Trailing edge or Leading edge for LED downlights?

[HappyHippyDad]

I am about to fit 9 5W GU10 LED's.

I realise that a Leading edge dimmer can be used but as I have not fitted a dimmer for LED's before I wondered if it was a much better idea to use a Trailing edge?

Also, I was under the impression that I had to meet the minimum wattage requirement for the dimmer switch, so my total wattage would be 45W, so a 250W dimmer would be ok as its minimum rating is 25W.

However I came across this on the web: LED Dimmer Switch Compatibility - Technical Information ? Light bulbs, LED bulbs, halogen lamps, spot lights and tubes

How Many Light Bulbs Can Be Used With A Dimmer?

All dimmer switches have a minimum and maximum rating (watts). Unfortunately LED loading cannot be calculated the same way as a traditional Incandescent or a Mains Halogen light bulb. A good ‘Rule Of Thumb’ to apply ,which is used by most manufacturers, is to take the maximum rating of the dimmer switch and divide by 10 (or in other words use 10% of the Max rating). As an example if you have a dimmer switch with a maximum rating of 400w (rule of thumb translates to 40w) you can run 5no 7.1w Dimmable LED light bulbs (see example below for further explanation). So this suggests I would need a 600W dimmer which cant be right!! (x10% = max 60W), have they got it wrong?

 

[topquark]

Leading edge, low load is generally what you'll need. The one thing I would say with the GU10 Leds is that they don't all fit! Some have slightly larger heatsinks around the lamp so won't quite fit in some fittings, try them out in the wholesalers if you can.

 

[D Skelton]

Leading edge, low load is generally what you'll need. The one thing I would say with the GU10 Leds is that they don't all fit! Some have slightly larger heatsinks around the lamp so won't quite fit in some fittings, try them out in the wholesalers if you can.

You mean trailing edge yeah?

 

[topquark]

In this case no, I meant what I said, they're mains voltage replacement lamps, so a resistive leading edge should be the tool of choice IMHO. Do you think that's wrong?

 

[topquark]

Something like one of these guys: 2 Way 120w LEDlite LED & Low Load Dimmer Switch - Module

 

[D Skelton]

In this case no, I meant what I said, they're mains voltage replacement lamps, so a resistive leading edge should be the tool of choice IMHO. Do you think that's wrong?

Most mains voltage GU10 replacement LEDs are manufactured in a way that makes them compatible with triac dimmers, however electronic dimming is still a far better solution. Capacitive loads such as electronic drivers and electronic transformers don't handle the high inrush current generated by triac dimmers very well. The triac dimmer will cause high current spikes at the start of the dimming half cycle as a result of capacitive leading current whereas a transition to a reverse phase dimming half cycle allows a smooth current increase leading to a drop, thus high inrush currents with a potential to damage the electronic elements within things such as LED drivers are non existant.

To summarise, GU10 LEDs will operate better and last longer with a trailing edge dimmer, other types of LED fitting will only work with a trailing edge dimmer.

 

[topquark]

I thought these were replacement lamps? I agree with most of what you've said (not sure what you meant by operate better), and it will allow for soft start etc. Trailing edge may need to be replaced should the lamps ever be substituted for GU10 50W halogens again. Also, when fault finding for eg, they will not provide continuity common-L1/L2 without voltage applied (so don't assume they're duff !).

So in answer to the OP trailing edge better (but more expensive), however not what I'd use in this case (assuming it is replacement lamps in existing GU10 mains fitting...which is the way I've read it).

 

[D Skelton]

I thought these were replacement lamps? I agree with most of what you've said (not sure what you meant by operate better), and it will allow for soft start etc. Trailing edge may need to be replaced should the lamps ever be substituted for GU10 50W halogens again. Also, when fault finding for eg, they will not provide continuity common-L1/L2 without voltage applied (so don't assume they're duff !).

So in answer to the OP trailing edge better (but more expensive), however not what I'd use in this case (assuming it is replacement lamps in existing GU10 mains fitting...which is the way I've read it).

They are replacement lamps, they will be designed to operate with a triac dimmer, but they will still perform (should have used this word instead of 'operate') better with an electronic dimmer. Common problems that can occur with dimmable GU10s on a triac dimmer are buzzing at the dimmer, flashing and flickering at low dim, reduced dimming range and variable performance based on the number of lamps controlled by one dimmer. Trailing edge is also better for filament lamps too (if ever the OP were to swap back to halogens), the soft start of a trailing edge dimmer exends lamp life.

 

[HappyHippyDad]

Thanks all,

Trailing edge it is!

As for the second part of the question, has that website got it wrong?

 

[D Skelton]

No, the maximum number of LEDs on a trailing or leading edge phase control dimmer should not exceed 10% of its maximum rating.

 

[HappyHippyDad]

No, the maximum number of LEDs on a trailing or leading edge phase control dimmer should not exceed 10% of its maximum rating.

Ok, thanks Mr D.

In that case 9x5w =45W which exceeds 10% of both the 250W and 400W dimmer switches? If I use a 600W dimmer the minimum load is 100W. This is getting confusing??

 

[D Skelton]

You will probably get away with it, but that is no guarantee, and as such will void any provided by the manufacturer of the lamps. I would fit a duel gang low load trailing edge dimmer.

 

[HappyHippyDad]

I started off just thinking I have to meet the minimum load requirement! Now I'm lost:

- If I had 4 LED's at 5W = 20W (This doesn't meet the min requirment for a 250W)
- If I had 5 LED's at 5W = 25W (This just squeezes into the min requirement of a 250W and just squeezes into the maximum of 10% of the 250).
- If I had 6 LED's at 5W = 30W (This meets min requirement of 25W for a 250 dimmer but is over 10% of the max - it also doesn't meet the minimum of 40W for a 400W dimmer)
- If I had 7 LED's at 5W = 35W (meets min requirement for 250W but not 10% of the max - doesn't meet min requirement for a 400W)

etc etc

Do I have to meet the minimum wattage requirement of the dimmer or 10% of the max or both (which seems impossible)?

- - - Updated - - -

 

[D Skelton]

Look at trailing edge dimmers man!

They have much lower minimum wattage values

This is the problem with dimmable LEDs in general! They are a PITA to design installations around!

 

[HappyHippyDad]

I have looked properly at Trailing edge dimmers now! :smiley2: Minimum is generally 10W which is great, just need to find a normal looking white one that switches up to 600W (10% = 60W) now!