3

I have a simple 120v range hood with two controls:

  1. Fan [High/Off/Low]
  2. Light [On/Off]

I'd like to convert the light switch to [High/Off/Low].

I know I'll need to change the switch from [SPST] to [SPDT], but I'm less clear on how to add load to the Low (dim) side to get the bulb to (roughly) 50% illumination -- the existing lighting load is a dimmable LED Edison bulb.

I suppose I could put a 120v pot inline on the low side (which would make it "settable") but this seems awfully hacky.

DaveL17
  • 153
  • 5
  • 5
    Can you maybe use two bulbs instead of one, and use the switch to select wheter only one is active or both. Similar like car headlights are configured for high/low beams. – jnovacho Jan 13 '23 at 12:17
  • @jnovacho Interesting idea; I'm not sure there's room in the hood for two Edison socket/bulb combos, but that is definitely something to think about. Thanks! – DaveL17 Jan 13 '23 at 12:47

4 Answers4

8

No, you can not put a pot inline with the bulb. Let's say the bulb uses 10W: if you dim it to half brightness you will end up with the bulb using 5W and the pot consuming 2W. That's going to mean a very hot and high capacity pot as it is dissipating the heat in a tiny area. (Thanks for keeping me honest - Ohm's Law isn't just a suggestion, after all.)

Usually these range hoods with a 50% setting have a dimmer circuit built in. The other old-school way was to use a diode in line, as that chops away half of the AC wave, but I don't know if that works with LED bulbs.

Unfortunately nearly anything you do here is going to be pretty hacky, so you're probably better off finding a unit that has the features you want.

KMJ
  • 4,915
  • 4
  • 29
  • Thanks for providing an answer. I've decided to look at replacing the entire hood insert, but hopefully your answer will help others who have a similar question. – DaveL17 Jan 13 '23 at 00:44
  • 1
    Not quite. If the bulb uses 10W and you dim it to 5W with a series pot/resistor, the resistor would have to dissipate about 2W, not 5W: the new load (bulb+pot) presents a higher resistance to the power supply, so the total power consumed is less than it was before. But your fundamental point (that it's going to get hot) is still valid :) – psmears Jan 13 '23 at 10:44
  • @psmears oh fair point, I didn't actually do the Ohms Law math. I'll revise to fix, thanks. – KMJ Jan 13 '23 at 18:01
  • It depends on the pot, I have a 100W ceramic one I use to test power supplies that would work, but it's really hard to turn, not something I would want to adjust daily... – dandavis Jan 14 '23 at 08:45
  • @dandavis that would work provided nobody ever put an incandescent bulb in the hood ... – KMJ Jan 14 '23 at 20:08
5

Any fix here is going to be a bit hacky. And with 120V, that can be dangerous. You need to somehow stuff a proper LED-compatible dimmer switch into the hood control panel. That hood control panel likely doesn't have a full-size switch, and the switch that is there may even be mounted directly to a circuit board. Running a switch loop from the switch to a separate junction box with a regular switch would work, but now you've got an extra cable running around above your cooktop.

You might be able to use a "smart bulb", with some sort of remote control to turn it on/off/low/high/etc. (and change color and whatever else...) But I'd be reluctant to put a smart bulb above a cooktop - heat and humidity won't help its longevity.

I'd save my money from this project and put it towards a new hood with multi-level (hi/lo or dimmable) integrated LED lighting.

manassehkatz-Moving 2 Codidact
  • 94,107
  • 9
  • 116
  • 276
  • Thanks for taking the time. As I mentioned above, I'm going to have to get in touch with the builder to find out if the unit can be swapped out. I accepted the other answer only because it was first. Cheers. – DaveL17 Jan 12 '23 at 21:48
  • 1
    Along the same lines, you can buy a Sabbath light bulb with a retractable cover. https://a.co/d/3LfaZpV. I guess it could work if you cover it half way. – erg0s4m Jan 13 '23 at 06:12
  • @erg0s4m A Sabbath bulb won't work in our case because the light is behind a filter/lens combo but it might be a good solution for others. Thanks for taking the time. – DaveL17 Jan 13 '23 at 12:55
5

It's not as simple as adding a dummy load.

LED dimming is highly complicated. Both due to the extremely non-linear nature of LEDs, and also the extremely hacky nature of triac dimming, for which your Edison LED was designed to work.

The upshot is that your Edison LED "bulb" isn't just a light bulb with 35 LEDs in series inside it with a rectifier. That wouldn't work for dimming, because with LEDs there's only about a 10% difference in voltage between "Full brightness" and "barely observable". As such, LED dimming is either done by adjusting the constant-current driver output, or doing PWM. Suffice it to say there's some real silicon in there doing fairly complex stuff to translate "triac dimming" into "meaningful LED dimming".

Since it's an Edison screw-in, you don't really have access to the LED innards to get to do things like add dummy resistors to divert some LED current.

Convert it back to incandescent so it works the way you expect.

Alternately, you could leave the on/off switch exactly as it is, and re-wire the thing to use low voltage 12V LED lights, with a PWM dimmer. These LEDs are very simple affairs - three LEDs in series with a resistor. PWM dimming is extremely reliable and "designed for LED", with full control. The dimmer module is quite small and you can fit it where you need. I would have the separate knob for dimming. Then have the 120V switch turn on the 12 V DC power supply, have that feed the 12V dimmer and then onward to the lights. These will never burn out and so won't ever need to be replaced, so there's no need for the LEDs to have sockets. . (the 12 volt power supply is the only thing ever likely to give out).

Harper - Reinstate Monica
  • 276,940
  • 24
  • 257
  • 671
  • Interestingly I've tried some LED bulbs with an old-school diode 'dimmer' and they dimmed about how you would expect. – KMJ Jan 13 '23 at 06:11
  • @KMJ There are LED bulbs that contain dimming circuitry, they'll be marked that way on the packaging if so. It's actually fairly common, now, as "Dimmable" bulbs are barely more expensive than non-dimmable. Most of the bulbs I see now are dimmable. But if you really cheap out and buy one that isn't, it'll definitely not do what you expect when attached to a dimmer. (Typically, they stay lit at full brightness regardless of the dimmer setting, until you get below a certain point where they promptly shut off.) – FeRD Jan 13 '23 at 10:10
  • 2
    Thanks for the explanation. In our case, LED vs. incandescent is somewhat immaterial because the circuitry within the hood is binary `on/off`. A commenter above suggested a bulb for a 3-way lamp which might be a suitable solution for us. Really want to avoid incandescent as it would use 60x+ energy in this application. Thanks for taking the time to provide an answer. – DaveL17 Jan 13 '23 at 13:01
  • @FeRD I know that and have a ton of them, basically all the ones I use nowadays are dimmable. I was just surprised they worked with a simple diode nearly as well as they do with a proper leading or trailing edge dimmer. – KMJ Jan 13 '23 at 18:04
  • @KMJ Yeah, sometimes you get lucky and dimming works reasonably well. But you can't ever count on it, unless you are recommending specific brands and models of LED and dimmer. – Harper - Reinstate Monica Jan 13 '23 at 22:39
  • @DaveL17 well, if you have enough design freedom, you can go to simple 12-volt DC "3 LEDs and a resistor in series" LED lighting, notably seen in LED strips but also many other form factors. Those dim extremely well using PWM dimming. The chop freqency is in the kilohertz so they don't visibly flicker, unlike sticking a diode on the AC supply line. – Harper - Reinstate Monica Jan 13 '23 at 22:41
  • @Harper-ReinstateMonica I've thought of adding independent LED accent lighting to the hood, using strips rated for outdoors. Then I'd be able to control their brightness fairly easily and use the original bulb as a work light. – DaveL17 Jan 14 '23 at 12:02
1

You might need to ask on the Electronics Stack Exchange for the precise part to use, but you can add an inline part (a capacitor?) off one side of the SPDT switch, inline with the light's power line.

Power comes in on the center contact of the switch. The high side switch contact wires directly to the light. The dimming part wires from the low side switch contact to the high side switch contact.

On high, the light receive full power. On low, power goes through the dimming part to the high contact then on to the light.

Caveats:

Such a change will break the UL listing for the hood. You'll run into trouble if you have a house fire and the insurance adjuster finds this change.

It's also illegal to do this if you're renting.

Such a dimming scheme will only work with an old-school incandescent filament light. Compact Fluorescent and LED lights won't work.

It may be much better to get a hood with dimming already built in.

Triplefault
  • 2,746
  • 4
  • 22
  • Appreciate the input. The hood is a custom unit, so I'm going to have to get in touch with the builder to find out if the unit can be swapped out. – DaveL17 Jan 12 '23 at 21:46
  • 2
    Was wondering if the new type of light bulbs would work. Do have a very old three way floor lamp that uses a 50/100/150w(?) lamp. – crip659 Jan 12 '23 at 21:50
  • @crip659 Heh, you make me sad by calling 3-way bulbs "very old". It's an interesting idea, but I don't know of any LED bulbs that work in 3-way sockets (there probably are--I'll have to do some research). It would also require a different kind of switch, but that's likely manageable (the hard part will probably be the form factor of the switch). Thanks for the idea. – DaveL17 Jan 13 '23 at 12:53
  • @DaveL17 Of course there are 3-way Edison bulbs. Duh. – DaveL17 Jan 13 '23 at 13:03
  • @DaveL17 Was calling the lamp old, since I think it is older than I am(which is old). The bulb is quite a bit younger/newer. – crip659 Jan 13 '23 at 13:22
  • 1
    Appliance bulbs are still mostly incandescent from what I've seen. Mainly because they are far better at working in extreme temperatures (inside ovens, or refrigerators). It's less efficient of course, but these lights are rarely on for very long, so the power drain is negligible, especially compared to the power drain from the appliances they're installed in. – Darrel Hoffman Jan 13 '23 at 15:20
  • @crip659 Yeah, but it's not older than I am. – DaveL17 Jan 13 '23 at 18:07
  • @DarrelHoffman Understood. The bulb is in an enclosed space within the hood vent and not subject to terribly harsh conditions (with the exception of water vapor, I suppose). We use the hood lamp for accent lighting and the extra energy is not insignificant. – DaveL17 Jan 13 '23 at 18:12
  • 1
    For incandescent bulbs, putting a diode in series is a useful hack to get half the brightness. – abligh Jan 13 '23 at 21:01