Can two LEDs share a resistor?

[crossthedog]

hon30critter

I would suggest that you wire in the resistors as close as possible to the individual LEDs. That will allow you to use just one pair of wires to power all the lights in the structure

Dave, I like the idea of having just two wires incoming to the structure, but again, having trouble picturing the wiring path and where the resistors would be in the diagram. After all the input I'm leaning toward separate resistors for each floor. Would that look like this?

[crossthedog]

CSX Robert

What betamax is describing is if you have two LEDs in parallel running off one resistor.

Robert, I always have trouble picturing parallel circuits; where would the resistor be in a parallel setup? I think of parallel as a loop with a bulb on it and a short-cut across the middle of that loop with another bulb on it. But where could a resistor be that it wouldn't affect both LEDs if it failed? If you feel an urge to draw a diagram, please indulge it.

Thanks,

-Matt

[CSX Robert]

wvgca

you can, but it's not really good practise to run more than one LED per resistor

No, actually, it's often a very good practice.  It's true that you can't adjust their brightness individually, but as long as you're ok running them at the same current then it's better to run multiple ones in series off a single resistor if you have the supply voltage to do it.  You get less current draw for the same light output and less power wasted as heat through the resistors.  With just a few LEDs, it won't make much difference, but when you get into dozens or hundreds of LEDs, it can make a big difference.

[CSX Robert]

crossthedog

Thanks for the warning about the surge in current should one LED fail

What betamax is describing is if you have two LEDs in parallel running off one resistor. If you have both LEDs and the resistor all in series (which is what I understaood you to mean), then if one fails it will break the circuit.  The other one still won't light until the bad one is replaced, but it won't get damaged.

[wvgca]

you can, but it's not really good practise to run more than one LED per resistor .. and with one resistor per LED you can easily adjust the brightness of each indivual LED, and resistors are really cheap, maybe a penny each ...

[gregc]

crossthedog

Is there any way to know from looking at the photo of my resistor above what strength it is?

i'm seeing blue red black gold which mean a 62 Ohm resistor which results in a 25 ma for a 1.4V LED driven by 3V or 16 ma for a 2V LED driven by 3V.    max LED current use to be 20ma and i think they are plenty bright today at much lower currents.

as i suggest, try different resistor (> 62) to lower the brightness.

 

[crossthedog]

CSX Robert

Actually, it may be too much resistance. With the LEDs dropping more volts, the resistor will drop less. Less volts through the same resistance gives less amps, which means dimmer LEDs, so you may have to lower the resistance.

Robert, this surprises me completely. I'd have thought it went the other way, that if I strung more LEDs in a series with one resistor, each one would be brighter. This is very enlightening. (<-- see what I did there?)

hon30critter

In your particular situation using one resistor per LED would be particularly advantageous. If I understand your comments correctly, you want relatively dim lights on the second story and brighter lighting on the main floor. If you use one resistor for each LED you will be able to control the brightness for each LED. If you use one resistor for both LEDs their brightness will be the same.

Well, not exactly the same brightness. I dipped the upstairs bulb in paint and then wiped it a bit, so it now has a warm gloaming look. But you're right in that when I tested it, the lower lamp had a brighter cafe color but it was not as bright as I thought it should be.

hon30critter

The other issue that you pointed out was that the wires were visible through the windows. There are ways to prevent this.

Wires showing through the window is not a problem for me; I showed that to demonstrate where I want to put the resistors, which is on the back lower walls. And your idea of painting them bright wall colors is a good one.

hon30critter

I suspect that you are installing the LEDs in the buildings and you are then installing the resistors in the wires leading to the building. I would suggest that you wire in the resistors as close as possible to the individual LEDs. That will allow you to use just one pair of wires to power all the lights in the structure.

No, I actually plan to have the resistors inside the building, but fastening them to something is problematic. Will epoxy damage the resistors? Thanks for the detailed ideas, as always.

betamax

Logic: The resistor is calculated to provide current limiting to two LEDs, which is twice the current as one LED. Should one LED fail, the current will increase to the remaining LED, which will soon fail as a result. Besides, how much money is saved by eliminating a resistor?

Max, I'm not trying to save money, I'm trying to avoid soldering in tight spaces with tiny wires, which activity I dis-love quite a bit. Thanks for the warning about the surge in current should one LED fail.

gmpullman

Looks like there's already a resistor under the shrink wrap in your second photo?

Ed, I was wondering the same thing, cuz it sure looks like they've wrapped a tiny resistor in there, but this package of LEDs I bought specifically because I wanted some withOUT built-in resistors. So I think that's just the soldered joint under there. But good to know I can gang the resistors up, as you've suggested. Thanks.

gregc

it seems today's LEDs are extremely bright and am using resistors in the 10s of Ks with 3.3V, so start with a 1K resistor (~10ma w/ 12V) and try other values until satisfied with the brightness.

Greg, funny you say that. I think the LEDs are often too bright for interiors, too. I like them for locomotive headlights, but yeah, normally way too bright for a lamp upstairs. However, in this case, I want the lower light to be a lot brighter than it is. When I tested the lamp in the cafe, it was the right hue, but it seemed a little dim. Is there any way to know from looking at the photo of my resistor above what strength it is? These came with some 3v loading dock lamps I bought from WeHonest; they're the only ones I "currently" have.

Thanks all.

-Matt

[gregc]

surely multiple LEDs can be wired in series with a single resistor if the supply voltage is high enough as CSX Robert said.   you could probably wire ~6 LEDs in series with a 12V supply, avoiding unnecessary work (soldering)

it seems today's LEDs are extremely bright and am using resistors in the 10s of Ks with 3.3V, so start with a 1K resistor (~10ma w/ 12V) and try other values until satisfied with the brightness.

[gmpullman]

Looks like there's already a resistor under the shrink wrap in your second photo? Probably a 680 or 1KΩ.

So you want to further reduce the intensity of the light from each individual LED by adding resistance in series?

I have 'ganged up' resistors for this purpose, especially in signal LEDs. The resistor can be in either leg but I usually try to keep them on the minus side as is a good practice for decoder function wiring since the anode is usually common. (blue + wire)

Good Luck, Ed

[betamax]

Use one resistor per LED.

Logic: The resistor is calculated to provide current limiting to two LEDs, which is twice the current as one LED. Should one LED fail, the current will increase to the remaining LED, which will soon fail as a result.

Besides, how much money is saved by eliminating a resistor?

LEDs and Resistors

[hon30critter]

crossthedog

So my question is, is there any way to do this with just one resistor?

Hi Matt,

As has been stated, yes you can use one resistor for two LEDs. In fact, you can use a single resistor for up to about six LEDs, but calculating the resistor value can be somewhat complex.

My suggestion is to use one resistor for each LED. In your particular situation using one resistor per LED would be particularly advantageous. If I understand your comments correctly, you want relatively dim lights on the second story and brighter lighting on the main floor. If you use one resistor for each LED you will be able to control the brightness for each LED. If you use one resistor for both LEDs their brightness will be the same. I would suggest trying a 10,000 ohm resistor for the upper floor and a 1000 ohm resistor for the main floor. If you find the second floor to still be too bright then try a 20,000 ohm resistor. I have used resistors up to 30,000 ohms where I wanted to simulate the light of a kerosene lantern.

The other issue that you pointed out was that the wires were visible through the windows. There are ways to prevent this. One is to simply paint the wires with the same colour as the walls. Or, you could run the wires so that they are not visible through the windows. That takes a bit of planning before you start to assemble the buildings, but it is easy to do.

I'm going to go out on a limb here, but I suspect that you are installing the LEDs in the buildings and you are then installing the resistors in the wires leading to the building. I would suggest that you wire in the resistors as close as possible to the individual LEDs. That will allow you to use just one pair of wires to power all the lights in the structure.

I may be totally out to lunch on all of the above. If so, I apologise.

Cheers!!

Dave

[CSX Robert]

crossthedog

Could I just make one long loop from the power source with a resistor anywhere in that loop?

Yes you can, as long as your supply voltage is enough.  Two 3 volt LEDs will drop 6 volts total, so it wouldn't work with 5 volts but would with 9 or 12 volts.

crossthedog

but I'm not sure whether this would be enough resistance

Actually, it may be too much resistance. With the LEDs dropping more volts, the resistor will drop less.  Less volts through the same resistance gives less amps, which means dimmer LEDs, so you may have to lower the resistance.  Note that it won't neccesarily be half the resistance, it will depend on the supply voltage. I would, however, just start with the resistor you have and see if it's bright enough.

[crossthedog]

Once upon a time there was a two story cafe.

Forget that, I'm ditching the storytime conceit. Here's the deal: I put a cardboard floor in to separate the two stories, and then separated some of the windows off with walls, so that a warm LED upstairs will light some but not all windows, while the cafe below will be brightly lit with another LED.

I've tested this holding two resistors and four wires with just my two hands (difficult? yup.) and it looks fantastic.

Now, take a look in the lower window. You can see the wires for both lights coming down the wall.

This is where I want to put resistance. In the other two buildings I've done so far, I put just one resistor in the basement because there was only LED that needed resistance.

They work fine. I know how to do it. But it seems like a lot of work and would be a mess of soldering and wiring to put two resistors in here.

So my question is, is there any way to do this with just one resistor? The resistor I have here is for 3v LEDs:

This is what I'm thinking, but I'm not sure whether this would be enough resistance, and if it is, where would the resistor be placed? Could I just make one long loop from the power source with a resistor anywhere in that loop?

Thanks in advance,

-Matt