Dimming 'dimmable' LED lamps

[DanSteely]

Hi All,

 

I have a question regarding 'dimmable' LED lamps, namely how are they dimmed.

 

As an example CPC have the following Philips lamp for sale: www.farnell.com/datasheets/2192699.pdf.

 

In the base of the lamp is there some sort of voltage to pulse width modulation type circuit or does the varying supply alter the LED brightness in a more direct way?

 

Also what sort of dimmer curve could you expect when connected to a professional dimmer.

[timsabre]

They are quite complex. There is a driver ic in the lamp which measures the phase angle (on time) of the dimmer and uses that to control the drive current to the LED. (dimmers do not very the voltage anyway, they just chop out parts of the mains waveform).

 

They don't work very well with professional triac dimmers. They will snap on at about 25% and will flicker, may stick on at full. They work much better with trailing edge (mosfet) dimmers but I don't know of any professional ones like that. Varilight make some domestic ones that work well and you can buy din-rail mount DMX ones on ebay.

[Don Allen]

I have found from experience that Philips dimmable led's need to be controlled from a compatible Philips dimmer. They cannot dim all the way to zero as they need enough voltage to drive their internal electronics, so the drivers will dim smoothly down to a low point, then although you continue to turn the dimmer control down, nothing happens until they snap off to avoid strobing. Only way to get an led to dim to zero is to use an external control such as DMX or DALI which can be expensive

[TomHoward]

I might be wrong but don't most domestically fitted dimmer switches not go all the way down to zero anyway - don't they usually dim between say 20% and 100%?

[alistermorton]

The bog standard ones we had when we still had predominantly tungsten lamps at home went pretty much to zero - you could set the lamp so it was a dull red "worm" glowing if you chose to do so.

[Tomo]

Retrofit "Dimmable" LED lamps rely on their internal electronics to dim. Thus they cannot dim to or from zero, because the internal electronics need power too.

 

There's three basic types of AC dimmer:

Forward phase (SCR), Reverse phase (IGBT), Sinewave (including electronic, resistive and variac).

 

Sinewave will not dim a dimmable LED retrofit at all.

- Older household/theatrical dimmers were resistive, some concert halls use variacs.

 

The best LED retrofit lamps will go down to around 10% apparent brightness or so when driven "reverse phase", eg from an ETC Sensor3 EELV module.

On a forward phase (SCR) dimmer they usually don't manage as well.

 

They will also will snap on much higher than the lowest they'll go when fading out.

 

Below that, they will either flicker or go out/don't come on at all.

 

Unfortunately, their behaviour depends on the exact load on the circuit - they draw current in a strange way that can affect each other and the dimmer.

 

Worse, within a given batch of lamps they will also turn on at a variety of slightly different levels (sometimes called "popcorning")

- So perhaps one or two look good enough, but ten looks horrendous.

 

In short, dimmable LED retrofit lamps are NOT suitable for theatrical houselighting, and probably never will be.

 

For the foreseeable future, the only suitable LED options for houselighting are controlled via DMX (or DMX over Ethernet)

 

The easiest to retrofit are the ones that have wireless control, or extra-low-voltage with a remote driver that can be back in the dimmer room using the original wiring.

Otherwise you'd need to run DMX up to the fixtures, which isn't always possible.

[Don Allen]

A local community theatre group had LED houselights fitted as part of a major refurbishment that were specified as DALI control in the contract, but the electrical contractor put in Philips dimmers to save money. The result was LED houselights that dimmed down to about 20% then strobed at the bottom end which would have had a detrimental effect on the audience.

 

If you are going to try dimmable LED's, try the LED and controller combination before installation.

[sunray]

A local community theatre group had LED houselights fitted as part of a major refurbishment that were specified as DALI control in the contract, but the electrical contractor put in Philips dimmers to save money.

 

so the contractors have been brought back in to fulfil their obligations under the terms of the contract?

[Don Allen]

A local community theatre group had LED houselights fitted as part of a major refurbishment that were specified as DALI control in the contract, but the electrical contractor put in Philips dimmers to save money.

 

so the contractors have been brought back in to fulfil their obligations under the terms of the contract?

 

As usual they were not unfortunately, as the tender specified DALI LED drivers but LED controllers, not LED DALI controllers. It is all in the wording and interpretation. They did supply DALI LED drivers, with hard power and control wiring to each LED houselight, but just LED controllers or dimmers.