nasty surprises.