# What's the difference between dimmable and non-dimmable LED lamps?

I have a large room in my house with 3 light fittings, with a total of ten 40W GU10 bulbs. So, this is using 400W when they're at max output (they're all on dimmer switches).

I'm interested in reducing this by replacing the bulbs with LED's (or some other dimmable low power alternative) as the ones I have fail over time.

So, I'm curious as to why dimmable GU10 LED lamps are so expensive. £20-£25 is the best price I can find.

• I think this is more of a manufacturing question. It has to do with supply vs. demand, as well as the cost of manufacturing, design, research & development, and quality control. As these lights become more widely used, and manufacturing techniques improve the cost will go down. Commented Dec 14, 2011 at 12:55
• there must be a difference if I can buy a non-dimmable LED lamp for £5-£10 but a dimmable one if £20-£25 Commented Dec 14, 2011 at 13:25
• There is a difference, more circuitry. which = greater cost. Commented Dec 14, 2011 at 13:59
• I have tried this on some lo cost China LED bulbs and lamps, and it seems to work on the absolutely cheapest. They are just made by a rectifier an a great number of LED's in series. Definitely not as good as an old lamp, but it works. Its working better when keeping one old-fashioned bulb in the system, it works like a stabilizing load.
– user13974
Commented Jul 13, 2013 at 14:57

The two differences are

• Cost
• It can be dimmed with a standard dimmer

A standard dimmer reduces the RMS voltage going to a light to dim it; this may be done by a simple resistor or by chopping of the top of the waveform. So a dimmable LED lamp must be able to:

• Cope with a big range of input voltage including part formed sine waves, which isn't easy for the control logic.
• Detect from the input voltage the light level that the user wants, then give that light level.

It would be a lot better if a dimmer switch could send a digital signal (radio, or over the main wiring) to the lamp, so that the lamp could know what was required without reverse engineering the voltage back to the user's wishes. However this would need a new standard to be used by all switch and lamp vendors, so may take a little time-:)

• There is a new open standard that does exactly that. It is called Ledotron. Developed by Osram and other large companies in 2012 and now in 2014 new lamps and dimmers have started to emerge, at least here in europe. Im about to install it in my kitchen, so soon I know if it is as good as the specification promises. :) Commented Aug 12, 2014 at 10:33
• Standard dimmers (triac) are leading edge cut which basically increasingly chop the leading edge of the sine wave off to reduce the power. Your power input becomes extremely quick rise on the leading edge and your LED power supply is being fed a series of power spikes instead of a smooth sine wave as the power is reduced. A dimmable LED power supply is configured to work with this adverse power environment. There are trailing edge cut (mosfet) dimmers available that are a little more compatible with LED and CFL technology. Commented Apr 12, 2015 at 0:50

Most LED bulbs on the market are in fact dimmable, IF you use a CFL/LED dimmer.

Your "standard" dial or track dimmer is either just a big potentiometer or Rheostat that reduces voltage through resistance, or a slightly more efficient design called a TRIAC dimmer, which uses a specialized transistor design to attenuate the line voltage by turning the line on and off based on where the AC is in its "rhythm" or "phase".

These dimmers work just fine for incandescents, but cause problems for CFLs and LEDs. A CFL needs a big power spike to get started, but then as long as it gets enough power (about 13w for a 60W-equivalent) it'll happily produce full light. LEDs are similar, but instead of needing a power spike to get started, they need DC current, produced by first "stepping down" the AC current and then running it through a diode and capacitors to restrict the flow to one-way. This is the same technology used in AC adapter "wall warts". The problem here is that the capacitors, which charge when the power is flowing the "right way" through the diode and discharge when the power is blocked by it, hold plenty of charge to keep the LEDs going when the power is cut off for slightly longer periods, such as by a TRIAC dimmer. In fact, many of these AC/DC converters have some sort of rectifier, like a TRIAC or SRC, built in to attenuate the power (the transformer will exchange voltage for amperage, or vice-versa, so you have to further cut down the current from line level), so the capacitor bank is DESIGNED to handle this type of current.

CFL/LED dimmers reduce power in a different way; they turn the power on, then off for set periods of time (i.e. "on for 10ms, off for 5ms"; as opposed to TRIAC dimmers' "on as long as the phase isn't x degrees or closer to peak power"). This still reduces RMS power over the entire waveform, but it does so in a way that CFLs and LEDs respond to more readily.

You still need special bulbs as well as special dimmers to dim energy-efficient bulbs. CFLs need a ballast that can charge and discharge very quickly (the technology found in "instant-on" CFLs that can "ignite" within just a few milliseconds) in order to turn on and off fast enough to make a difference. LEDs, for their part, react more naturally to the longer "off" periods of a CFL dimmer than a TRAIC, and so most LEDs can be dimmed to some degree, but you don't get a very smooth dimming across the full range. Many newer ones instead use integrated circuitry (yes, there are computers in your freakin' light bulbs now) that will "sense" the on-off pattern and translate it to a more rapid succession of "clock pulses" to "blink" the LEDs. This bypasses any problems with the capacitors keeping the LEDs going and results in more gradual dimming.

Now, there are brand-new LED bulbs that are dimmable with TRIAC dimmers; the IC controller in the bulb is designed not only to correctly distinguish CFL dimmer "on-off" patterns, but TRIAC "on-off" patterns as well, so it can translate both of them into the rapid pulsing used to control LED light levels.

Dimmer switches work by cutting off some of the voltage for a period of time: http://home.howstuffworks.com/dimmer-switch2.htm and I believe that this sort of on/off is bad for traditional CFLs, so that they have to make CFLs that can "interpret" this voltage as a signal to dim.

• Put it through a full wave rectifier and smooth the output is the usual difference. The chopped sine wave is pretty rough on AC circuitry that's expecting full sine wave. Commented Jul 13, 2013 at 17:36