Minimum load for light bulb dimmer, why does it have it, and does it matter?

[Okto1984]

I'm curious about light bulb dimmers. Some years ago I installed some in-line dimmers on various table lamps, all of which I used with 60w bulbs as this is apparently the minimum load they should have according to the label on the dimmer. However, my 60w bulbs ran out and I put in 42w halogen GLS bulbs, and even some 12w philips LED dimmable bulbs in a few. It's been a long while before I even noticed the 60w minimum load label again, but all seems to work fine. I suppose what I'm wonder is, does it matter? Any explanation would be interesting and very welcome as I've not been able to find anything about this yet anywhere else.

[JohnBHanson]

Most lamp dimmers are just connected via two wires in series with the lamp. Thus the power the
lamp dimmer needs to work itself is obtained by taking a small current through the lamp when it
is switched off. With the new efficiency led lights this small current can cause the light to illuminate. It is actually made worse by the fact that led lights are constant power and thus will decrease their effective
resistance if they need to to make sure they have the power they need.

[Radio Wrangler]

The regenerative current gain in triacs isn't well controlled and depends a lot on temperature and the rate of crime. Leakage currents are no better, they also suffer from dv/dt false triggering and several other malaises. Put all the specs together, stir and add seasoning, and you come out with a minimum load value that a simple phase shift dimmer circuit will work with over the full spread of devices and temperatures. The Diac is as bad (so are SCRs)

Unless your triacs are at the unfavourable end of gain and leakage specs, and things are hot, you're unlikely to see problems. It would most likely fail to trigger or stick full on which would not put you at any risk.

David

[emeritus]

I use a dimmer, with a minimum specified load of 40W, to control my bedside lamp, an anglepoise-type with an opaque reflector. I replaced the existing 60W GLS bulb with a smaller golfball-type 40W GLS bulb that sits deeper in the reflector to reduce light spill and to get a lower light level that I can get with the 60W. While it works OK at the max and min brightnesses, it is unusable over a range of intermediate brightness levels because it flickers at a few Hertz. Sometimes it doesn't come on at all if switched on with the dimmer at the low setting until it it turned up to near maximum. No problems when using a 60W bulb. Because I normally only need to use it on either max or min brightness I have retained the 40W bulb.

[vidjoman]

I have several dimmers that quote min. 60 watt that are currently used with a 200 watt load. Question is, will they work with LED lamps where the load will only be 24 watts? I can't find any definite answers anywhere and don't want to replace all 8 dimmers as well as the lamps.

[emeritus]

I just tried a 15W pygmy GLS lamp in my bedside light, and it behaves exactly that same as with the 40W: Ok at top and bottom of range, but flickers in a range of intermediate brightness settings. Of course, a GLS lamp is essentially a purely resistive load: how it would behave with what I assume would be the complex load impedance of a LED lamp. I do not have any mains LEDs myself to try the experiment.

[julie_m]

The problem with the usual type of dimmer switch, which is wired in series with the lamp load as though it were a simple on-off switch, is that there is no neutral connection present at the dimmer; so the only possible charging path for the timing capacitor is via the load. And if this isn't a simple, consistent, predictable resistance, then weird things can happen .....

[vidjoman]

I would have expected the LED lamp makers to give some clue about dimmers. All you get on some lamps is that they are 'dimmable' and more expensive, but no info as the what dimmer is required.

For LEDs and CFLs you need a "leading edge" dimmer https://www.tlc-direct.co.uk/Main_In...ers/index.html

[Nicklyons2]

Note that some dimmers have a little preset pot hidden round the back often referred to as an 'anti-hysteresis' pot; it can be adjusted to give less flicker problem at the end of the range with the load in use.

[vidjoman]

Originally Posted by merlinmaxwell

For LEDs and CFLs you need a "leading edge" dimmer

Thanks for that MM. I'll try and find out what we have fitted before rushing out to get new ones. They have only been there a couple of years.

[matthewhouse]

Originally Posted by merlinmaxwell

For LEDs and CFLs you need a "leading edge" dimmer https://www.tlc-direct.co.uk/Main_In...ers/index.html

The instruction manual for the dimmer in the link says 'Do not use this dimmerswitch for LED lighting'! However leading edge is preferred for some types of LED lamps but trailing edge is my first choice. I use these as they can be set to either leading or trailing edge and are suitable for almost all loads except inductive. They are also silent and are soft start. Modules are available if you want to fit them into your existing decorative switch plates.

Branded bulbs will often tell you what type is required. Most normal off the shelf dimmers will be leading edge.

Adding a resistive load, even as small as a 15W pigmy lamp will often allow many types of dimmable lamps to work with many dimmers. Danlers make a box with a 10W wirewound resistor on a heatsink in it, which does the same thing as the 15W bulb, providing a stable resistive load for the dimmer.

[Okto1984]

Thank you each for the replies. I've been a little busy to reply, but have been following the replies with interest. Seems that the minimum load is the amount at which it's assured to operate ok, but some, like mine may be fine with less. Possibly even a lot less. I don't have any flickering issues. I can't dim quite as low with lower wattage bulbs, they just go out at some point, but I can get more than dim enough for my needs.

Originally Posted by vidjoman

I would have expected the LED lamp makers to give some clue about dimmers.

Philips give some details here about the bulbs I bought:

http://www.lighting.philips.co.uk/pw...EDlamps_MV.pdf

However, they do state that dimmers with neon indicators are not suitable, yet mine have worked fine for years, all on dimmers with a neon indicator. How or why I don't know for certain, but they certainly do work. I suspect the issue may be that these types of dimmers rely on current passing through the light bulb to power the neon indicator. Perhaps this does something to the LED bulb after a while? I do however turn the lamps off with the switch in the bulb holder when not in use (the dimmers were added later, so they have switches already), and just use the dimmer as a pre-set light level. Perhaps doing that has avoided any problems?

[broadgage]

I have largely given up on dimmers due to instability at low loads, burning out at higher loads that are still well within the alleged rating, and RFI.

Instead I use where possible multiple switches that control lamps of differing output. Beside my bed I have a 3 gang switch that controls a twin fluorescent light for "bright" a couple of 9 watt CFLs for "medium" and a couple of 1 watt LEDs for "dim" All the lamps are concealed atop a high shelf and give an indirect light reflected from the ceiling.

In other areas I fitted LED lamps that are switched, dimmed, and varied in colour by means of an infra red remote control.

When re wiring a friends house, we installed numerous wall bracket light fittings each of which used 3 lamps.
The middle lamp is one circuit and the outer two lamps on another circuit. Use of a 2 gang switch then gives 3 lighting levels, by lighting one, two, or all three lamps in each fitting.

A large chandelier containing numerous lamps was wired to 3 switches so as to permit of 5 lamps, or 9 lamps, or 15 lamps being lit, or all 29.