Dimmable LEDs - 8ch CBus Dimmer - Lights flicker when off

Hi Guys,

I know... same ol subject but I cant seem to find a solution in other posts

Situation:

- 8 ch CBus dimmer
- Max Dimmable Downlights per channel = 8@5w (Total 40w)
- Lights flicker like a discoteque when channels are turned off

Obviously the channels on the relay are always closed when off, so there must be a miniumum load per channel, so ive looked around and aparently the minimum load is 25w?? so why am I still getting a flash on the ch loaded with 40w? So does anyone know the true minimum load? Or does anyone has a solution for this already? If I have the mimimum load I could put a resistor in series to bring the load to mimimum

Has anyone had this problem and found a solution? cheers guys

 

Hello MVV
Could you post which LED and driver are giving you this problem for future reference?
I can't help much with your solution but I have found that Pierlite Starburst works and there are other brands mentioned on the forum.
There has been some ideas of using a CBus relay or an external relay running off a dimmer channel which drops out at around the minimum brightness.
I dont know if anyone has tried it but it doesn't seem a great idea to make things too complex for a specific led's problems.
I have read that one halogen lamp on the channel is enough load to stop the flashing but havent tried this myself.

 

This has very little to do with minimum load. The load type is the issue.

This is a common problem with certain LED and CFL load types when connected to dimmers. This has been covered many times on the forum before but the short version is that there are capacitors in the dimmer for EMC purposes that allow a small amount of current to trickle through the channel even when it's off. If the LED lamps haven't been designed for use with dimmers, then this trickle current can cause the electronics in the lamp to try and operate when there isn't sufficient power to do so.

The phenomenon is not typically seen on 2-wire circuits because there is typically a mechanical switch that is flicked when the channel is turned off. This is why sometimes people wire a dimmer channel in series with a relay channel and assign them the same group address to solve this issue.

What is required is a device that will collapse the voltage across the load whilst the dimmer is off. This prevents the LED power supply from trying to operate when it shouldn't. LED lamps that have been designed from the outset as dimmable, such as the Starburst, have devices internally that do this. Some other dimmable LED lamps don't. Non-dimmable types for sure do not.

To test the theory, if you put an incandescent light bulb in parallel with your LED lamps, even a small one of 15W or so, you should find that the flickering stops. You can also put a halogen downlight in parallel. The most elegant and long-term reliable solution is to fit a Clipsal 31CAP in parallel with the load on the dimmer channel. It has no filament to fail, so should last indefinitely, and solves the off-state flickering issue in lots of cases.

If you search the forum for posts on dimmer leakage current you will find a swag of posts on this topic.

One last point is that the above advice should solve any flashing or flickering you have when the dimmer channel is turned off. It's unlikely to help with any flickering or flashing observed whilst the dimmer channel is turned on.

 

Further to the above post, the use of the 31CAP in parallel with the dimmer channel is really only suitable for use with the C-Bus DIN Universal dimmer, not the leading edge dimmer. This is because the leading edge dimmer has a capacitance limit of 300nF and the 31CAP would put it over this limit.

 

Is there anything that can be used with the leading edge dimmers

 

You need to connect another load in parallel with the ones you have. The easiest solution is a single halogen downlight and associated transformer. however when the halogen fails you will have to replace it. Another option is to replace one of your existing LED downlights with another type of LED downlight that is designed for use on dimmers, as it will look after the bleed current.

Technically it can be done with mains-rated resistors, but you need to watch the resistor type fairly carefully and you need to make sure it's safe, can't overheat, etc.

Also, make sure you're under the 300nF limit for the leading edge dimmer. Some of the cheaper LED products have particularly large front-end filter capacitors.

 

Thanks for your reply I have not yet got my lights so I will hopefully be able to get ones that are suitable with out any other devices. My one issue is that a few of my chanels only have one lamp on it

 

If this is the case then you can instantly rule out all the "cheapies". You'll need to buy a good quality downlight. There are a few available that will happily run with 1 lamp on a C-Bus Leading Edge dimmer.

 

Thanks alot Newman, your info is much apprieciated!!

The LEDs are Chinese MR16 Dimmable 5w, transformer unknown, I didnt pay too much attention, the are CE rated thats all I know, they were brought by the customer, I'll have a close look on Thursday, as for the Din Dimmer, to tell you the truth I dont know the exact model number, when i ordered it I just asked for a 8ch Dim, so ill have to have a look at that on Thursday too, fingers crossed its a universal, then ill get 8 caps and put them in across Load CH & N and hopefully that fixes it.

Thanks alot again for the information! LESSON LEARNT!

 

is there a 8ch Universal Dimmer??? the 8ch Cbus Dimmer im using is the L5508D1A, max capacitance is 300nF, so i figure ive got to find a 100nf (.1uF) 500v ac capacitor? and somehow make it safe to put in the switchboard, heatshrink it?

 

Unless I have missed some new products the 8 channel din rail dimmers are leading edge.
The universal is the same size but 4 channel.

 

Correct. The L5508D1A and L5504D2A are 8 and 4 channel Leading Edge dimmers. The L5504D2U is 4-channel and the only DIN Universal Dimmer in the C-Bus range.

 

You have the Leading Edge dimmer, so you have to work with the 300nF limit of that dimmer. Before you can determine what value capacitor to fit, you need to determine the input capacitance of the LED lamps your customer has bought. You may be able to measure it with a multimeter, but some circuit topologies can affect the reading. The only way to be 100% sure would be to contact the manufacturer or open one up and have a look.

If, for example, each of the transformers has a 33nF capacitor at it's input, and there are 8 of them, that puts you at 264nF, leaving you only 36nF before you hit the limit. If the input capacitance of the transformers is greater than 37.5nF then you cannot connect 8 of them to a C-Bus Leading Edge dimmer, regardless of whether they flicker in the off state or not.

I actually suspect that the input capacitance of the transformers will be such that you can't fit a sufficiently large bypass capacitor, and that you'll end up having to wire the dimmer channel through a relay. Al alternative is to slap the owner with a wet trout for buying cheap crap that isn't compatible.

 

yeah i know! long story is the customer bought this stuff off a guy importing the crap, his mate is my old boss and is saying it will be fine, the reason hes not my boss anymore is because of bull**** like this, always talking bull****, ill let you guys know how i go!

 

At least now you know if it has problems you can go and grab some Starburst fittings which will work.
The time and effort you spent researching shows you are wiser than the other fellow.