3 mm LED lights Series or Parallel

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modelbldr_207582
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3 mm LED lights Series or Parallel

Post by modelbldr_207582 » Sat Aug 19, 2017 2:36 pm

I'm wiring a number of 3 mm LEDs and I'm wondering if I will need resistors for 3 plus LEDs? As I understand it when wiring a number of LEDS on the same circuit resistors are not needed.

Is this true or false?

I have an understanding how to wiring them in Parallel and Series, I just don't know of the number of resistors needed when wiring these LEDs #-o

Thanks so much

Ant
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Re: 3 mm LED lights Series or Parallel

Post by Ant » Sun Aug 20, 2017 2:00 am

In series, one resistor per set of LEDs.
In parallel, one resistor for each LED.

EVApodman
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Re: 3 mm LED lights Series or Parallel

Post by EVApodman » Sun Aug 20, 2017 11:20 am

In a series circuit the current is the same and voltage varies. The same current will flow thru LED and they will all have the same ( approximate) brightness. the down side is that if one goes they all go, but with correct resistor values they will probably outlast you.

In a parallel circuit each LED has its own resistor, but the current draw is cumulative, causing larger current drains on your power supply. The good news is that if one LED goes out, all the others stay lit.
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TurkeyVolumeGuessingMan
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Re: 3 mm LED lights Series or Parallel

Post by TurkeyVolumeGuessingMan » Mon Aug 21, 2017 1:53 am

In my limited experience and from the advice of so many other modelers, always wire your stuff in parallel and never in series. It's better only one LED burns out than all of them going dead.
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Rocketeer
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Re: 3 mm LED lights Series or Parallel

Post by Rocketeer » Mon Aug 21, 2017 10:30 am

The point of the resistor is to limit the current going through the LED (typically to 20 milliamps). The formula for finding the resistor size is (Voltage of the battery -minus- voltage drop of the resistors / amperage). So for a typical single LED (usually about 2 or 3 volts forward voltage) running off a 9-volt battery, that would work out to:

(9-3)/0.020 = 300 ohms

Two LEDs in series would give:

(9-6)/0.02 = 150 ohms

It's conceivable that if you wired three LEDs in series you'd luck out and your battery voltage and your LED forward voltage would just happen to be such that the current would be 20 milliamps or less, and you wouldn't need a resistor--but as others have said, if it were me I'd wire the LEDs in parallel, each with its own resistor.

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Re: 3 mm LED lights Series or Parallel

Post by TurkeyVolumeGuessingMan » Mon Aug 21, 2017 9:02 pm

I was watching Jay Chladek's YouTube channel and he used 3V LEDs in his Moebius Cylon raider build. Since he powered it with a 3V battery pack (two AA batteries) and wired everything in parallel, it was all 3V so no resistors were necessary.

When I did my Revell Cylon Raider recently, I used one resistor per parallel cluster of LEDs. So that isn't good, huh? I was just BSing my way with resistors and just got it to the point where the lights weren't bright and no heat was emitted.
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Re: 3 mm LED lights Series or Parallel

Post by tetsujin » Fri Aug 25, 2017 5:38 pm

TurkeyVolumeGuessingMan wrote:
Mon Aug 21, 2017 9:02 pm
I was watching Jay Chladek's YouTube channel and he used 3V LEDs in his Moebius Cylon raider build. Since he powered it with a 3V battery pack (two AA batteries) and wired everything in parallel, it was all 3V so no resistors were necessary.
I did the math on this at one point - but the problem with this is that the battery voltage decreases as the battery discharges, and as the voltage across the LED decreases a little, the current through it decreases a lot. The LED winds up turning off far before the battery is truly depleted.

Frequently that's still OK (frequently people only light models for a few hours at a time anyway), but it is kind of wasteful. If you double the battery voltage, and use a series resistor, you waste a lot of that battery power, but you wind up getting more than double the battery life before the LEDs fail to light.
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Rocketeer
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Re: 3 mm LED lights Series or Parallel

Post by Rocketeer » Tue Aug 29, 2017 7:16 pm

Rocketeer wrote:
Mon Aug 21, 2017 10:30 am
...if it were me I'd wire the LEDs in parallel, each with its own resistor.
(cough cough cough) I'm a bit of a hypocrite, here. I just finished this model, which I'd had 95% completed for the longest time, and back when I rigged up the floodlights (couple years ago?), I made up three strings, each of three white LEDs in series, with a small resistor. Power is a 9V battery.

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Re: 3 mm LED lights Series or Parallel

Post by Ant » Wed Aug 30, 2017 2:41 am

There is absolutely no problem with using one method (series / parallel) over the other, *provided* the specification of the devices are respected. If your circuit is designed correctly, you will get a lifetime of use.

LEDs just randomly failing or "burning out" under correct operational parameters is so rare as to not be worth considering. You are probably more likely to drop the model first.

It's only when they are operated outside specification, usually insufficient current limitation, that you will start to see shortened lifespans and ultimately failure. Just because a circuit appears to work, doesn't mean it's operating in spec and will continue to work.

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Rocketeer
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Re: 3 mm LED lights Series or Parallel

Post by Rocketeer » Wed Aug 30, 2017 10:17 am

One thing I've resolved to do: Prior to sealing up the model, do a burn-in test of the lighting, for an hour or so, to see if any problems are going to arise. Far better to find a faulty LED or a bad connection when you can fix it, than have it emerge after everything's glued, puttied, and painted.

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