Ultimate constant-current DMX dimmer?

[Joakim Arfvidsson]

My lightning desire is for excellent dimming to 0.2%, with no flicker or even pwm artefacts. When I flick my eyes or move my head, I don't want a strobed light on my retina.

My lights are high-cri COB LEDs (bridgelux or cree), and so often require drive current of 30V or more.

The ultimate driver would:

- Use a logarithmic curve so that DMX's 256 steps are stretched out at the top end, allowing you to access 0.2 % at the bottom

- Output current of at least 1.2 A

- Output voltage of at least 32 V

- Cost per channel < $50 (about the cost of my downlights)

I had some false starts:

Dmx4all BB4. Great, customizable driver. Mainly limited by output voltage (24V).

Eldoled PowerDrive 561/A. While claiming to be constant-current, it actually uses PWM albeit at >500Hz so it's not that bad.

Ordering these for testing is becoming expensive:) But my house has ~120 downlights so I will need 40-120 channels depending on voltage of the driver and the LEDs I end up picking.

Any advice for devices to test would be appreciated!

[Duncan]

the log curve is a function of the dmx controller (ie loxones perception correction) not the driver so you dont need to worry about that bit
all CC dimmers still use pwm, delivering the stated current all the time but with pwm on/off -  linear cc drivers would be too inefficient otherwise.

http://www.ltechonline.com/html/en/Dimmable-LED-Driver/DMX-RDM-Series/DMX-CC-Driver/

take a look at these - these models have an inbuilt psu as well, with a wide range of current and voltage

They also do multichannel models without the psu

http://www.ltechonline.com/html/en/LED-Controller/DMX-RDM-Controller/DMX-PWM-CC-type/LT-858-CC.html

although these drivers seem to be limited to 1050ma

this one goes upto 1750ma

http://www.ltechonline.com/html/en/products/DMX-RDM-Controller/DMX-CC-Driver/DMX-50-500-1750-F1P1.html

as a matter of interest for me, what leds were you looking at?

[seb303]

I've used Eldoled PowerDrive for all my CC downlights (Orluna units).  Very happy with the dimming - perfect right down to zero to my eyes.  No visible flicker due to the high frequency PWM.

The drivers themselves can be configured with a logarithmic curve, or you can do it in Loxone config.  I don't think the bit quantisation would be visible whichever way you do it.

You'll need the Eldoled tool to program the drivers - to set the constant current level, DMX channel, etc.

Seb

[Rob_in]

Sorry I can't help (researching myself) but this is pretty interesting.

I assume you don't like the 'stock' Loxone RGBW DMX dimmer? PWM frequency too low? Won't dim low enough for you?

[Duncan]

the stock loxone rgbw dimmers are constant voltage, not contant current so wont fit this requirement

[Andrew B]

Once you're over 300-400 Hz, visible (even subliminal) flicker shouldn't be possible.  I am inordinately sensitive to it, and my system at 450 Hz seems fine.

Note that something many people don't seem to realize is that PWM frequency doesn't change depending on dimming level... the pulses just get shorter, and the gaps longer.

[Joakim Arfvidsson]

On Sunday, December 4, 2016 at 11:33:50 PM UTC-8, Duncan wrote:

the log curve is a function of the dmx controller (ie loxones perception correction) not the driver so you dont need to worry about that bit
all CC dimmers still use pwm, delivering the stated current all the time but with pwm on/off -  linear cc drivers would be too inefficient otherwise.

Using PWM internally is fine, but a good constant-current dimmer would then smooth the output to produce a near-constant current.

It's better to place the log curve in the driver, so that you get more resolution at the low end. I don't know for sure (have yet to set it up) but I expect I'll want a super-dim path light for walking around at night when wife is asleep. And fading looks much smoother when the 256 steps are spaced in a way that they're all useful (equally space perception-wise).

 

http://www.ltechonline.com/html/en/Dimmable-LED-Driver/DMX-RDM-Series/DMX-CC-Driver/

take a look at these - these models have an inbuilt psu as well, with a wide range of current and voltage

They also do multichannel models without the psu

http://www.ltechonline.com/html/en/LED-Controller/DMX-RDM-Controller/DMX-PWM-CC-type/LT-858-CC.html

 

although these drivers seem to be limited to 1050ma

this one goes upto 1750ma

http://www.ltechonline.com/html/en/products/DMX-RDM-Controller/DMX-CC-Driver/DMX-50-500-1750-F1P1.html

Sweet! These look promising!

 

as a matter of interest for me, what leds were you looking at?

I'm looking at using this: https://www.superbrightleds.com/moreinfo/recessed-downlights/led-recessed-light-engine-w-round-75mm-aimable-ring-115-watt-equivalent/2641/

(the driver can be snipped)

But, I will modify it to replace the LED with 3000K, 93-95 CRI, XLamp CXA1512  http://www.digikey.com/product-detail/en/cree-inc/CXA1512-0000-000F0YJ430H/CXA1512-0000-000F0YJ430H-ND/4928738

I'll use some other decorative fixtures as well, but these will be my main lighting source for the whole house.

[Joakim Arfvidsson]

On Monday, December 5, 2016 at 12:24:15 PM UTC-8, seb303 wrote:

I've used Eldoled PowerDrive for all my CC downlights (Orluna units).  Very happy with the dimming - perfect right down to zero to my eyes.  No visible flicker due to the high frequency PWM.

Glad to hear the positive review… they do seem acceptable to me. But I'm hoping for better.

Btw, here is a video showing demonstrating the PWM problems: https://goo.gl/photos/yDuzAfQdnEg6inro8

 

that's PowerDRIVE, Loxone RGBW (with their own WW spot), and dmx4all BB4 (TRUE constant current)

The drivers themselves can be configured with a logarithmic curve, or you can do it in Loxone config.  I don't think the bit quantisation would be visible whichever way you do it.

 

You'll need the Eldoled tool to program the drivers - to set the constant current level, DMX channel, etc.

Yeah I have that cable and I played around with this. I get better results by using the driver's curve. Notably, the driver also does *way* smoother fading than letting loxone do fading. So the right config is to do both curve and fade at the driver, not on the loxone side.

 

Seb

[Joakim Arfvidsson]

On Monday, December 5, 2016 at 4:27:15 PM UTC-8, Andrew B wrote:

Once you're over 300-400 Hz, visible (even subliminal) flicker shouldn't be possible.  I am inordinately sensitive to it, and my system at 450 Hz seems fine.

That's true so long as you don't move your eyes or the light, or if the light is diffuse. And you don't use a video recorder.

I have a high quality CV dimmer that has 244 Hz, 2 kHz, and 4 kHz. At 244 you can see plenty of PWM evidence (when moving eye or light source). At 4kHz this seems to be gone as well. But the Loxone WW spot can't handle 4kHz… its internal current regulator is not fast enough and it will be quite a bit brighter than your duty cycle commands. No low dimming at all.

 

Note that something many people don't seem to realize is that PWM frequency doesn't change depending on dimming level... the pulses just get shorter, and the gaps longer.

Actually, the PowerDrive does change its frequency it seems. They call it "hybrid drive" or something. At lower levels, the frequency is lower. This may be an engineering solution to handle minimum turn-on times in lights or in the internal hardware. At higher brightness the frequency goes up to what seems to be kHz+, before becoming seemingly solid.

[seb303]

On Tuesday, December 6, 2016 at 12:29:07 AM UTC, Joakim Arfvidsson wrote:

Using PWM internally is fine, but a good constant-current dimmer would then smooth the output to produce a near-constant current.

I'm not sure that variable current dimming is a good idea if you want to maintain a high CRI light output.  The LED colour characteristics will change as the current drops, whereas PWM gives consistent colour all the way down.  Of course, high frequency PWM is important to get pleasant dimming.

[seb303]

On Tuesday, December 6, 2016 at 12:40:45 AM UTC, Joakim Arfvidsson wrote:

Actually, the PowerDrive does change its frequency it seems. They call it "hybrid drive" or something. At lower levels, the frequency is lower. This may be an engineering solution to handle minimum turn-on times in lights or in the internal hardware. At higher brightness the frequency goes up to what seems to be kHz+, before becoming seemingly solid.

I think the type of lamp probably makes a difference too.  I have Orluna downlights which have an optical mixing chamber, rather than raw LEDs on the surface.  These dim right down to 0 without any perceivable flicker.  Whereas my linear LED strips, also using EldoLED drivers (albeit the constant voltage models), do have a slightly visible flicker at the very lowest dim levels when the PWM frequency is not so high.

[seb303]

On Tuesday, December 6, 2016 at 12:33:47 AM UTC, Joakim Arfvidsson wrote:

Yeah I have that cable and I played around with this. I get better results by using the driver's curve. Notably, the driver also does *way* smoother fading than letting loxone do fading. So the right config is to do both curve and fade at the driver, not on the loxone side.

 

The Loxone fading is acceptable I would say, but definitely slightly stepped.  This may be to do with the limits of the internal frame rate in the Miniserver, or the limit of the DMX protocol (which is pretty slow by modern standards).

[Duncan]

the filtering and exact circuitry of constant current dimmers/generators varies widely and there is a tradeoff between various parameters

see; http://www.eetimes.com/document.asp?doc_id=1273252 for a technical overview of the tradeoff between dimming speed and accuracy vs cost and size

that 2nd led cob chip you linked to has a maximum current of 1.2A, but all the rated parameters are measured at 700ma, so this cob module requires a 700ma constant current driver, or possibly 1050ma with sufficient heatsinking, but not 1.2A

[Clegger]

Hi Seb

I recently gave up on the Chinese mains-dimming units that have been discussed on here and have been looking at alternatives. The Eldoled CC units seem to be good quality at a reasonable price.

I got hold of an Eldoled Powerdrive 562/S (https://www.eldoled.com/led-drivers/powerdrive/50-watt/ac-562s/) this week and have had a play with it connected to the computer via their programming tool. As you say, the dimming quality is wonderful, and the tool seems decent.

The difficulty I have is that when I then connect it to my Loxone DMX cabling, nothing happens. To confirm, in the test room I've removed the drive units that were originally driven by the old dimmer and rewired the two lights in parallel. When that didn't work, I connected a single LED downlight directly to the Powerdrive, controlled by the Loxone DMX extension, but again it isn't working.

I'm aware that there are settings that might need tweaking (16 v 8 bit, for example), but am reticent to play around too much at this stage in case I break something.

Do you recall what changes, if any, you had to make to the default settings in order to get Loxone's DMX extension to talk to the Eldoled dimmer? 

Many thanks

Jeff

[Viktor Granbom]

Hi, have you tried the eldoled drivers with dali/knx controlling? Much smoother dimming performance. In my eyes perfect but when i got forced to use the loxone light controller the flickering started.

If you have a dmx/knx bridge try mapping a knx dimmer and try that dimming curve instead. I think it's what you want (but maybe don't want to pay for)....

/V

[Clegger]

I've now had a chance to play with this and it's working.

Well, I may need to define "working". I should have paid a little more attention to the maximum drive voltage per channel. While each channel will do up to 50W, the drive voltage maxes out at 50V. 

With this, I can drive up to 3 x 500mA Photonstar downlights with an EldoLED POWERdrive 562/s (https://www.eldoled.com/led-drivers/powerdrive/50-watt/ac-562s/). Unfortunately, when I add a fourth downlight it doesn't work, and the downlights just sit there briefly flashing every couple of seconds. I assume this because there isn't enough voltage to drive all four of them in series. Worse, nearly every room in the house has more than 4 downlights!

I've looked at the higher power units, but they only output 7 more volts per channel, which isn't enough to help. 

I could, in theory, wire the downlights in each room in series-parallel (eg, 2 downlights in series, in parallel with another 2 in series). I seem to recall my electrician thinking paralleling was potentially problematic due to mismatches between downlights possibly resulting in slightly different amounts of light being generated in each parallel branch at the same voltage. Might be wrong about that reasoning though - it was a long time ago.

Any thoughts on whether potential downsides to the series-parallel approach? The total power would still be well within each channel's capability.   

Any other ideas that lets me drive >3 downlights starting with a single supply cable to each room? I'd really like to avoid mains current dimming based on my experiences with it to date.

Thanks

Jeff

On Monday, December 5, 2016 at 8:24:15 PM UTC, seb303 wrote:

[Duncan]

You shouldn't run series in parallel with a constant current source. The risk is a much larger current going through one string and burning out that led string. If the number of LEDs in each string is higher then it becomes lower risk.

If you measure the voltage required to drive each string at max current you can add a series current limiting resistor and run them in constant voltage mode but that would require different drivers.

[Jeff S]

Thanks Duncan, that rings a bell with what the electrician was saying. It was trying to avoid running signal wires to every fitting that eventually led me down the mains-dimming road, but plainly that isn't working out very well. 

So option-wise, what am I left with? Is anyone aware of a high-quality, mains powered CC dimmer that can supply 500mA at more than 50V? I've had a bit of a look around and can't see anything that puts out more than 57V - most stop at 48 or 50V.

In the absence of CC dimmer packs that output more than 57V, what are my realistic alternatives (if any) to mains dimming for dimming a house full of Photonstar 500mA and 350mA downlights? 

Thanks

Jeff

On 9 October 2017 at 12:51, Duncan <joanne...@gmail.com> wrote:

You shouldn't run series in parallel with a constant current source. The risk is a much larger current going through one string and burning out that led string. If the number of LEDs in each string is higher then it becomes lower risk.

If you measure the voltage required to drive each string at max current you can add a series current limiting resistor and run them in constant voltage mode but that would require different drivers.

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[Duncan]

What's the voltage required at these max currents per fitting? I would suggest linking pairs in series with a resistor and using constant voltage dmx dimmers. For example I've just converted some RGB outdoor floodlights with remotes. Each led module has a string of 6x 300ma LEDs per colour. I measured the voltage required for each string at full output, picked a psu with a slightly higher voltage and added a resistor to each string, then the whole thing can be driven from a constant voltage dmx driver.

You could do this for each lamp rather than pairs but it depends on their voltage and is slightly less efficient, but wiring is easier as singles. Since they are cob LEDs each one is already a series string of LEDs hence the highvoltage per unit.

[206el...@gmail.com]

The resistor method to balance the current is possible - but when you calculate the resistor value for this to actually balance the LEDs, you will discover that to balance the current within 5% - you will need burn about 1 watt per 600 mA LED - so you might be wasting 10 or 20% in the balancing resistors, and still not have good balance.

We found this device recently to exactly balance the LEDs - it takes a constant current source and splits it into two matched strings of LED's. ATX-LED AL-Doubler https://www.shop.atxled.com/products/al-doubler?variant=17623340384371 - up to 22 watts of LEDs can be balanced.

They also have DMX wall Switches - that look like regular wall switches AL-DMX-WS https://www.shop.atxled.com/products/al-dmx-switch-dmx-wall-dimmer