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1feralengineer t1_j2mkahc wrote

Your dimmer is fine.

The division has already been done for you.


Ok_Television_9348 t1_j2mvkuw wrote

I agree. You only need to divide if the dimmer didn’t explicitly say for LED. Divide by 10 is a quick and dirty conservative way of converting incandescent rating to LED rating.


cannondave t1_j2n02io wrote

If the dimmer has a range of 5w to 100w, can I only dim the load down to 5w, for example a 10w light down to 50% (which is 5w)?


Redthemagnificent t1_j2n2uhn wrote

Dimmers don't work by limiting the number of watts. They work by reducing the AC voltage going to the lights. That's how you can dim a 100W incandescent and a 10W LED with the same dimmer switch.

That's also why some LEDs are not dimmable. They need the circuitry required to convert lower "dimmed" AC voltage to the right DC voltage so the light dimms as expected.


jzooor t1_j2n56uu wrote

Dimmers work by "chopping" out the lower voltage parts of the AC waveform. Every time there's a zero voltage crossing the circuit turns off until the voltage has increased enough again to turn back on at whatever set point the dimmer is set. The average (RMS) voltage is reduced because the circuit is off for some short period, but the peak voltage stays the same.


HanzG t1_j2n8w03 wrote

Worth noting that old style dimmers with turn knobs and slider-paddle type are a variable resistor and induce a voltage drop. These are still for sale in stores today and some LED bulbs will work with them, but some will not.


Riegel_Haribo t1_j2nieqv wrote

No they aren't, not any that go in a normal outlet box. Older dimmers are also a SCR chopper, but they work on leading edge of the phase which doesn't work well with LED. They also may need more minimum current draw or lower impedance than an LED fixture takes in order to operate correctly. They latch "on" starting at a particular voltage to the end of phase zero crossing.

LED+ dimmers (aka HED technology, C-L technology) have more circuitry ensuring that even slight loads will still get proper regulation. They can be reverse phase, that starts "on" and turn off at the correct duty cycle point.

Early rotary rheostats or variacs capable of controlling lighting were huge.


velifer t1_j2nx0n4 wrote

>No they aren't

How strange that some I've removed from my older house were rheostats, exactly as described. Maybe you could consider there are houses built before 2010.

You're ill-informed.

rotary rheostats were extremely common and fit into standard work boxes.


asr t1_j2pfg4c wrote

Sorry, but it's you that is wrong. No one uses rheostat dimmers, they would radiate a ridiculous amount of heat - basically the entire heat load of a lamp, inside a tiny box.

/u/Riegel_Haribo is correct.

Maybe you are confused because they look like rheostats, but they are not.


velifer t1_j2psnxx wrote

If you don't think the multi-wound rheostat dimmer for home lighting and fans was ever a thing, then I'm sorry that facts are hard for you. Yes they sucked. Yes, they could get hot. Yes, they absolutely were used in houses and in a form factor that fit into standard work boxes.


asr t1_j2q5rr4 wrote

No they were not. Sorry, but rheostat dimmers simply get too hot to be placed inside electrical boxes on the wall.

You are just mistaken about this. They can only be used in specialized applications with the load exactly matched to the rheostat.

They were never used for general purpose lighting in homes.

I don't even get how you think they were supposed to work - the rheostat has to match the load exactly, how exactly would you even install them in a home where the homeowner can change the light bulb?

(Fans are a different story.)


EliminateTimeZones t1_j2qafne wrote

Variable autotransformers do not need such close load matching and definitely have been used in residential applications. Bigger than your standard wall switch, but sold in a form factor to be installed in a standard 2x4 studwall


TrainerNaive t1_j2r5rl7 wrote

Your house would soon be on fire if you fitted a simple rheostat in a regular wall box and used it to control more than a few watts.

The other folks are right, the earliest domestic lighting dimmers (late 60s onward) used semiconductor switches. Commercial premises, school halls and theatres had rheostat dimmers before that but they were in huge cabinets and dissipated huge amounts of heat.

Very low power fans might have been controlled by rheostat. More likely rotary variacs (variable auto transformers).

I am a chartered electrical and electronics engineer with over 50 years' experience.


sudo_mksandwhich t1_j2q5r9b wrote

But did you actually take one apart and confirm that there is not an SCR inside?


EliminateTimeZones t1_j2qaz0q wrote

The ones installed in a standard workbox are not purely a rheostat/resistor/autotransformer. Older dimmers that don't involve a chopper circuit are much bigger, but certainly do exist. It's possible that a 100-150W unit could be fit into 2 gang box, but I've never personally seen one.


Riegel_Haribo t1_j2o3132 wrote

Maybe YOU should consider solid-state SCR dimmers were in use since 1961:

edit: amazing how many button-pushers don't realize the the nonsense they are upvoting above. Dimming lighting with a variable resistor in a utility box would be a ridiculous hazard. At particular setting you can have made a voltage divider with as much power dissipation in the box as from the total of incandescent light fixtures.


purrcthrowa t1_j2ob6o3 wrote

Exactly. I remember my dad installing a solid state dimmer in the extension to our house in about 1973, and they were common then. The only problem was that he found it quite difficult to source as he needed a pretty high power one (750W or 1kW - something like that). We also had a couple of wire-wound linear rheostats which were probably about 1kW capacity each, and which my dad used for amateur theatre lighting and these things were huge - about a foot long and about 4" square in section. They also needed a lot of ventilation to keep cool.


asr t1_j2q6evz wrote

Thank you. /u/velifer is simply wrong about this. Your memory of wire wound rheostats matches what I know - they are huge, used for theater lighting, and need cooling.

They were never installed in general purpose home lighting.


[deleted] t1_j2or2bk wrote



asr t1_j2pfkb0 wrote

Except that he's right, and you are wrong. And your childish insults doesn't help your case at all.


velifer t1_j2q2z45 wrote

Interesting. There's an old one for sale.

One of those things you say doesn't exist.


Here's the thing: your tone policing doesn't make you correct.


asr t1_j2q65y8 wrote

That is not a rheostat for lighting, it's most likely removed from the speed control of a motor.


EliminateTimeZones t1_j2q9v50 wrote

That is a dimmer. It says right on the cover dimmer. It's a variable autotransformer in a box,.which will dim incandescent bulbs just fine. But maybe only a hundred or so watts looking at it's size.

I have in my possession several Luxtrol WBD series lighting dimmers with a similar electrical configuration that were removed during a dining room remodel.


Doctor-Funkenstein t1_j2psms4 wrote

Lol I'm glad this is the rabbit hole I went down tonight. The true electrical nerds know whatsup while the hive-mind goes towards what seems like an obvious answer. I feel like this dimmer switch thing is a metaphor for so many other things. Voltage phase control is most definitely NOT a new technology. Those light switches would put out a tremendous amount of heat with even a couple amps, and incandescents are Amp suckers.

A dimmer looks like a pot, but it's not!!!


EliminateTimeZones t1_j2q66xf wrote

I have some actual rheostat dimmers I removed from an old dining room. They weigh about 10lbs each and are in cast iron boxes that are about 5x4x3 inches in size.


asr t1_j2pfp3j wrote

That is not worth noting because that's not true.

Older dimmers are NOT variable resistors! That's simply not true.


HanzG t1_j2pnupv wrote

"Modern dimmers are built from semiconductors instead of variable resistors, because they have higher efficiency. A variable resistor would dissipate power as heat and acts as a voltage divider. "

Is this quote not true? Were old ones were not resistor types that induce voltage drops?


EliminateTimeZones t1_j2q94xj wrote

I have some Luxtrol WBD series variable autotransformers that were removed from residential dining room applications. They are much bigger than your standard wall switch, but designed to fit into a wall cavity.


asr t1_j2q53km wrote

Correct, the old resistors based ones were not used in homes, only in commercial applications (theaters).

They were too large and got too hot to be useful in homes.


EliminateTimeZones t1_j2q6h6n wrote

I've seen them in homes, typically dining room chandeliers. They are big and got hot, just like you state.


asr t1_j2q6qxc wrote

Those are not rheostat (resistor) based though. They use thyristors to chop the electrical current (you can hear them humming if you listen closely).

Basically they are switches that turn on and off very quickly.


EliminateTimeZones t1_j2q8tba wrote

Nope. Definitely a thing. Also I'm corroborating your claims that they are too big to fit in a standard electrical box.

I just pulled mine out of basement storage and took a look. Inside is a variable auto transformer. Lookup "Luxtrol WBD" on eBay to get an idea.


EliminateTimeZones t1_j2qb548 wrote

Nope no thyristors in these. Just checked. Just a variable autotransformer. (Not a resistor or rheostat).


Fine-Team-4296 t1_j2sr1fj wrote

There are 4 general types:

The only ime I've seen a variable resistor in real life is in a car. It was for the high medium and low select on a fan speed for heater and a/c. I doubt they would have used them in a house because it would be dangerous due to the heat it creates but who knows.


fomoco94 t1_j2oel6m wrote

That's not true. The peak voltage also will vary for the typical phase shift dimmer. It's just not a linear function of the RMS voltage.


asr t1_j2pf145 wrote

TRIAC based dimmers do that. But newer (and better) MOSFET types cut out the high parts of the waveform instead.

They work better for LED's - more compatibility, and the ability to dim to lower levels.


cannondave t1_j2wf5yu wrote

Very interesting. So if a dimmer supports minimum 3w, I can put a 3w dimmable led light, and dim it all the way down from 100% to 0% (give or take)?


ShuRugal t1_j2o4b9l wrote

>Dimmers don't work by limiting the number of watts

Unless it's a constant power dimmer.


jrabel1 t1_j2rcwdh wrote

Yes, like the first Ford's, "you can dim to any level as long as it's 67%..."


InvincibleJellyfish t1_j2o3byl wrote

It could also lead to flickering in LEDs even if they are "dimmable" as the dimmer just chops up the AC waveform. At least some of them do. This leads to a lot of spikes which can lead to both flicker and an annoying buzzing sound.

Those wifi or bluetooth controllable LED bulbs are much nicer, although way too expensive imo.


created4this t1_j2o9h5c wrote

It’s worse than that, a standard (cheap) dc supply as used in a LED lamp uses a capacitor to limit the current as the capacitor charges and discharges on the AC cycle it lets a small amount of current flow, but if the AC signal is not smooth then the capacitor and the rest of the circuit get the electronic equivalent of jerk on every cycle, which stresses everything and will rapidly lead to failure.


Raul_McCai t1_j2nx74n wrote

> Dimmers don't work by limiting the number of watts. They work by reducing the AC voltage going to the lights.

That might be an old rheostat dimmer. Modern ones turn the power on and off at a rate so fast you don't see it to produce the dimming effect.


StoneTemplePilates t1_j2pnhq5 wrote

What is the division necessary for? If the dimmer is good for 600W then why can't it handle ~40 x 14W?


1feralengineer t1_j2qbvvq wrote

The 600w rating is for a pure resistive load.

LEDs are a nonlinear load. They would fall in the capacitive category (the current leads the voltage by about a factor of about 0.8 full load/brightness). Dimming them can shift the power factor significantly (just because the wattage is reduced doesn't mean the current is reduced - especially peak currents that interplay with the semiconductors that do the heavy lifting of the dimmer).


StoneTemplePilates t1_j2rhr67 wrote

>just because the wattage is reduced doesn't mean the current is reduced

How come? The LEDs still run on 120volts so if the wattage is reduced and the voltage is the same, then the current has to be lower. What am I missing?


1feralengineer t1_j2rnsfk wrote

>What am I missing?

Power factor.

VA only equals watts in a purely resistive circuit (PF=1). In an AC circuit if the current sine wave leads the voltage sine wave (capacitive load), or the current sine wave follows the voltage sine wave (inductive load) the apparent power (VA) is higher than the actual power (W). PF=W/VA


StoneTemplePilates t1_j2s3det wrote

I'm gonna need to do some googlage to understand this, but thanks for the explanation!


According_Instance42 t1_j2tr932 wrote

The dimmer rating for 5 -100w means that if the load is lower than 5 watts, the dimmer cannot work, because it cannot detect the low watts. If the watt is too high, it can blow up the fuse in the dimmer, so it doesnt work either.