Are LEDs and dimmers fundamentally imcompatible?
March 3, 2020 10:37 PM   Subscribe

I really enjoy the aesthetics of old-timey lanterns at night in my apartment, but I'm on my second LED lamp and already I'm having the same issues as the first.

My first lantern had a cheap plastic body and ran on AAs. After a few months, the following issues cropped up nearly every use:
-turn the dimmer dial, doesn't turn on
-turn the dimmer dial, starts to light up, goes dark after hitting FULL brightness on the dial
-manage to turn it on, will suddenly go dark, but whacking it against a hand or thigh will (sometimes) turn it back on

Yes, I changed the batteries. No, the problems didn't stop. So I bought a slightly more expensive one (photo of actual model) with a metal body and actual glass. It uses two giant D batteries. Nearing a year of owning it, same issues.

Is this a Thing with dimming LEDs, or am I just unlucky twice in a row? I love my little lamps but I'm tired of fighting them to stay lit.
posted by lesser weasel to Home & Garden (15 answers total) 7 users marked this as a favorite
Not incompatible fundamentally, but not all LEDs and/or all dimmers are compatible. It sounds whoever produced your lamp doesn't know that.

I don't know how to filter or search for battery-powered things that are built correctly. When I replaced some recessed lighting in my house with LEDs, it was a Thing to make sure the switches and lights were both compatible.
posted by Dashy at 10:51 PM on March 3, 2020 [1 favorite]

Might be worth trying getting into the thing's innards and giving the track on the dimmer potentiometer a spray with contact cleaner.

Other than that, then assuming no fundamental design fault (perhaps an unsafe assumption if extremely cheap) the most likely thing that's going to kill that class of cheap battery powered electronics is shitty assembly.

The circuit board will be very simple and the most likely thing to be wrong with it is cold or dry solder joints, so having somebody with decent soldering skills rework the board ought to make it more reliable.
posted by flabdablet at 11:01 PM on March 3, 2020 [2 favorites]

Battery holder contacts and springs are also often not terribly reliable and might benefit from a touch up with fine sandpaper and/or contact cleaner. Also pay attention to the way the button at the positive end of the battery fits into the holder. Sometimes there is a slot in the battery holder that the + end button has to protrude through to touch the contact behind it, and some batteries (especially rechargeable ones, for some reason) have + buttons that are not quite as tall as usual and struggle to make the distance, or slightly wider than usual and don't quite fit into the slot. Shaving the plastic down a bit and/or bending the battery holder contact some can help with this.
posted by flabdablet at 11:05 PM on March 3, 2020

It is possible to make LED lights which dim beautifully. The dimming may involve them simply becoming less bright or it might also involve a change of colour temperature to mimic incandescent filaments (redder when cooler).

However both the light and the dimmer have to be of a type which will work with each other. The overall mechanism is much more complex and nuanced than it is with incandescent bulbs (where you just reduce the current with a potentiometer). If you buy an LED bulb which is marked as being dimmable (some are not) - and you use a dimmer which is "trailing edge" - then you are probably going to be OK. But no guarantees because this depends on the circuitry in the dimmer, the circuitry in the bulbs, the number of bulbs and your expectations of what good dimming should look like. Generally the situation has improved a huge amount in the past 5 years or so - but you can still have problems with cheaper or older fittings.

Here is a troubleshooting guide to getting LEDs to dim properly.
posted by rongorongo at 11:26 PM on March 3, 2020 [6 favorites]

Much of the advice you're getting here appears to be based on an assumption that the LED lighting you're using and the dimmer you're controlling it with are separate products.

But the question is not about mains-powered LED lighting being controlled by a standard wall-mounted dimmer; it's about battery powered LED lanterns with integrated dimmer controls (see the linked picture). This is not a compatibility issue, despite the question's title. This is about faults in a single product.

And that product is of a class that's usually built down to a price rather than up to a quality spec, because it's a low voltage battery powered thing, there is no risk of electric shock and the risk of fire is low. The things that go wrong with it will be more like the things that go wrong with hand torches and head-mounted battery lamps than with mains powered lighting.
posted by flabdablet at 11:57 PM on March 3, 2020 [8 favorites]

If the dimmer knob and the LEDs are all things that come with the lantern, they should "just work" together (otherwise it's definitely a defective product), but I suspect that they don't use the highest-quality electronics in those lanterns, probably because they assume that you won't be using them as often as you are.
posted by Aleyn at 12:00 AM on March 4, 2020

I mentioned compatibility in case the answer was "good concept, but LEDs don't play well with dimmers in general." I have a few leads now and I'll take a better look at the battery holders when I get home, but unfortunately it's looking like cheap quality could be a main culprit. :(
posted by lesser weasel at 12:08 AM on March 4, 2020

The key thing to understand here is that a battery powered dimmable LED lamp is not going to involve anywhere near as many compatibility layers, and will therefore have many fewer potential failure modes, than a mains powered LED lamp running off a wall-type dimmer originally designed for mains powered incandescent lighting.

LEDs are inherently a low voltage direct current technology, as are AA and D cells. Mains power, on the other hand, is relatively high voltage (nominally 110V in the US, up to 240V elsewhere) and also low frequency (50 or 60 cycles per second) alternating current.

A single white LED usually needs to be supplied with around three volts before it will turn on even slightly; above that voltage, the amount of current it draws and the amount of light it makes both increase very rapidly with voltage and by the time it's seeing maybe four volts it's in danger of burning out. The simplest feasible power supply circuit for running a LED is a single lithium coin cell, whose open circuit voltage when new is about 3.3V but whose internal resistance is such that drawing heavy currents from it will easily drop its terminal voltage below 3V. A coin cell, a LED and a magnet to make it stick to things is all it takes to make a LED Throwie.

Hand torches and head lamps are usually designed around rather gruntier batteries than coin cells to get more brightness and better run times. Designs built around three AAA cells are really common. The open circuit voltage on a new carbon-zinc or alkaline AAA cell is about 1.5V so three of them in series yields 4.5V, and a small series resistor or PTC thermistor is incorporated to prevent the LEDs drawing too much current and burning out.

Two cells in series yields about 3V for alkaline cells or 2.5V for NiMH rechargeables, which is not quite enough to fire up a white LED. So any battery powered LED design with less than three AAA or AA or D cells is going to need a small voltage boosting circuit, usually built around a single transistor. And it's pretty easy to incorporate a variable resistor into such a circuit to control the average LED current and therefore the brightness. Typically there will be a circuit board with maybe half a dozen small components on it at most.

So there's very little to go wrong in a battery powered LED brightness control circuit. Really the only thing that's going to kill it is bad internal contact, and the fact that yours have proven themselves fixable via percussive maintenance is pretty good evidence that that's what's going on with them.

Battery holders, substandard solder joints and potentiometer tracks with grit or dust on them are all reasonable suspects.

Mains powered LED lighting is a whole other thing. Inside a mains powered LED lamp you will typically find a small switch-mode power supply circuit whose job is to convert the incoming high voltage AC into a well controlled DC current to drive multiple series-connected strings of LED elements. These power supplies usually do a pretty good job of regulating the current they feed to the LEDs regardless of mains supply voltage variations, which is a good thing for providing nice steady light but doesn't play nice with external dimmers.

Mains powered dimmers have been a thing since incandescent lighting was the norm, and they all work basically the same way: by letting through only some controllable part of each cycle of the alternating current mains supply. And although this works just fine for limiting the amount of power delivered to an incandescent bulb, it plays merry hell with the input stages of a naively designed switch mode power supply.

What the power supply in a dimmable LED lamp needs to do is recognize the kind of chopped-up power that a dimmer will feed it and adjust its LED output current accordingly, rather than trying to regulate the supply variations away as if it were just dealing with a brownout. Some designs are better at doing this than others, and there are also dimmer designs available that try to ease that job for LED lamps (and compact fluorescents, to which similar considerations apply) downstream of them. There's very little standardization here and loads of potential for incompatibility.

But none of that makes LEDs inherently non-dimmable; they absolutely are. It's not the LEDs themselves that don't play well with dimmers in general, it's the power supply circuits built into certain mains-powered LED and compact fluorescent lamps.

Reliable dimmable battery-powered LED lighting ought to be easy to achieve.
posted by flabdablet at 4:04 AM on March 4, 2020 [7 favorites]

it's looking like cheap quality could be a main culprit

As long as the electronic design is basically sound, and it seems that it is because they do work sometimes, then build quality issues are fixable with a bit of rebuilding. Could even be a good project for training up your own soldering skills if that's not something you already do.

Old-school 60/40 tin/lead solder is a hell of a lot easier for the home gamer to work with than the new lead-free types commonly used now on commercially built PC boards. And a lot of manufacturing lines originally designed to bulk-solder cheap and simple boards with leaded solder just don't make reliable joints when they're used with lead-free solder, which has a higher melting point, instead.

I've seen a lot of cheap tiny single-layer non-through-hole-plated brown phenolic PC boards in battery powered equipment where every solder joint just looks dull and awful. Slurping the old solder off with the help of flux paste and wick, and re-doing every joint with hobbyist-grade 60/40 rosin-core solder instead, can make them work much more reliably.
posted by flabdablet at 4:42 AM on March 4, 2020

You might look at finding an LED source that in it's 'ON' state is what you want for "dim", and retrofit that into the shape you desire.

Or, replace the rotary dial with a simple resistor and on/off switch. Rotary dials are mostly very cheap components, and and their lower settings, suffer from spotty quality.
posted by nickggully at 5:55 AM on March 4, 2020

If you have several that no longer work, wire them as plug-in lamps, put in an LED 'Edison' bulb, and enjoy them. You can get wiring kits at the hardware store, you might prefer one that takes a smaller chandelier bulb.
posted by theora55 at 6:37 AM on March 4, 2020

Just to reinforce flabdablet's point, the reality is that you are pretty much constantly surrounded by working dimmable LEDs and they work so reliably you don't even notice or think about it at all. When you adjust the backlight in a smartphone you are dimming LEDs. Many TVs and monitors use the same technology.

It's not a hard problem to solve technically, but implementing it robustly in a cheapo device is not easy and lots of manufacturers end up cutting corners in low-end (or even better) devices and that's what leads to the unreliability you're most likely seeing.
posted by range at 9:48 AM on March 4, 2020

For cheap led lanterns in specific, there may be another problem in play: LED brightness and lifetime are determined how much current goes through the led. It is easy to build a light that runs at 150% of recommended current, and doesn’t last very long. That also lets you use cheap low lux leds and sell them as a high lux lamp. This is a popular strategy for super cheap bike lights and lanterns, where they use analog circuitry (no microprocessor) to connect the led to the battery. Today this is the under 5$ price point, but it used to be more common. If built that way, sooner or later the voltage or current will go too high and fry the led. Anything that strobes is a better bet.
posted by unknown knowns at 5:34 PM on March 4, 2020

This is not a fundamental problem or a matter of bad compatibility. The issue isn't LED burn-out and doesn't sound like bad solder joints either. It does very much sound like the kind of errors you get with cheap carbon-track potentiometers, in this case maybe used at currents beyond their specification. Decent pots are pricey compared with everything else in this circuit, so it's expected that they skimp on it.

It's easy to fix this by replacing the pot with a better one, but perhaps not worth it unless the tinkering itself appeals to you. The contact cleaner suggested above is worth a shot, at least for a short-term fix!
posted by Herr Zebrurka at 8:49 PM on March 4, 2020 [1 favorite]

the kind of errors you get with cheap carbon-track potentiometers, in this case maybe used at currents beyond their specification

If the design does something crude and gross like just putting a carbon-track potentiometer in series with the LEDs, thereby using the pot itself as a current limiting resistor, then yes this will absolutely burn it out over time, and the most likely place to get burnt spots is exactly where the wiper contact sits on the track at full brightness - because that's where the pot needs to conduct the highest current, and it will be doing that over the smallest possible length of carbon track. That will cause a lot of localised heating inside the pot and could easily burn holes into the track along the arc traversed by the wiper contact.

If that's what's going on, putting your nose very close to the dimmer control on a lamp that still works will probably reveal that telltale whiff of magic smoke being let out.

Burnt spots should also be completely obvious if you open up the pot from a dead lantern and visually inspect the carbon track.

Redesign options include replacing the carbon track pot with a wirewound version having the same resistance (crude, ick) or designing in a little power control board, perhaps based on some variant of a joule thief for maximum battery runtime.

Potentiometer abuse of this kind would be more likely in a lantern that runs off more than two cells, because the maximum voltage you can get out of two cells is only a whisker over 3V and that's going to struggle to light a white LED even before the batteries go flat. And even though D cells are big, their output voltage is no higher than that of AA or AAA cells; the increased size just lets them keep working for longer than the smaller cells. So a lantern based on two D cells almost certainly already has some kind of active power control electronics built, in and is less likely to be abusing a carbon track pot in this way. Not impossible - nothing's impossible - just less likely.
posted by flabdablet at 10:49 PM on March 4, 2020

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