Can you run enough LED's in series to not need a resistor?

Affiliate Disclosure: We may receive a commision from some of the links and ads shown on this website (Learn More Here)

Welcome to your Virtual Model Railroad Club on the web. Want join in the convesations?

Register for your free membership today!



KB02

Well-Known Member
When I built my round house and was putting lights in it, I discovered that the more LED's I put in series, the more power they needed to light them all. I could get a max of 3 2mm LED's in series behind a 1K resistor and still have them lit. If I ran 4, they wouldn't light, and if I ran just 2 they were much brighter.

So, following this logic, if I added, say, 10 LED's to a series, would I be able to light them all behind a 0.2k resistor? Or 20 with no resistor at all? (No real math went into these calculations - just making the numbers up.)

Or would the lead LED just get toasted by the extra voltage leading to a cascade of failures down the series?
 

Burlington Bob

Well-Known Member
I didn't run any numbers either, but all the LED's would share the load equally, meaning that the first and the last in the series would be of equal brightness, or burn out at the same time. No cascading effect.
 

NYC_George

Well-Known Member
I had recently bought at least 100 3 volt 3mm, 5mm and SMD LEDS. I did not want to solder a resistor to each led so I also bought a couple 3 volt DC power supplies. The supplies were I think $4.00 each. Saved me a lot of extra soldering. There are some examples in my Getting Stuff Lit thread. I have no idea why more modelers wouldn't do this. If anyone has any argument doing this let me know.

George
 

kjd

Go make something!
You can also use different resistor values to have stings with different numbers of LEDs light evenly.
There wouldn't be any cascading failure because if one fails the circuit is broken.
 

John P

Member
Now wait a minute, guys, that's not how LEDs work. Any diode has a non-linear relationship between voltage and current. The way it works is that there's almost no current up to a certain threshold of voltage, and then the current becomes almost unlimited. If you put a bunch of LEDs in series, you just make that effect happen at some higher voltage, which will be about 2 Volts per LED. What gives you a reasonable relationship of current to voltage is putting a resistor in series with the LEDs. There's no way you can say you don't need a resistor just because there are more LEDs! It might be that this setup is working because the voltage on the power supply is being pulled down (effectively, it has an internal resistor in series already) or some such behavior. But you really do need some way to control the current through an LED.
 

wvg_ca

Active Member
you don't need a resistor if you have enough leds ....how ever there are some drawbacks to this idea ... 1] if the leds are not out of the same run or batch you might get some varying intensities 2] if a led 'opens' they all go dark .. 3] if a led 'shorts' it can affect brightness, and eventually over drive most of the leds
 

John P

Member
I won't get into an argument here, but this just isn't good design. You really do need a resistor or some kind of current-control circuit to operate LEDs. It may be possible to make them work without that, but it won't give consistent results.
 

Burlington Bob

Well-Known Member
I know it's not good practice but I figured that the total load (enough LEDs) would act like a series of incandescent bulbs; as bulbs are added to the circuit, they would get dimmer, depending on the number and wattage of the bulbs.

As was stated, wiring lights in series can be an exercise in frustration. Remember the old Christmas tree lights.........when one went out, they all went out because they were wired in series. A burned out bulb acts just like a blown fuse or tripped circuit breaker; it interrupts the circuit, thereby not allowing a complete flow of electrons.

I can wire a house, but electronics is something I have to do research on before I do anything physical. Thanks for the lesson in electronics, John. I usually get out my how-to model railroad books before I do much of anything. Maybe I should devote a little more time to them!
 

John P

Member
I was ready to say that current-control component could easily overheat, but then I realized it's not likely. Drop a full 12V across it and at 20mA, you're still only losing .25W, so it would be warm, but not melting. Another idea would be an LM317 voltage regulator used as a current regulator, which only takes a single resistor.
 

NCRC5315

Member
I was ready to say that current-control component could easily overheat, but then I realized it's not likely. Drop a full 12V across it and at 20mA, you're still only losing .25W, so it would be warm, but not melting. Another idea would be an LM317 voltage regulator used as a current regulator, which only takes a single resistor.
I routinely run 24VDC though these, without issue. They can be used with up to 90VDC.
 

Iron Horseman

Well-Known Member
...t hat's not how LEDs work. Any diode has a non-linear relationship between voltage and current. The way it works is that there's almost no current up to a certain threshold of voltage, and then the current becomes almost unlimited.
John P is correct. A LED is a diode that just happens to emit light. It is not a light bulb and should not be treated as such. Restating ... One shouldn't attempt to apply normal Ohms law series and parallel equations to diodes since there is no fixed value for R. For all intentions R is infinity until the voltage reaches the threshold. After the threshold the R becomes zero.

If one uses them in series with no resistors and it works I would guess they aren't really straight LEDs but have some sort of resistor built in.
 

Rico

BN Modeller
Это немного не по теме, но если мне понадобится лестница, я буду об этом помнить.
лол
 

Frank

Member
You are correct about the mystery, but I did like your answer for whatever reason it was given!:)
It's somewhat less of a mystery when you know that google.com/translate is a thing. I can't vouch for the accuracy, but the translation is at least actual English as opposed to whatever computer translations used to give you.
 

Burlington Bob

Well-Known Member
I just copied and pasted, found out it was Russian, as I suspected. Then searched for a Russian to English translation.

The reference to the ladder was the intriguing part of the mystery.
 



ModelRailroadForums.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com

RailroadBookstore.com - An online railroad bookstore featuring a curated selection of new and used railroad books. Railroad pictorials, railroad history, steam locomotives, passenger trains, modern railroading. Hundreds of titles available, most at discount prices! We also have a video and children's book section.

ModelRailroadBookstore.com - An online model railroad bookstore featuring a curated selection of new and used books. Layout design, track plans, scenery and structure building, wiring, DCC, Tinplate, Toy Trains, Price Guides and more.



Top