Dimmers not working - leading, trailing, TRIAC - very confused!

Joined
15 Dec 2008
Messages
199
Reaction score
0
Location
London
Country
United Kingdom
This must be a hated topic, but please bear with me – dimmer switches!

I have had a full flat rewire. I liked the look of the Schneider ultimate flat plate white metal series, so selected that. Have a bunch of the LED dimmers used for Philips GU10 lamps.

They work great.

I also purchased some LED wall lights from Italy (built-in LEDs, not replaceable). The website for the lights showed they are TRIAC. But I checked with the UK company I bought them from (before buying!), and they said they were compatible with trailing edge dimmers (a la Schneider).

Now the problem…..

I have 4 of these wall lights in the lounge – 2 high level ones (30w each), 2 low level (17w each). There will be another floor lamp (not yet purchased), and therefore a 3g, 2 way dimmer. Following Schneider’s instructions, the electrician fitted retractive switches to enable dimming from 2 locations. There are no other lights in the room.

High-level lights – seem to work perfectly.

Low-level lights – dim perfectly, but don’t fully switch off. They retain a (permanent) glow.

I have 2 more wall lights in the hallway (17w each), with the same issue. Dim fine, but don’t turn off (I can see about ¼ of the small individual LEDs remain lit and never switch off unless I turn off at the main fusebox.

Does anyone have any ideas?! Schneider do a standard (presumably leading-edge) dimmer, but it explicitly states “not suitable for LEDs.

Varilight have the V-pro range that has both leading and trailing edge settings. But on the website it says that it is NOT compatible for TRIAC dimming, and you have to use the V-Com range. I’m confused, as I thought TRIAC and leading-edge are one and the same thing.

Sorry for length of post, but would love to hear any thoughts/ideas!
 
Sponsored Links
Joined
28 Jan 2011
Messages
50,114
Reaction score
3,351
Location
Buckinghamshire
Country
United Kingdom
The website for the lights showed they are TRIAC.
I don't understand what that means ....
Varilight have the V-pro range that has both leading and trailing edge settings. But on the website it says that it is NOT compatible for TRIAC dimming, and you have to use the V-Com range. I’m confused, as I thought TRIAC and leading-edge are one and the same thing.
I am again rather confused. A "TRIAC" is a semiconductor device (basically a semiconductor 'switch') which I thought that virtually all dimmers, whether leading-edge or trailing edge, used to control the output.

As you imply, compatibility between dimmers and LEDs is a nightmare subject, but that does not help me to understand the above.

Kind Regards, John
 
Joined
15 Dec 2008
Messages
199
Reaction score
0
Location
London
Country
United Kingdom
Thanks, John. My knowledge is at best limited, so I'm none the wiser. Looking at the Varilight FAQ:

Is the V-Pro a TRIAC based dimmer?
  • The V-Pro is not a TRIAC based dimmer. For applications that require TRIAC dimmers, select our V-Com series of dimmers.
The link to one of the lights is below - and you can see a symbol saying 'Dim Triac'

Just very frustrating as I can't understand why the 2 lights aren't switching off whereas the other one is!
 
Joined
3 Nov 2006
Messages
26,587
Reaction score
2,812
Location
Bedfordshire
Country
United Kingdom
The website for the lights showed they are TRIAC

I don't understand what that means ....

The TRIAC is the switching device in the dimmer. It is triggered to be ON some time after the voltage has gone through the zero crossing point and remains ON ( latched ) until the voltage returns to zero. Once triggered ON a triac cannot be turned OFF ( unlatched ) until the voltage goes to zero which causes the triac to un-latch itself.

The alternative switch is a power transistor which can be switched ON and OFF at any time in the mains cycle. Most often these are a MOSFET which is turned ON at the start of the main cycle and then turned OFF some time during the cycle.

They can also controlled in a way that simulates the action of a triac by being turned ON ( and held ON ) some time after the zero crossing point and then turned OFF at the next zero crossing point.
 
Joined
3 Nov 2006
Messages
26,587
Reaction score
2,812
Location
Bedfordshire
Country
United Kingdom
"Triac" equates to Leading Edge while not Triac equates to Trailing Edge ( or may the other way round if I am wrong )
 
Joined
15 Dec 2008
Messages
199
Reaction score
0
Location
London
Country
United Kingdom
Thanks both. I see that the link I posted didn't show up for some reason - but simply put, under the description of the light, it has various symbols (IP rating etc), and one of the symbols shows 'DIM TRIAC'
 
Joined
28 Jan 2011
Messages
50,114
Reaction score
3,351
Location
Buckinghamshire
Country
United Kingdom
"Triac" equates to Leading Edge while not Triac equates to Trailing Edge ( or may the other way round if I am wrong )
No, I don't think you've got it the wrong way around, although your statement perhaps does not tell the whole story ...

For reasons you explained, I think that a TRIAC-based dimmer inevitably result in leading-edge dimming. As you said, the control circuitry triggers the TRIAC 'on' at some point after the start of a half-cycle, and the TRIAC then maintains itself 'on' until the end of the half-cycle - hence "leading-edge dimming.

With a MOSFET (or other transistor) based dimmer, the control circuitry can presumably switch the transistor on and off whenever it wants, so could producing leading-edge or trailing-edge (or, I suppose 'both-edge'!) dimming. However, since they tend to be more complicated/ expensive to make than TRIAC-based dimmers, I doubt that this technology would be used to emulate a TRIAC-based dimmer - so, in practice, I would imagine that most/all transistor-switched dimmers are probably trailing-edge ones.

What confuses me a bit is that, although both leading- and trailing-edge dimmers have their individual (different) potential problems when dimming LEDs, I though that trailing-edge (i.e. not TRIAC-based) ones were more often the more satisfactory for dimming LEDs - which seems to be the opposite of what the manufacturer of the OP's lights seems to be saying.

Kind Regards, John
 
Sponsored Links
Joined
3 Nov 2006
Messages
26,587
Reaction score
2,812
Location
Bedfordshire
Country
United Kingdom
Most "retrofit " dimmers have to be designed to work without a Neutral they take power from the Live and the Switched Live which, wen the lamp is OFF is a pseudo "neutral".

This requires a small current to flow through the lamp when the lamp is OFF. For LED lamps this small current is enough to make the LED glow when OFF

Fitting a "bypass" across the LED to allow the small current to flow should solve the problem

Copyied and pasted to save typing again

A capacitor in series with a resistor across the supply to the LED will in almost all cases solve the problem. The capacitor absorbs the capacitively coupled power without the voltage being high enough to affect the LED.

0.1 microfarad 400 volt capacitor polyester and 100 ohm 1 watt resistor have proved sucessful. I would also fit a 1 amp fuse in series as well

400 volt as it has to cope with the approx 320 volt peaks of 230 v AC It cannot be an electrolytic or polarised capacitor.

Contact suppressors such as the one below provide the necessary circuit in a single convenient package. The use across LED lamps is not their intended purpose but they do provide the function required.

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0209241

Read more: http://www.diynot.com/diy/threads/gu10-led-still-glow-whwn-off.197054/#ixzz4LFoN1qCB

Maplins also provide a similar item http://www.maplin.co.uk/p/rc-contact-suppressor-rg22y
 
Joined
15 Dec 2008
Messages
199
Reaction score
0
Location
London
Country
United Kingdom
Thanks to all.

Bernardgreen - I'd seen similar threads to this suggesting inserting a contact compressor. To clarify - this would need to be installed on EVERY individual wall light that remains on?
 
Joined
3 Nov 2006
Messages
26,587
Reaction score
2,812
Location
Bedfordshire
Country
United Kingdom
this would need to be installed on EVERY individual wall light that remains on?

One suppressor per switched circuit. If a switch controls more than one lamp then it is only necessary for one suppressor which can be fitted on any one of the lamps on that circuit.
 
Joined
15 Dec 2008
Messages
199
Reaction score
0
Location
London
Country
United Kingdom
I just checked out the description of the Maplin product - should I be concerned by its reference to Triac?

"Sudden interruption of current in an inductive circuit normally produces a high voltage surge. Damage to contacts occurs due to arcing. In thyristor or Triac Circuits incorrect functioning may result due to high dv/dt."
 
Joined
27 Aug 2003
Messages
69,757
Reaction score
2,884
Location
London
Country
United Kingdom
Did they really need to use a term more commonly encountered in differential calculus?
 
Joined
28 Jan 2011
Messages
50,114
Reaction score
3,351
Location
Buckinghamshire
Country
United Kingdom
Did they really need to use a term more commonly encountered in differential calculus?
I would have thought that many/most people familiar with differential calculus would be likely to write "dv/dt" as shorthand for "the rate of change of voltage".

Kind Regards, John
 
Sponsored Links
Top