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When an LED driver is described as “non-dimmable”, does this mean that only the input power must be constant, or that both input and output power must be constant?

Disclaimer: I’m not an electrical engineer and I’m probably not using the correct terms for everything—but I’m willing to learn.


Background

Analysis

The word “non-dimmable” should say it all, right? What is stopping me from heading out and purchasing replacement “dimmable” drivers?

  • Most information I’ve found paraphrases to:

    If you want dimming, you need a dimmable LED driver.

    This advice seems oversimplified: it doesn’t explicitly acknowledge the possibility of the dimming occurring on the output side of the driver. Since dimmable LED drivers all appear to expect variations on the input side, it is almost certainly not being made with any consideration for output-side dimming at all.

  • I can find only one source (on Reddit) acknowledging the variation on the output side, and it goes on to state that a non-dimmable driver is okay in this situation:

    Dimmer compatible power supplies refer to their ability to take a dimmed input. In this case, the input to the driver is never dimmed so no need for it to be compatible with a dimmer input.

    However, this truly is the only source I’ve been able to find in support of using a non-dimmable LED driver in this way. Going off numbers, the odds are (sadly) in favour of this being bad advice.

  • One of my non-dimmable LED drivers starts making noises as soon as the lights are either switched off or dimmed to around 40% or less, and I’m unsure whether this is caused by the driver being non-dimmable, or by an unrelated issue.

Questions

  • What exactly does “non-dimmable” mean (and not mean) in the context of LED drivers?
  • Can a non-dimmable LED driver be used to dim LED lighting if the dimming occurs on only the output side of the driver?
  • Do (or can) non-dimmable LED drivers have any minimum output load requirements—and if so, what happens if those requirements are not met (such as a light being switched off)?
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    \$\begingroup\$ Hello and welcome: it's very unhelpful of Shelly to describe those RGBW2 units as "relays" on the web site. There are lots of wifi/similar controllable relays which are pure on-off switches. These devices are clearly LED controllers, as written on their box! \$\endgroup\$
    – jonathanjo
    Jun 20, 2023 at 11:52
  • \$\begingroup\$ Hi, and thank you. I did feel odd writing “relay” as I’ve yet to come across one that isn’t just on/off. I’ve now edited my post to call it what it really is. \$\endgroup\$ Jun 22, 2023 at 15:57

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Yes.

Based on the problem description and linked parts, and assuming your existing installation is sized correctly (not too many LED's/fixtures for the chosen power supply/driver), you can use that "relay" to control your existing LED fixture(s) with your existing "driver".

You install the Shelly controller between the "driver" and the LED fixture(s). Everything should work per the Shelly's user manual.

enter image description here

  • (A) -- Replace this with your existing voltage-mode LED driver
  • (B) -- This is your new Shelly PWM controller
  • (C) -- These wires go to your LED fixture(s)

As to your questions in detail...

What exactly does “non-dimmable” mean (and not mean) in the context of LED drivers?

There is no standard definition, but, typically, dimmable drivers offer an electrical (or mechanical) control port to adjust the output of the driven LED fixtures. Electrical ports can be wire terminals or wireless interfaces, while mechanical ports are things like knobs and switches.

Non-dimmable drivers output either a constant current (meaning, they try to always output a constant current, this isn't always possible) or a constant voltage. In your case, your driver is the constant voltage type.

enter image description here

You can determine this from (A) the specification is for a constant 24V DC output -- a single number is specified here, and (B) the specification is for a range of output currents 0 to 2 Amps.

Constant voltage sources vary the current to make sure the voltage stays constant and constant current sources do the opposite (vary the voltage to keep the current constant).

Most low-cost constant voltage LED drivers are indistinguishable from a "normal" power supply. They're branded as LED drivers to appeal to buyers looking for LED applications, but the electronics aren't any different from any other voltage-mode power supply (the most common type).

Can a non-dimmable LED driver be used to dim LED lighting if the dimming occurs on only the output side of the driver?

If you implement the dimming with Pulse-Width-Modulation ("PWM") then this works perfectly fine in theory. The Shelly website doesn't say how they implemented the dimming function, but PWM is the most cost effective and common way of doing it. I'm pretty sure that's how it works in their device.

PWM is about varying the amount of time the LED is on vs. off over a given interval. The driver doesn't really know (or care) how long the LED is off and the driver is capable of handling the LED when it is either on or off.

Keep in mind, that no one here has complete access to the schematics and internal design of the devices you are asking about and there are some esoteric failure modes for rapidly switching a load on the output of a power supply. That said, be careful (keep an eye on things for a while in the beginning), but I wouldn't worry too much about problems that are unlikely to affect you in this scenario.

Do (or can) non-dimmable LED drivers have any minimum output load requirements

Practically, no. All feedback-based power supplies (meaning anything that specifies, and therefore "guarantees" a stable output) have a minimum output requirement in order to ensure stability, but this requirement will be handled inside the power supply/driver itself.

—and if so, what happens if those requirements are not met (such as a light being switched off)?

This isn't how the minimum load requirement works. Minimum load is about the light fixture when it is on.

If you violate the minimum load of a constant voltage driver, you are providing it a fixture whose voltage requirements are below that of the driver -- and if you do that (say connect a 12VDC LED string to a 24VDC driver) the LED's will be destroyed by the excessive current that will flow through them.

If you violate the minimum load of a constant current driver, you are providing it a fixture whose maximum safe operation current is less than the driver wants to provide. Exactly what results in this case depends on the internal details of the LED fixture and driver architecture. Either the result is that the driver will output a voltage that is too high and cause more than the intended current to flow damaging the LED's, or the driver will be unable to operate resulting in the LED's being off (without damage to either component).

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    \$\begingroup\$ Thank you very much for your incredibly detailed, thought-out response. I especially appreciate your annotated pictures, your direct answers to all of my questions, and your advice about constant-voltage LED drivers being “rebranded” regular power supplies. Your answer prompted me to more thoroughly research whether the Shelly uses PWM: thankfully, it does. I’m keen to learn more about the potential risks to the power supply by rapidly varying the load on the output (as will obviously happen using PWM). \$\endgroup\$ Jun 22, 2023 at 15:54
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How LED strip dimming works

First, before any dimming will be possible, you need to investigate those so-called LED "light strips" you have there.

There's a quasi-standard design for LED light strips, which involves cuttable segments every 25-100mm (1-4") and each segment has 3 LEDs and a resistor in series. Very simple design. (the 24V variant has 6 LEDs and 2 resistors in series). These have no (intended) capacitance, and so they respond very well to PWM Dimming - or simply pulsing the power supply at a square wave of, say, 3000 Hz, varying the duty cycle to change brightness. Your LEDs, however, do not look like the quasi-standard, so you'll need to figure out how they work and if they are compatible with PWM dimming.

Note that PWM dimming happens at the 12/24 volt level - the "dimmer" inputs 12/24V and outputs pulsed 12/24V. The vast majority of this PWM dimming kit is built to be voltage-tolerant and so the same unit can do 12V or 24V.

How AC mains dimming works

Dimming on AC, however, harkens back to the early age of electronics where lights were presumed to be incandescent. Variacs would be too heavy; rheostats would make too much heat; so dimmers used the earliest electronics that would make that possible. They used a very simple back-to-back thyristor called a triac. When gated, it allows power to pass forever (can't be turned off) until current stops or reverses. Which happens every 10ms or so with AC power. Isn't that clever?

So the dimmer is simply a timer that waits 0-10ms before gating the triac. How long it waits controls how bright, making a wacky sawtooth sinewave that controls an incandescent bulb well. Deliciously obsolete. But as of today, triac dimmers dominate the store shelves at your local Wickes or Home Depot, so it's still the lingua franca of home dimming.

And as you can gather, completely incompatible with LED strip PWM dimming.

So when you have a dumb but rich architect who wants their dimmers to aesthetically match, even though some drive LED strips? There's a "smart" LED power supply that accepts the ugly triac-dimmed sinewave as its power, decrypts it to determine the dimming level being requested, and then outputs 12/24V PWM dimming at that dim level to drive your LED strips. It's a nice piece of kit for those stubborn customers. (And some screw-in LEDs have that same feature built into them. That's what "dimmable" means in the AC mains space.)

These installations have their own separate non-dimmable 24V/48W LED "driver".

That means it's NOT the fancy piece of kit I just described in the last paragraph. They're just setting your expectations. You don't need to worry about that.

Once you're on the DC side, dim your heart out

with 12/24V dimming or color controller kit made for LED strips.

And that Shelly kit looks fine for that.

Normally I give a seething condemnation of Shelly for their cheap, unsafe and illegal AC mains equipment with fake CE instead of proper UL, TUV, BSI etc. certification... but in the 12/24V world, that's par for the course.

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  • \$\begingroup\$ Thank you very much for the detailed descriptions of how the dimming works on either side, and for confirming that a non-dimmable power supply is fine for output-side dimming. Sadly I don’t have any info on the LEDs themselves, but I’m willing to take the chance that they’re PWM-compatible (and replace them with something PWM-compatible if I turn out to be wrong). \$\endgroup\$ Jun 22, 2023 at 15:35
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Non dimmable in this context means that they cannot be combined with a dimming light switch. Essentially the AC input must not be dimmed.

You have constant voltage power supplies, they put out a continuous voltage just like any other power supply. This can be dimmed using PWM or a voltage converter - it's just a power supply. That low side dimmer will work.

In other contexts non dimmable can mean other things. For example a constant current driver should not be combined with that PWM dimmer.

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  • \$\begingroup\$ Thank you. It’s good to know that “non-dimmable” in this context refers only to the input side, and to know the importance of PWM dimmers being paired with only constant-voltage power supplies—not constant-current, which would seemingly compensate by increasing voltage (and frying the LEDs in the process). \$\endgroup\$ Jun 22, 2023 at 15:38
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Firstly, leds themselves are either constant voltage or constant current. This is why the manufacturers/suppliers will bundle or recommend suitable drivers.

A dimmable driver will reduce current or voltage, depending on whether it is supplying a constant current led or a constant voltage led. It is instructed or driven to do this either by supplying it with a leading edge or a trailing edge phase control. The driver is designed to sense this and work accordingly.

A non-dimmable driver is not designed to sense the leading edge or trailing edge voltage input. But you can adjust its output to do dimming, the driver does not notice this. You can dim the led using PWM (or just switching it off and on), the frequency is normally held above 120Hz to avoid noticeable flicker.

Some references

  1. DIMMERS: LEADING EDGE VS TRAILING EDGE
  2. Dimming Basics – Types of LED Dimming Explained
  3. What's the difference between Leading and Trailing Edge Dimmers?

I am not associated with any of these sites.

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  • \$\begingroup\$ Thank you for confirming that a non-dimmable driver is okay for PWM-based dimming on the output side, and for your suggestion to ensure a PWM frequency above 120Hz where possible. I was relieved that your references, while still useful, don’t cover whether a non-dimmable driver is okay for output-side dimming—which you have covered in your answer. \$\endgroup\$ Jun 22, 2023 at 15:45

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