1 866 609 0282 toll free
Select Page

ELV, MLV, Forward Phase, Trailing Edge, Triac – what’s it all mean?

The terminology around dimmers can be confusing. Luckily, it’s just different ways of saying the same thing.

Here’s the scoop. First of all, let’s be clear that this post is talking only about the common wall-slider-type dimmers made by companies like Lutron, Leviton and Legrand. This type of dimmer has been in use for decades, originally used for dimming incandescent bulbs.

How do they work? Well, they chop the AC power waveform. What’s that? The power to your outlets and fixtures is AC power (“Alternating Current”), (generally 120 volts in a house). It’s called that because it “alternates” like a sine wave, 60 times a second.

Dimmers change the shape of the AC wave by chopping off part of it. This reduces the power available to the bulb, which becomes less bright. Magic!

What does Triac mean?
The key electronic component in a dimmer that makes wave-chopping possible is called a “triac”. So all these type of dimmers are “triac dimmers”.

What’s Leading Edge and Trailing Edge?
When chopping a sine wave, you can chop the beginning of the wave, or the end of the wave. In both cases you achieve dimming by reducing the outgoing power. The more you chop, the dimmer the light.

So if the triac chops the beginning of the sine wave, it’s “Leading Edge” dimming. And guess what it’s called if you chop the end of the wave? Right! “Trailing Edge”.

What’s Forward Phase dimming and Reverse Phase dimming?
Forward Phase is another term for Leading Edge dimming. It’s just another way of saying it. And, Reverse Phase dimming is another way of saying Trailing Edge. That was easy!

Now the fun part begins.

What’s an MLV dimmer?

MLV stands for Magnetic Low Voltage. Hmm. So that must mean the dimmer is magnetic, and it’s low voltage right?


It actually means it’s a dimmer for Magnetic Low Voltage lights.
Huh? How can a light be magnetic? This is really confusing.

It’s a bit of a long story. After the incandescent light bulb, a “new” type of lighting was introduced called halogen. It became very popular starting from the 1970s onward. The most common type of halogen bulb was an MR16, about 2″ in diameter. The big advantage was they could be spotlights, small and bright. This enabled much more subtle and controlled lighting effects than simple incandescent bulbs that shine all around.

For technical reasons, using 12 volt power at high current allowed halogen MR16s to be much brighter than if they ran from the 120 volt / low(-ish) current coming from the wall. The 12 volt conversion was easily achieved with a simple transformer to reduce the voltage and increase the current.

These transformers were very effective. But also heavy and bulky, and because they were just a couple of coils, worked like an electromagnet. So they became known as “magnetic transformers”.

So the combination of a 12 volt MR16 halogen bulb and a “magnetic” transformer became known as “Magnetic Low-Voltage” Lighting. Or MLV.


Coming back to dimmers … how do you dim a 12 volt MR16 bulb powered by a magnetic transformer? Well, exactly the same way you’d dim a regular incandescent bulb! Just chop the beginning of the AC waveform. In other words, a “Leading Edge Dimmer”. Or a “Forward Phase” dimmer. Or, an MLV dimmer! They’re all the same thing.

Still hanging in? One more to go.

What’s an ELV dimmer?

Back to the MR16 halogen light story. When track lighting came out, people wanted to put MR16 bulbs on tracks so they could highlight different areas easily. But their bulky heavy transformers weren’t suitable for this. So a streamlined electronic version was developed to save space and cost. Basically it was an electronic power supply for MR16 12-volt (low voltage) lights, that could mount on a track system. Hence, Electronic Low Voltage lighting.

And how do you dim a combination of a 12 volt MR16 bulb and an “electronic” transformer? The same way as an incandescent bulb, right?
Well, not quite. Turns out that if you chop the beginning of the AC wave, it causes the electronics to hum and buzz unpleasantly because of the sudden wall of AC power coming into the device. So the solution was to chop the trailing edge of the AC wave instead of the beginning, giving it a more gentle start to the power.

So this was a new type of dimmer on the market. An “ELV” dimmer, made for a 12 volt MR 16 and its electronic power supply.

“ELV dimmer” doesn’t mean the dimmer is electronic and low voltage, it means it’s for dimming an electronic power supply of a low voltage halogen light.

An ELV dimmer cuts the trailing edge of the power wave, to reduce hum.

So ELV, Trailing Edge and Reverse Phase actually all mean the same thing.

Even though most halogen MR16s have been replaced by LED lights by now, the terms MLV and ELV persist in the marketplace. But the meaning has become a bit blurry.

Aren’t there any dimmers that can do both Leading Edge and Trailing Edge?

Yes, they are usually called something like “universal dimmers”, and they include a way to switch them from Leading Edge/Forward Phase/MLV to Trailing Edge/Reverse Phase/ELV.

What about LED lights? They’re all electronic, so they need ELV right?

Not necessarily. Some lights can work equally well with leading edge and trailing edge, some prefer one or the other. Same goes for LED power supplies for LED flexible strip lights. It depends on the device, there is no hard-and-fast rule.

Is there such a thing as a Magnetic LED Driver/Power Supply?

Yes, there are some dimmable power supplies for LED flexible strip which are mostly just a large transformer and a few other components. These are called “Magnetic” power supplies. They should be dimmed with leading edge/forward phase/MLV dimmers.

Dimming of LEDs

Dimming of LEDs can be confusing because there are several ways to dim and many different products for each method. Using the wrong products together can cause many issues.

When you think of dimming you probably think of slider-type controls mounted in the wall, like those by Lutron or Leviton, found in most homes and offices. But it’s not the only way.

They are “triac-based” dimmers, though you won’t see those words on the package. They work by chopping the 120 volt AC power signal, which basically reduces the amount of power getting to the light.
Traditional incandescent bulbs with tungsten filaments dimmed very easily by this method. However, LED lights are electronic devices, more complex than an incandescent bulb.

LED lights can come in different forms. They might be bulb-shaped with a screw base, designed to replace traditional bulbs in lamps and light fixtures. They might be linear flexible strips that are used for under-cabinet and cove lighting and many other effects. They might be “LED bars” which are similar to LED strips but in a rigid form. They could also be “LED panel” lights designed to replace fluorescent fixtures in offices.

LED screw-in bulbs are generally designed to work with wall-slider dimmers. However because the electronics inside each company’s bulb are a bit different, they may work better with some dimmers than others.

That’s a subject for another article.

Each bulb has a built-in power supply (“driver”) that converts the 120 volt AC wall power into a lower-voltage DC power to power the LEDs. That built-in driver is also responsible for converting the “chopped” AC power from the wall dimmer into a lower light output.

LED flexible strips generally rely on a separate power supply / driver to convert the 120 V AC wall power into the lower voltage suitable for LEDs. Can these be dimmed the same way, with a common wall-dimmer?

The answer is, it depends. If you use a “triac dimmable” power supply (driver), the answer is yes.

But is it the only way? No. You can use a non-dimmable power supply, and add a separate low-voltage dimming module to the 12 v or 24 V DC output to dim the output side, often using a PWM (pulse width modulation) technology.

These dimmers may include a knob, buttons or slider to set the dimming level. Some are available with a remote control so you can put the control on the wall or carry it with you, within a certain distance range. They can communicate with the dimming module by infrared (IR) light similar to a TV remote, or by radio-frequency (RF) signal.

But wait, there’s more!

There are drivers (power supplies) on the market that support 0-10 volt dimming, and other methods. What’s that?

For 0-10 volt dimming, a separate pair of low-voltage wires must be run from a special 0-10 volt dimmer to the LED driver. That seems like a lot of extra work, and it is. Why does this even exist? Basically it wasn’t that easy to design a high-power “triac” type driver, so this system was developed. It’s commonly found on drivers for LED panel lights, warehouse-type high-bay lights, but other lights might use it too. Note: some automation systems such as Lutron RA2, Vantage etc. offer modules that support 0-10 volt dimmable power supplies. However, they also offer modules for “normal” incandescent or LED screw-base bulbs – those are the modules sthat hould be used for “triac (AC) dimmable” LED power supplies (drivers).

You can buy drivers that support 0-10 volt control signals, and connect them to power LED flexible strips and bars. BUT be careful: The connections on those drivers are often just marked “DIM”.

Do not connect a 120 volt AC wall dimmer to those inputs. It won’t dim, and may even damage the driver or dimmer. Why do they just put “DIM” then? Because some of these drivers can also accept a varying resistance as a dimming control on the DIM input. That’s normally supplied by a “potentiometer” (POT) which is like a knob on a stereo system, also found on things like electric guitars. So, technically this is a “dimmable” driver, but it’s not dimmable by the common wall sliders, only by more esoteric methods!

Seems weird? Maybe it’s weird for lighting, but these power supplies are also designed for industrial applications, not just residential lighting. And they may be used all over the world, where different types of controls may be more commonly used. There are even different control protocols such as DALI, Zigbee, Zwave and more that can be used to achieve dimming and other effects.

Having read this far, you might be more confused than ever.

Don’t worry, we’re here for you. Call us, or use the contact form on the website and ask us your specific lighting and dimming questions!

“Daylight” does not mean full spectrum

Often we hear people say that they are using “daylight” bulbs to light their studio workspaces because they want to see the colours accurately.

Unfortunately, this is generally a mistake, resulting from a simple misunderstanding of the terminology. Everyone knows that sunlight is a full-spectrum light source. We’ve all been taught than when looking at paint samples in a hardware store, it’s a good idea to take them outdoors to see the true color of the swatch. Or if you’re buying clothes, you might want to look at them under natural daylight.

So why are typical “daylight” bulbs a bad idea? Don’t we want a light source that’s as much like the sun as possible?

The short answer is yes, you DO want bulbs that provide full spectrum lighting, as the sun does. However, that’s not what “daylight” bulbs do!

The term “daylight” relates to the “colour temperature” of the light. That is, whether it’s a “warm-colour” or a “cool-colour” light.

“Warm” light mean yellowish light, and “cool” light means light that has more blue content. Incandescent light bulbs give off “warm light” that’s rather yellowish. Yes, those bulbs are also very HOT, but that’s not why it’s called warm light.

Sunlight has much more blue content, so it’s considered a “cool” colored light source (even though sunlight can be pretty hot too!)

So what’s the problem?

Your typical “daylight” bulb does have lots of blue spectrum, like the sun. BUT it doesn’t have enough red spectrum. Natural sunlight has both.

And what’s more, if you are using a fluorescent bulb as your “daylight” source, not only are you missing out on red spectrum, you’re missing a lot more too. Fluorescent lights are notorious for having many peaks and valleys in their light spectrum. Each one of those valleys is a weak spot in the spectrum. That’s bad.


Because all the colour you see is actually just reflected light! Grass looks green because it reflects the green spectrum of light. Its not really a green colour, it’s a green reflector.

And we’ve all been taught that black objects absorb light. Which is true … but it’s more accurate and useful to realize that objects look black because they absorb light. If it reflected some or all of the spectrum, it wouldn’t be black!

So “daylight” bulbs usually give you the bluish cast of sunlight, at the expense of the rest of the spectrum, which is vital. You can’t see paint colors properly (or clothing, furniture etc) if those colour are not fully present in your light source.

Take a green ball and shine a red light on it. What do you see? A red ball? No, it’s impossible. The red light is absorbed by the green ball. Instead you see a grey ball! That’s an extreme example of what happens when your light source doesn’t have the spectrum required to render colours accurately.

And if you are an artist trying to paint under a deficient light source, you might as well be working blind, because you really won’t be able to see what you are doing properly. Unless of course, all your works are only blue, then you might manage. But for the rest of us, a good full spectrum light source is the way to go.

The warmness / coolness of the light source is called the “colour temperature” or CCT. That’s what “daylight” is all about. It means it’s a “cool” light.

The spectral accuracy of the light source is something different. Its measured by the “colour rendering index” or CRI. With LEDs you can get full spectrum lights in virtually any colour temperature, including daylight. But most of the time, you don’t.

Most LED manufacturers sacrifice some colour rendering accuracy in order to make their lights brighter and more efficient. Sometimes that’s ok, and sometimes it’s not.

Lighting artwork is definitely one of those times when it’s not OK to sacrifice light quality!

In case it’s not obvious yet, at Lumicrest we specialize in full spectrum, High CRI LED lighting.