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For my wedding reception, we are building ~25 lighting fixtures with 4 bulbs each (fixture per table for dinner). For aesthetic and budget reasons, these are 60w "Edison-style" bulbs which we want to dim in order to give a cozy, intimate feeling .

If budget were no question, I would wire a dimmer into each fixture and wire the bulbs in parallel from the dimmer (or just put them all on DMX...) BUT, budget is already tight and the cheapest dimmers I can find are $6.

What I am considering now is wiring each fixture in series which will cut power/brightness by 4 to start but I believe we will want to go less bright.

Reading the following: Permanently dimming an incandescent light bulb, it seems like using an appropriately sized capacitor would be safer than using a resistor which will get hot but that's where I get a bit lost - size of capacitor and how many I'd need.

To cut brightness by half, do I assume a bulb resistance of 220ohm (60W / 115V)? Which then leads to a 1.2uf capacitor, yes? Or should I be calculating for 4x that since there are 4 bulbs in series?

Or is there a better/safer/cheaper way to go about this?

Jonas Stein
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JackTraore
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    Wiring in series will cut power by more than a factor of 4; it's hard to say exactly due to the temperature dependence but it'd be more like a factor of 16. I'd not be surprised if the bulbs didn't light at all. Edit: [simulation](http://tinyurl.com/qqxbrm6) – Hearth Nov 23 '19 at 17:14
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    just use 230v bulbs instead of 120v – Jasen Слава Україні Nov 23 '19 at 22:45
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    If you are thinking of a capacitor you are not needing to vary the dimming. Why not buy 40W bulbs? – Ross Millikan Nov 24 '19 at 05:29
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    Just FYI. There are Edison style LED lamps, as well, with lots of LEDs in the "wire" that are strung up so that they operate on 120 VAC mains, directly. They come in somewhat different color temperatures, too. And they aren't expensive (though 100 of them does add up, I grant.) Have you considered them, at all? Just curious. – jonk Nov 24 '19 at 07:21
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    Just to confirm, you are not planning on driving all of them from a single 15 or 20A circuit, correct? I would use a minimum of 5 circuits. I realize you are planning to have them dimmed but the circuit should be rated for full load. – MooseBoys Nov 24 '19 at 18:53
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    Have you thought about candles or lanterns as an alternative? People often use them at weddings to create that special intimate feeling for some special part of it. – Daniel Frużyński Nov 25 '19 at 15:10
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    $6 dimmer? Oh, [it's for a wedding, you say?](https://www.youtube.com/watch?v=gimiDBAK2wA) This $60 dimmer is our best dimmer... – Aaron F Nov 25 '19 at 17:25
  • Chances are a suitably rated capacitor would cost more than $6 and it won't be adjustable. Why don't you try a couple bulbs in series and see if it looks okay? You have 4 cheap options- bulb + rectifier, 2 series bulbs + rectifier, 2 bulbs series, 2 bulbs parallel (normal brightness). Also, 100 bulbs in series pairs will probably draw something like 16-20A so maybe above the capacity of one outlet. – Spehro Pefhany Nov 25 '19 at 17:30
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    Considering you'll be having a lot of guests and there's a safety component to your DIY question, are you *sure* this is the route you want to take? – Mast Nov 26 '19 at 11:22
  • honestly when I first read the question I thought "East Frisian" dimming (a famous joke here goes about an east frisian taking a stone and a match to bed - "dim" the light with the stone (one-way-process) then check with the match) – eagle275 Nov 26 '19 at 14:59
  • Strings of LED christmas lights would probably be more manageable. They're available with warm bulbs, don't draw an unmanageable amount of power, are already outdoor rated, and can be bought for a very reasonable price. You can make chandeliers out of them. Don't kill yourself or your guests with some crazy DIY hack job. – J... Nov 26 '19 at 17:33

9 Answers9

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Mains guy here. Nothing in LV electronics prepares you for mains.

Don't be cheap

Doing anything with mains electrical plays rather badly with "cheap". Or should I say "inexperienced people thinking cheap", as mains electrical equipment is very reasonably priced.

Start at the end

It sounds like you are just starting to work through the implementation of this, and starting at the lamps. Do it the other direction. Start by working out how you're going to get electrical cables out to each of 25 tables without creating a trip hazard. Then figure out how you are going to distribute 6000 watts nominal of power to the tables, and where you will source that (as it's way too much for a branch circuit).

Note that conventional dimming (or a diode approach) does not reduce the power that must be provisioned, because it does nothing to change the current draw (think VA) of the loads, and distribution equipment is sized based on current, not wattage. In your wiring, you must provision VA, not watts.

Lamps in series within the fixture

I gather your goal is to permanently reduce brightness. The right way is to use smaller bulbs, at the risk of stating the obvious.

However, within a fixture, the idea of series wiring has merit. You can do it easily, using mechanical execution of work that the fire marshal and insurers won't have a problem with. Mismatching bulbs will only make it look bad, not be unsafe. You don't have to worry about wiring methods to an electronic component, nor double insulating the component or grounding the fixture, nor mismatch if someone screws a 5W or 200W bulb into it.

Keep in mind that effective output will fall faster than you think. Incandescent bulbs are not constant current... They act like resistors. Put two 60W bulbs in series, you get two 16 watt bulbs, not two 30s. Put four 60W bulbs in series, you get four 5W bulbs. These are the only choices of arrangement given 4 bulbs. But light is logarithmic, so try it.

On the other hand, this is very good news for provisioning. Since the fixture is permanently wired like this, and the sinewave isn't tampered with (so VA==watts), you can use this lower figure in your provisioning calcs. Four "16 watt" bulbs is 64W per lamp or 1600W for all. Four "5 watt" bulbs means 20W per lamp or 500W all. We're actually in the reach of consumer dimmers here.

Low voltage/LED is where you want to be, though

Honestly, in this day and age, I have no idea why you are still fooling around with incandescents, especiall on an arty project.

Because this thing you're doing cries out to be implemented as low voltage LED. You're looking for lumens in the hundreds per fixture, so just a few watts of LED. Let's be generous and say 1000 lumens, that's 10 watts of LED, x 3 hours, 30Wh, that's a stack of eight AA batteries that costs $8. Or even if you do hardwire off, say, a 24V power supply, you can use the cheapest wires available - gaffer tape down a couple of #18 individual wires to the floor, no trip hazard there! No need for bulky 16/3 cordage, you can get away with a lot with low voltage because it isn't going to bite anyone.

Want plugs and sockets, use Ideal fluorescent ballast disconnects at about 75 cents per mating pair. Can't use those on mains, except inside a fluorescent fixture, but fine for a temporary LV installation.

And low voltage LED puts you into the insanely cheap LED strips (which you can cut to any length, even as little as 1 inch), PWM dimming, all that jazz. You could even have color and centrally control it.

Or if you buy monochrome, any color you want, as long as it's not black. With monochrome, you can put two equal 12V strip segments in series and drive it with 24V.

You might be able to do all 25 fixtures by cutting down a single $18 roll of LED strip. That's where you find cost savings, and you get more design freedom to boot!

Either battery power it locally at 12V, or distribute 24V power (for much lower voltage drop).

Harper - Reinstate Monica
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    "I have no idea why you are still fooling around with incandescents" — I would guess either because they like the aesthetics of incandescent bulbs (which some modern LED filament bulbs do imitate pretty closely, but not perfectly) and/or because they have a bunch of old 60W incandescent bulbs lying around that they want to make use of, instead of paying money to buy LEDs. – Ilmari Karonen Nov 24 '19 at 17:30
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    That may not feel like such a good reason when explaining it to the fire marshal. As far as aesthetic, you mean color temperature. Cheap LED strips are available in any color temp. You can even get low-pressure-sodium yellow, *if you really want it*... – Harper - Reinstate Monica Nov 24 '19 at 17:34
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    Not just the color temperature, or even the spectrum. A LED strip with any color temperature still looks like a LED strip, not a light bulb. (And yes, I'm aware that there are ornamental LED filament bulbs that _do_ look a lot like vintage incandescent light bulbs. They cost more than cheap dimmers, though.) – Ilmari Karonen Nov 24 '19 at 17:57
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    @IlmariKaronen: possibly it could look nice an classy inside a fully-enclosed fixture so you're seeing light through frosted glass. – Peter Cordes Nov 24 '19 at 21:42
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    I like the black LEDs – Volker Siegel Nov 25 '19 at 08:40
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    "Edison" style LED bulbs look very much like the incandescent versions, and they make this a much easier problem to solve. – JPhi1618 Nov 25 '19 at 16:20
  • "Put two 60W bulbs in series, you get two 16 watt bulbs, not two 30s. Put four 60W bulbs in series, you get four 5W bulbs." I'm curious, how did you come up with these figures? Did you already take the logarithmic perception of light of humans into account into those figures, or is that extra? – Alexander Nov 25 '19 at 16:21
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    @Alexander As a power guy I'm only talking about power, and I mean actual power, not "watt equivalents" or whatevs. Crunch it through Ohm's Law and you'll see why halving the voltage quarters the wattage. That should give you 15W. I'm throwing in another 1W to allow for incandescents being *not quite* linear. It's a SWAG, I haven't actually measured it. – Harper - Reinstate Monica Nov 25 '19 at 16:25
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    @VolkerSiegel the best thing about black LEDs is their efficiency... I put one on an AA battery last year, still going! – Harper - Reinstate Monica Nov 25 '19 at 16:26
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    @Harper-ReinstateMonica Ah okay, I came up with 15, so I was wondering where the 16 came from :p – Alexander Nov 25 '19 at 18:12
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    @Harper-ReinstateMonica they run on any voltage too! I just wired one up directly to mains and it works just as well! No silly electronics involved! (this is a joke; kids, don't try this at home) – Doktor J Nov 25 '19 at 21:18
  • @Harper-ReinstateMonica, you can get any *color temperature* you want from cheap LEDs, but not any *spectrum*. Cheap white LEDs are typically a mix of blue and orange-yellow that give an approximation to white; if you want a full blackbody spectrum, you either need a blackbody light source (ie. an incandescent) or a *very* expensive LED setup. – Mark Nov 25 '19 at 22:32
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    @Harper-ReinstateMonica: Put two 60-watt incandescent bulbs in series and they'll give less than 15 watts of light each while drawing more than 15 (but less than 30) watts of power each; the effect may be perfect for this application. Note, however, that this approach is for incandescent (including halogen) lamps only. LED and CFL lamp assemblies are unlikely to work well if wired in this way. Depending how they are constructed, they might harmlessly do nothing, or they might destroy each other. – supercat Nov 25 '19 at 23:59
  • What will happen if he gets one bulb before the rest of the bulbs? Will this work instead of connecting each pair in parallel? – akostadinov Nov 26 '19 at 11:17
  • @IlmariKaronen I recently bought a couple of 1700K lightbulbs that look a lot like incandescents do from the outside and have a bit of a yellow/orange glow. With modern leds, you can do anything and everything. – Mast Nov 26 '19 at 11:24
  • @akostadinov series string won't light til they're all in. – Harper - Reinstate Monica Nov 26 '19 at 14:07
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To use a 9uF film Cap to match the impedance of a 60 W bulb drawing only 22W at 60Vrms/60Hz is actually more expensive than buying a dimmable LED bulb before they became fairly cheap.

First of all a 60W bulb draws about 500W and up to 2.5kW cold for the 1st cycle.

Second, the visible temperature of a 60W bulb is around 2800'K and Lava Red starts around 1000'K to 1200'K with Orange being around 1400'K ~ 1500'K then Yellow 1600'K to Warm White at 2800'K.

So a warm glow of 1200'K would be cozy red warm.

This turns out to be about 900'K rise instead of the normal 2500'K rise above 300'K room temperature.

900/2500 =36% power shared among 4x 60W bulbs means each bulb draws 22W or 88W per fixture.

Since the PTC effect on tungsten resistance is proportional to power \$Pd=I^2R\$ this can be achieved at 1/2 of the voltage or as Dave says 2S2P or 2P2S.

Total power = 100 fixtures * 88W = 8.8kW with a 4x to 5x initial surge.

enter image description here

It all comes down to WHAT are your SPECS?

Colour temp, power, flicker, buzz volume budget including extension cords and fixture?

Community
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Tony Stewart EE75
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You should try wiring one of the fixtures as 2S2P. I'm betting that this will give you the nice warm glow you're looking for, and it's the easiest thing to implement.

Dave Tweed
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One time event? 25 fixtures? For this do hire companies exist!

While series strings and such are possible, the elephant in the room is most likely power distribution, specifically all that cable (Which then needs to be 'drunk uncle' proofed, not trivial). You can hire lighting fixtures designed for use as table centre pieces that are LED based and BATTERY powered, the win being that you don't have to mess with the power distribution (Besides, building 25 of ANYTHING is a pain in the arse).

Dan Mills
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I would suggest that you try putting a single diode in series with the bulb(s). This will cut the rms current in half right away, then you can try various series/parallel combinations of bulbs to get the desired level of glow.

If you used four 60W bulbs (240W total) we would expect the average diode current to be on the order of 1A (at 120V, half-wave rectified). The current might be a bit higher because the filaments are a bit cooler, but I think a diode rated at 400V and 3A should be OK. The power dissipation will be about 1W, which might be pretty warm.

Elliot Alderson
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  • 60Hz buzz is more noticeable than 120Hz – Tony Stewart EE75 Nov 23 '19 at 20:17
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    A diode is a mistake. it will actually *increase* the bulb's current draw, from 0.5 to 0.6 amps, while cutting its light output from 60W to 36W or so. That's because VA isn't watts. VA is the whole sinewave which must be delivered, watts is the actual draw. – Harper - Reinstate Monica Nov 24 '19 at 09:13
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    Actually, VA is **apparent power**, the product of rms current and rms voltage. Watts are **real** power, the product of instantaneous current and instantaneous voltage. For a non-reactive load, such as an incandescent bulb, they are the same. I do agree that the peak current will increase. – Elliot Alderson Nov 24 '19 at 13:04
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    Watts are *useful* power. Harmonic effects and wire heating are quite real, as is their effect on distribution equipment. Theory aside, VA defines the sizing of distribution equipment. Here's what you can't do: put 3840 VA of load (1920 watts) on a 2400W-nominal circuit. – Harper - Reinstate Monica Nov 24 '19 at 15:30
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    @Harper-ReinstateMonica: For the wiring, overheating effects should still be based on average current, not just peak. Or are you arguing that with less heating in the bulbs, their resistance will be lower leading to larger *average* currents? Anyway, you can use 1 diode per fixture and mix them up so your main power-distribution point sees about the same current draw over each half of the cycle. (Or just two big diodes and plug half the tables into each, although that concentrates all the half-cycle currents together in that wiring). – Peter Cordes Nov 24 '19 at 21:36
  • I'm arguing that 200% current half the time makes more heat than 100% current all the time, *and we're still ignoring harmonics*. But the stuff with VA/PF is all addressed in NEC; if that doesn't conform to theory it's probably because NEC is written in ash and blood, not chalk :) Your first idea (per fixture) would work, your second idea (per string) would be fine up to the subpanel, but would require provisioning twice the branch circuits. – Harper - Reinstate Monica Nov 24 '19 at 21:47
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    If you are going the diode route, make sure half the lamps have the diode pointing one way, and the other have the diode pointing the other way. Otherwise a substantial half-wave rectified load may play havoc with the electricity supply. – Simon B Nov 25 '19 at 11:12
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Go with LED bulbs

You can get 12V filament-style LED bulbs for about $5 apiece. At 12V/4W apiece, a four-bulb fixture will draw ~1.34 amps. Stick a 1.5 amp or higher power supply under the table, replace the fixture's plug with a female DC jack that matches your power supply, and you're off to the races.

If you like to do your shopping on Amazon:

If you're not a fan of them, a little Google searching should turn something up; remember, if you're looking for 12V or 24V LED bulbs with the standard E26/E27 base, try RV stores.

At those prices, total cost per table (not counting the fixtures themselves and any extension cords, both of which you'd need in your current setup) is about $27. When you consider that you'd be using more expensive (and easier to trip over) heavier gauge cords for incandescent bulbs, you can save there too.

The power supplies I linked are rated at 0.8A input, so for 25 tables you're looking at ~20 amps input. Two 15A circuits will run that easy peasy. I know power factor correction is a thing, which means these could theoretically draw more than 20 amps peak, but your fixtures aren't going to be drawing the full 2A on the load side; only about 2/3 that.

Extra Credit

Make it all battery powered! You can get 7Ah-9Ah 12 volt sealed lead acid batteries for about $20 apiece. Strap them under the table somewhere out of the way (one per table; they're not that big, about 6 x 3 x 4 inches), and each table has its own power supply! At 7Ah, you're looking at at least 4 hours of runtime off a fully charged battery. Sure the batteries seem expensive... until you realize you're saving 100% on extension cords and reducing your trip hazard by approximately 100%.

Still not "warm" enough?

Cheap solution, throw a lamp shade on the fixture! A warm neutral tone shade (think beige or tan) will fit almost any decor, or if compatible with your decor, an orange or even red shade! These will impart their warmer tones to the light and also eliminate the harshness of a direct bulb. You are also no longer constrained to filament-style bulbs if you use a shade because the bulbs are no longer directly visible!

If you really need warmer bulbs, they make them in 2200K varieties (these will usually have golden or amber tinted glass), but you'll end up paying more -- cheaper just to get some run of the mill 2700K bulbs and slap a shade on them!

Doktor J
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  • P.S.: a pack of 100 spade connectors for those batteries runs about $10 and would be enough to refit the fixtures for *every* table! – Doktor J Nov 25 '19 at 22:02
  • The problem with LEDs is that they don't warm when they dim, so you end up with less light, but it's still relatively 'cool' 2700K light, even if you get the warmest "warm" bulbs you can find. There are LED bulbs that warm when they dim to even lower colour temperatures and OP seems to want this very warm lighting effect. The only option there would be to spring for those, more expensive, warm-dim type of bulbs. And it seems OP is trying to do this on a shoestring budget. – J... Nov 26 '19 at 14:13
  • @J... doing any mains-powered project on a "shoestring budget" is asking for trouble. heck, doing any mains-powered project if you're not *absolutely* sure what you're doing is asking for trouble. Cheap trick: put beige or tan lampshades on the fixtures. Classier look, and adds extra warmth to the light, while the neutral color goes with almost any decor. – Doktor J Nov 26 '19 at 17:10
  • Agreed completely - OP is certainly at risk of all manner of unfortunate consequences with this project. The shade idea is good. It should go into the answer because it seems a critical part of OP's problem (getting the right warm light tone they're after). – J... Nov 26 '19 at 17:27
  • The only modification I'd do the the LV idea is to use LED strips if that is compatible with the fixture design. A standard 5-metre LED strip gives exactly 100 or 200 cuttable segments, *just enough* for the entire project. Downside: no spares. Figure about $25 for a more expensive/high power/high density/exotic kind. – Harper - Reinstate Monica Nov 26 '19 at 17:53
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    @J... integrated that into the answer! – Doktor J Nov 26 '19 at 21:56
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A capacitor in series with one or more bulbs would be a reasonable low power way to dim them.

Note that incandescent filament bulbs have a strong temperature coefficient of resistance, the resistance drops significantly as they become dimmer, so 60W@115V will not give the resistance when dimmed.

'Brightness in half' is not very meaningful, as their colour will change with temperature, and the eye is logarithmic to brightness. It would be best simply to experiment with a few different values of capacitance and see what looks good on one fixture, then replicate that another 24 times. While there are approximations for resistance versus voltage for filament bulbs, it doesn't help a lot if you don't know what temperature is actually going to look good.

To get your first try of capacitor value in the right ballpark, by all means set the impedance to 220 ohms which is about 10uF. Buy a few capacitors of that value, and put a few in series or parallel to vary the value as you observe the lights. I tend to use 'motor run' capacitors for this sort of thing, which are already rated for mains connection.

A somewhat cheaper way than using extra components is to find out how many lamps in series give you the requisite brightness / colour, and build your fixtures to that size, rather than specifying 4 ahead of time.

Neil_UK
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  • Yeah, motor run caps would be a good safe choice. Obviously do not use polarized caps unless you want fireworks at the wedding reception. – John D Nov 23 '19 at 17:31
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    And presumably these motor caps will be more expensive than the dimmers... – pipe Nov 23 '19 at 20:15
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How, exactly, do you propose to run power to your fixtures? If you're using something other than existing power outlets (one at each table), you can run multiple fixtures from one dimmer. Let's say you're using 120 VAC, 15 amps. That's nominally 1800 watts per circuit, and that's 30 60 watt bulbs.

Then you only need 4 dimmers to control 120 bulbs.

And don't think that you can use more bulbs per dimmer, since the bulbs will be running at lower power - you must keep in mind that Murphy's Law says that some yoyo is going to turn the dimmers all the way up.

Plus, inrush current for 30 60 watt bulbs is something you'd need to look into. It's entirely possible that you could not simply turn power on and off for any circuit.

Of course, running separate cabling from a few dimmers to multiple fixtures is not something I'd recommend. That cabling is almost certainly vulnerable to accidental compromise, with embarrassing (not to mention potentially deadly) consequences. Think about what happens when a guest steps on a cable while wearing spike heels.

WhatRoughBeast
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This all seems overly complicated. Here are 2 easy options:

  1. Buy 240V bulbs and use them in your 120V fixture. They will be quite a bit dimmer and is perfectly safe.

  2. Buy 15-25 watt 120V bulbs instead of 60 watt bulbs.

Fuffer
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    This. Use higher voltage bulbs and power them with lower voltage. I was using this trick not to dim the light but to protect power sources against shortcut. Multiple bulbs in series to dampen the voltage spike and multiple parallel branches to assure the current throughput. it was protecting 700V/5A power for glow discharge. – Crowley Nov 26 '19 at 02:17