I have a warehouse with a double side to it one side will be a warehouse the other a paint factory, the building itself is quite large for ho and will wrap around a corner, I love modeling the inside of structures and this one had 7 stories with many areas lit up with led, i have about 80 led in parallel , i have them running off a 12 volt dc supply 1000 Ma 9 watts, when I attach the power to the leds they light up nice and bright, they don’t get warm at all and seem to be fine, I don’t know why as they are in parallel mode, but as soon as I throw one 100 ohm resistor the resistor gets very hot, do I even need a resistor? I don’t know the rating on these leds, I know they are 3mm diffused white., im concerned that I will eventually blow out all the leds? They look nice and bright, which is what I would want, but how do I know how to keep them safe from overloading. the resistort i used was a 100 ohm 1/4 watt 5% tolernace, does it get hot cause its only 1/4 watt and my dc supply is 9? if so how do i use a 100 Ohn resistor with a higher watt? and would that stop it from heating up? Id liike to dim the led’s a little to keep them from bunring out.
At a minimum, you’ll need at least a 1/2 watt rated resistor. At 12 volts, something like 470 ohms or somewhat more as a minimum to protect them. I think what’s worked so far is that you have so many they act as a load. Being parallel isn’t the problem, that’s the way you want them. You’re just trying to put 10 pounds of sausage into a 5 pound casing with that 1/4 watt resistor.
I’m a bit of a barnyard electronic tech, so someone who can calculate this out may give you a better answer.
Another solution is to divide the power feed into two circuits or more. That may not be so easy with them installed though.
Its weird, i put a 480 Ohm 1/2 watt I get no lights at all, I put no resistor I get everything lit up nice and bright, would maybe need to tone down a little bit, so i use the 100 Ohm it works great ,just that the reistor gets hot, i cant touch it too long onlt for a few seconds. do they make a 1/2 watt 100 ohm? id like to keep the brightness going , bt not have the reistor get so hot. maybe that resistor is suppose to get hot like that , Im not sure I never had one get hot like that.
Try 200 ohms and see what happens. Just solder 2 of the 100 ohm resistors end to end in series. That will give you 200.
Generally with resistors, heating is a sign that they are not a high enough wattage rating. To increase the rating, you could put 2 1/4 watt resistors in parellel and effeictive get a 1/2 watt rating IIRC, but they do make all the common resistor in 1/8, 1/4, and 1/2 watt versions. may take some tracking down to find a particular combination of resistance and power rating.
Hi mike, tried that it takes the heat off but the lights become dim, i figure if i can find a power supply with less watts or a resistor thats 100 ohm or a little less thats 1/2 watt that may work. Thanks for your input, i do appreciate it.
Yep, 100 ohm, 1/2 watt resistors are readily available and will be less hot to the touch.
I use 100 ohm, 1 watt resistors for my bus wire snubbers. The 100 ohm, 1 watt resistors are readily available as well from Radio Shack and on line electronic stores.
If they are in parallel, then each LED has its own resistor. You would have adjust each resistor separately.
If you have 80 LEDs in parellel on a single circuit and want to make them dimmer, reduce the voltage. A large ceramic variabable resistor might work for you.
Are the leds in parallel and just one resistor or are you using one resistor for each led? You need a resistor for each led i.e. a led and resistor in series wired to the 12 volt buss.
Parallel LEDs can be tricky, but if they are all from the same batch it might work. If they all are lit evenly, then perhaps you’ve lucked out. 80 LEDs in parallel on a 12V 9 watt power supply is about 9mA per LED, which is a safe level, but if ant any time part of that circuit becomes disconnected, the LEDs will see a huge current spike and you could have a chain reaction of them blowing out. What you want is one of those 100 ohm resistors on EACH LED. 1/4 watt will be more than enough to not get hot. I’m kind of surprised the 1/4 watt resistor didn’t melt when subject to the full power of the LED string. 1/2 watt won’t be enough either. But if you put one resistor per LED, then you’ll want a 1K resistor, not a 100 ohm, with a 12V power supply. That will make each LED/resistor combo draw 8.5mA, making the whole string of 80 in parallel draw .68 amps, which the 9 watt (.75 amps) power supply can handle. Those 1K resistors can even be as small as 1/8 watt without getting hot.
Unfortunately, I have one resistor at the power source for the entire building, pulling all the leds out and adding a resistor to each one would be quite a task, I’m ho
Yes, wiring in parallel can be pretty confusing. It does work, but you’re probably better off not doing 80 in parallel. With my larger buildings, I build multiple “sticks” of parallel LEDs, then wire their leads together at a common feed point. That way any failures are limited in their impact, while wiring is simplified and the need for a resistor on each LED is reduced. Generally I limit each “stick” to 10 or fewer LEDs IIRC. I test them as I build them before they go into the structure, which let’s me verify each one works as intended, as well as getting the resistor value set right for the specific use it see in each building. I’ve had no failures and the lights are on every day, often all day.
It might be too late for your current project but for lighting large structures in the future these LED strips would be just the solution you might be looking for. They even have peel-n-stick tape already applied.
You just cut them to the length you need at the “scissors” emblem (in groups of 3 LEDs) then add leads to supply 12v DC.
There is no problem at all with PARALLEL LEDs. You can add 100s of them without problem.
SERIES is different, and has LEDs wired one after the other on a single wire. I have tried that too, and you do not want to do that! If you take apart a set and use parts of it here and there, you are on your own.
Wire in parallel with one leg of each LED going directly to the buss.
Yeah, I was playing fast and lose with terminolgy here. Technically, my arrangement is often a hybrid series-parallel circuit, sometimes just a parallel circuit wired back to the bus in multiples at the power entry point to the structure.
Series connection of LEDs can be problematic. If you get too many in series, they simply won’t light, as the voltage drop across each added up to more the the voltage output of your power supply. So it’s also technically limited. But it does work if you understand the limitations.
What can still be problematic about parallel LED circuits is when they are guarded by a single resistor, instead of by resistors on each LED. That’s where breaking down large groups of LEDs into the “sticks” I described above helps, as a single resistor is on the feed line to eaxh stick. That limits the chance of short or something else blowing a bunch of LEDs all at once to just the affected “stick.”
There’s a reason all those LED strips can be cut apart into groups of 3 LEDs - they use a series/parallel wiring. The 3 are in series (about 10.5 volts) and then they have an appropriate resistor for 12V, and on the whole reel of them you have the groups of 3 in parallel with each other. Trace the wiring on them, you’ll see.
The problem here is that even a 1/2 watt resistor is not enough. Assuming the LED drop is 3.5, average for white LEDs, that leave 12-3.8, or 8.5V to be dropped by the resistor. Current = voltage/resistance, so the current in the resistor will be .085 amps, 85mA. Power is current squared times resistance, or .7225 watts. About 3/4 of a watt - you need at least a 1 watt resistor for this.
As for the rest - currents in series add, and a 9 watt 12V power supply is .75 amps. So the LEDs get .75 - .085, .665 amps, divided by the 80 LEDs (current in parallel adds) gives each LED 8.3mA - assuming they all are close enough in spec that one doesn’t get the lion’s share (no relation to the subway engineer). This is compared to the 9.3mA they get with no resistor.
Randy’s numbers do help settle things. I wonder if there’s anyway to split the circuit in half, then wire them to the supply as two circuits? I know that may be impractical, depending on how the building is put together with the lighting. This could be a case where putting two 1/2 watt resistors in parallel might be the simple solution.
Actually I did make small sections of leds using copper tape on styrene for each room, so I could probably make small little separate modules of leds and tie them to one bus. i also went out and bought a 100 Ohm 5 watt resistor and its working fine, no heat up at all and things seem to be ok, ill probably leave it like that, in the event the thing blows Ill have no choice but to redo and then i think moving forward with my buildings I will use the copper tape also as a bus wire and break my LED up into smaller units with a resistor, that does make sense. Thanks to everyone for all your help, great ideas!!
