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.

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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!

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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?

Nope!

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.

Whew!

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.

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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.

By default, each AVStream queue contains a leading edge stream pointer. The leading edge points to new frames as they arrive into the queue. Specifically, the leading edge initially points to the first frame to arrive into the queue and does not move until the minidriver moves it. AVStream creates the leading edge, which then exists for the lifetime of the queue. Minidrivers can manipulate the leading edge using functions provided by Microsoft.

When a new frame arrives in a queue, AVStream sets the leading edge to point to this frame, provided that the leading edge does not already point to a frame.

To obtain a pointer to the leading edge stream pointer, the minidriver calls KsPinGetLeadingEdgeStreamPointer.

The minidriver is responsible for advancing the leading edge in all but the two situations summarized in the following table.

See Introduction to Stream Pointers For more information about advancing stream pointers.

Specifying a Trailing Edge Stream Pointer

Minidrivers can specify that a queue have a trailing edge stream pointer. The trailing edge usually indicates the oldest frame of interest to the minidriver. To specify a trailing edge, set the KSPIN_FLAG_DISTINCT_TRAILING_EDGE flag in the Flags member of the relevant KSPIN_DESCRIPTOR_EX structure. Then call KsPinGetTrailingEdgeStreamPointer to obtain a pointer to the trailing edge stream pointer.

When the trailing edge advances, the reference count on the frame to which it previously pointed drops to zero and the frame completes. If the frame is the last contained within its IRP, a sink pin completes the IRP to the caller; a source pin sends the IRP to the pin to which it is connected.

Maintaining a Frame Window

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As a result of the frame reference count rules described in Introduction to Stream Pointers, a frame between the leading and trailing edge remains in the queue until it is canceled, even if the frame is not referenced by a stream pointer. As such, a minidriver can use the leading and trailing edge pointers to maintain a working window of multiple contiguous frames. Frames in the window might be awaiting processing or filling, for example.

In the following diagram, the oldest frames are at the bottom. New frames arrive at the top. The number in each frame is the reference count for that frame. When the stream pointers advance, they move up in this diagram.

Drivers Leading Edge Laptops & Desktops -

The leftmost queue shows how the minidriver can use a trailing edge to create a working set of frames. Each frame between the leading and trailing edge has a reference count of one despite the fact that no stream pointers reference these frames.

The middle queue is an example of Cloning Stream Pointers. The driver has repeatedly cloned and then advanced the leading edge, as described in the pin process steps in AVStream DMA Services.

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The rightmost queue shows how the minidriver can maintain reference count for a frame behind the trailing edge by using a stream pointer clone.