Dimming Constant current drivers for led lighting

Hi all,

I have got 2 banks of 10 x 6w LED downlighters which have mains fed constant current dimmable drivers on each light but want to control them through my velbus system.

Do I:

A/ dim the mains to the drivers ( tried it with the 400w dimmer on velbus but the lights are jumpy in progression and dont fully turn off)

B/ use the 0-10v vmb4dc through the vmbrgbdc and purchase a constant current driver that will run 10 x 6w LED downlighters and connect these 2 modules on the outgoing side of the said driver

C/ rewire the leds to run in series with each other but not sure if its a safe way of doing so as the voltage would have to be high in series

Any thoughts or suggestions would be deeply appreciated
Thankyou in advance



Speaking from experience and lots of testing.

My advice would always go for your plan B.

That said, some low voltage LED lamps Dim better than others.

The majority of one’s I’ve tested work well, but I have had experience of some that just don’t Dim, or burn out really quickly.

Testing is knowing :wink:

Just be aware that there are three methods to achieve dimming.

Constant current

Constant voltage


PWM (which is what the VMBRGBDC module uses) tends be the most common and most compatible.

You’ll soon know if the lamps you’ve got don’t like it.

One other thing to consider is the frequency of the PWM.

The VMBRGBDC dimmers run at 90Hz, which is really slow.
The Dim curve isn’t great.

However, there is a 500Hz dimmer available now, which should have a much nicer dimming curve.

Maybe one of the Velbus team would be kind enough to put the link in here for this dimmer :wink:

Similarly, there are other manufacturers that produce low voltage dimmers that work very well with the VMB4DC 10v output.

Please let us know how you get on.



Hi Stuart,

Thanks for your input on this. The only thing that made me hesitate going down the B/ root was a guy said to me at an electrical store.
“you cant do it that way because as you dim down the voltage then the driver tries to increase the current”. I’m pretty sure he’s wrong
on that as the LED will only be able to draw what the voltage is governing it to and a constant current driver will always produce the
same current regardless of load that’s the whole point. Or am I talking rubbish.

Thanks again Stuart I’m going to find a suitable driver and give it a bash. Am I right in thinking only use one channel out the 3 as I need
to set the current to how many lamps I’m drawing off it and wont be able to use the other circuits as it is a common power input for all 3
Plus if I’m only running one channel and have a larger capacity driver to cater for all 3, when i’ve only got one bank of lights on they would get the hit of current from the other 2 channels?

Just realised something, do you mean you can get a constant current driver on the output side of each channel which is 12v in and 12v out? that would be cool as I could then buy 3 constant current drivers for RGB module


Hello Gizmo

Just to defend the “guy down the shop”, in true terms he is spot on.

If you “dim down” (which down in a leading edge phase shopping fashion by the VMBDMI), all that the switch mode power supply will do is be more efficient and attempt to maintain the constant output voltage.
Which is why you got the terrible results when you tried it.

Your method B suggested that you wanted to follow the true lines of low voltage dimming, as per the diagram I’ve created for you.

This way, you are taking your mains supply down to a constant voltage (12V or 24V) and feed it into the common input of the VMBRGBDC dimmer module.

As you have noted, it does have a common + output, and 3 x 0V “dimmed” PWM outputs.

You can use these as feeds for 3 channels of lighting, assuming that you are driving these 3 dimming channels (at 0~10V) from 3 channels of a VMB4DC.

Common anode dimming modules are very useful if you are intending to dim RGB Led lamps, which have a common Anode.

While it’s often cheaper to create common cathode dimming modules, doing so would have caused problems for those of us who need to dim RGB lighting.

From what I’ve discovered over the years, there is no harm in PWM dimming multiple single channel LEDs with a Common Anode dimmer (IE Dimming the Cathode)

I hope this helps.



Hi Stuart , wow thanks for the flow chart that explains it nice and easy.

The only thing I cant get my head round is the lights I have do not have built in resistors to control current through them, like the 5 meter strip lights you see, its the CC driver doing that part
so that’s why I waffle on about using the loads across all 3

So the question is and I will try to make this as least painful as possible:

Do I use a constant current driver where you have it in your flow chart as lighting power but can only use one channel or can I wire 3 of them on the output of the RGB controller?

Sorry to be a pain but I don’t want to blow the controller trying it

Thanks for your time



Here comes a simple answer…

You can’t use a constant current ‘driver’ / power supply in that combination, because the load will vary.

I am curious why you think your led lamps don’t have a load resistor in them? Are they very special ones that run at <3V?

Good luck


Just as extra information

These are the only dimmers I’ve found that can handle ‘Dimmerable mains drivers’


It’s not a solution for controlling via Velbus, as there is no way to remotely control these dimmers, but they do work.

Due to all the mess and confusion with LED Dimmerable mains replacements etc, I’ve simply adopted the following rule;

Mains dimming is for true mains filament lamps only.

LED dimming should only be done at low voltage, using something like the plan I’ve posted.

It’s made my life a lot easier :wink:

Hi Stuart thanks for that

Ive gone for do or die approach as this is driving me insane and holding up my progress


Hopefully these will work if not im out of pocket by £70 for 20 of them but better than paying £30 for one due to minimum order and 20 was postage free

Wish me luck mate or reply and tell me it wont work…lol



I think you need a refund

I don’t see how that’s going to work for you.

Unless there is something you’re not disclosing.

That seems to be a LED dimming driver to run from a low voltage input.

Where (in my diagram) do you think you’d be putting these?

What I would have recommended is a 90w or 120w 12V constant voltage 240V power supply.

Good luck

Oh bugger,

My intentions were to put them in after the RGB unit

Don’t forget the RGB unit output is PWM and that your Current LED driver needs constant voltage input!

I’m also working with various LED models using solution B or MDAR one. It works very well, even using different models and power of 12V AC/DC LED on the same RGB channel! There is a disatvantage with the dim curve and it’s not possible to dim very low, some LEDs need to have a minimum of 20% or more dimming signal from VMB4DC otherwise it will flash.
I often use a Velleman RS-75-12 power supply connected directly to the 12/24v SUPPLY of the VMBRGBDC

Hello Virago

It’s interesting that you comment on the Dim curve.

Which dimmers are you using?

Are you using the VMBRGBDC, which is a 90Hz PWM?

I only ask because I tested a 200Hz dimmer from another supplier and the results were amazing.

I’m really looking forward to testing the new Velbus dimmer, as it is apparently a 500Hz PWM ! :slight_smile:

Yes, I use VMBRGBDC modules to drive the LEDs. Even if I switch on the LED to 100% and then dimming it down, it will begin flashing at 20% (depending of the LED model). The dimming module is a VMB4DC. Meanwhile I have no problem using VMB4DC controling a Niko dimmer (0-10v control) to dim halogen lamps through an electronic halogen transfo, dimming at 1% works.
I’m also waiting for the new LED driver from Velbus!
What kind of “another supplier” driver are you using? I’m curious to try it

Ok folks I think I’m going to learn the hard way with this one. I have even considered now ripping out the LED strips inside these units I have bought and replacing them with the ones with resistors on cos I really don’t know if its worth all the hassle and cost to get it right. It seems a dark science to me. I understand the principles but cant waste much time on it any more. The units I bought were recommended by a company that manufacture and design LED lighting but are not familiar with Velbus but I did send them the links to the kit I would be using including data sheets and this was what they came back with. Mmmmm. Im seeing the easy solution of adding one halogen in the circuit somewhere which will smooth everything out by past experience.

Once again though folks, thanks for your input on this. I will let you know how rubbish it is once they arrive

Hi Gizmo,

You sound like a man on the edge :frowning:

Before you rip out the guts of these units, just try a short strip hanging from a low voltage dimmer and see what happens.

What’s the worst that can happen?

I feel your pain regarding led suppliers, I’ve only been dealing with them for 2 years and already I’ve got some ‘stories’ to tell.

I took note of what someone said when I started selling Velbus products…

“Testing is knowing”

So now I only recommend units that I’ve had on soak test for a while.


These are the dimmers that I’ve had great success with.
They come in various channels and either common anode, or common cathode.

Lewis even has a rather clever solid state relay / dimmer that can take a PWM feed and mirror it precisely.

So a single PWM output can drive up to 100 of these slave 20amp dimmers, at 12V or 24V!

Now that’s some serious low voltage dimming.


I’ll go back to Velbus dimmers once I’ve tried the new 500 Hz versions :wink:

Revised findings:

I have now realised that the original supply voltage to these LEDs is higher than I thought. Im getting a reading of 18v from the mains driver. I have just connected up a 5S lipo pack and we are in business. All I have to do now is purchase an 18v PSU after I have tested it on the Dimmer first. Had a positive boost now to continue :slight_smile:

ok, parts have just arrived this evening and yes MDAR I feel like im on the edge. AAAARRRRRGGGGGGGGGHHHHHHHHHH!!! …LOL

I have just wired one up and on first contact off a 13.8v supply I get a little flash of brightness for a split second then you can just barely see a tiny dot of light from the centre of the LED
Im guessing this is to do with the ON/OFF / Dimming white lead of which im not sure what to do with. it mentions on the meanwell site 5th page down see link:


I have shorted this to -Vin which completely shuts the tiny light output off as stated but not sure why i am not getting full on with it being open circuit as suggested

My intentions were to use my RGB dimmer for that part and was expecting this to be on full brightness when open circuit.

I knew this was going to be trouble.

I appreciate the help you guys have given me.

The one part that is driving me insane is that the LEDs are run from a constant current driver so I cant use one common power supply eg: constant current to feed all 3 channels on RGB due to the fact that I will be trying to control each channel separately so the other the driver would be feeding that extra current to the channel which is in use that will overload the LEDs. If im right in my thinking. I know this might sound a bit lazy and im not expecting you folks to do all the donkey work for me but could you help some more on this before I have no finger nails left 8o(

P.S I have measured the output voltage and it is giving the same as the input of 13.8v is it the current which is being restricted here for the light output. this is not set up through any dimming control by the way, I have just sat that module between the PSU and Light just for testing purposes

Hi MDAR and Virago,


Sussed it im up and running and its sweet as a nut surprised to have full range dimming too

Thank you ever so much for your inputs I would of given up ages ago without that


Hi Gizmo

I’m sorry to say that I’ve got some really bad news for you.

On your behalf I’ve spoken to a couple of electronics engineers about Constant Current dimming of LEDs.

One guy said it was impossible and that dimming constant voltage LEDs with a PWM dimmer was the only realistic solution.

Whereas another guy said…

Ummmmm, yes it’s possible, just really expensive and complex.

So I’m afraid that it looks like your only option is to rip out the constant current strips and replace them with constant voltage strips.

You’ll get much better results.

I really hope it doesn’t cost you too much in time or money.

Good luck



If you need any more Velbus kit, please drop me a line.


I’ve just seen your last post, not sure why it didn’t display whilst I typed my previous post.

For the sake of our sanity and to help everyone else, can you detail exactly what you’ve done to achieve your goal.

I’m fascinated, especially as I’ve been told that it can’t be done.



In case anyone is interested, this is the software I used to create the diagram.

The auto arrange feature is really useful.

I haven’t tried adding images of Velbus modules into it yet, but I’ll try that soon.


Thanks Stuart,

Basically I have inserted the Meanwell LDD-350LW in the circuit between the RGB output and the LED strips.

My biggest problem that I could not work out was the faint light output from my 13.8v supply feeding it so
had to go to Meanwell direct for advise and he pinned it down to not enough voltage to drive my strips based on the
info I gave him.
Apparently the voltage is not so critical on constant current drivers as it is limiting the current drain to the LEDS
but a minimum voltage is expected through it. I might be explaining this horribly wrong but I hope you get what I mean

The range on the chip is 2-36v I think. I supplied it with an 18v supply which gave me a nearly full brightness comparison to
the original mains constant driver and 24v made it perfect same light output. Lifespan results may take time to conclude but
we are looking good so far.

It has cost me £85 for 20 modules but no one seems to have them in stock so I was limited on shopping around

Ordered my 24v 150w Din Rail PSU which should be here tomorrow so I can finally get this complete.

There is one thing I have noticed, when it gets to the low end of dimming, it has a very slight flicker which I assume is the 90hz
on the RGB dimmer but not noticeable in the grand scheme of things. Im guessing the new unit coming out will resolve this issue?

Im assuming by your comments about Velbus products, you can get them?

I do need another white touchscreen VMBGPOD and im considering the VMBHIS if you could provide a price that would be good