Resistor Question

[zarnold16]

Im making a LED christmas tree that will be powered off an 5V power supply with 1Amp. The LEDs FV is 3.2V and use 24mA. I was gonna use 25 leds for the tree and was wondering what resistor to use. Or do i need one resistor for each led if so then i will have to redo the design.

Thanks in advanced,
zarnold

[IanB]

You need one resistor per LED. Each resistor should be about 100 ohms.

(By the way, twenty five 100 ohm resistors won't cost very much if you buy them in bulk.)

[VEGETA]

yes, one resistor per LED. Or, you can use a current source instead of a voltage source. I saw this from one of Dave's videos about modifying a magnifier scope or something.

[nixfu]

There resistors are needed to limit the current to EACH LED.  So, yes you do need them on each one.

[c4757p]

Slightly more in depth: if you share a resistor among multiple LEDs, like this:



It will work in a perfect, simulated world, but not this one. When the LEDs glow, they warm up. As they warm up, they drop in impedance, taking more current for a given voltage. They all have the same voltage applied to them, because they're directly in parallel, so any LED which has lower equivalent impedance (due to variations in manufacturing or temperature or whatever) will draw more current. This one will heat up more, dropping in impedance even more, and this unlucky LED will soon end up consuming most of the power while the others consume hardly any.

He pops, and the next one follows. Give it a short while and they're all dead.

In a lower-power scenario, he might not pop, but he'll glow much more brightly than the others and the whole thing will look like crap.

If each one has a resistor, then if it starts to draw more current, its resistor will drop the voltage, providing a negative feedback effect which limits the current to a reasonable level:



The voltage drop across a resistor is V which is equal to I, the current through it, times R, its resistance. V=IR, the definition of resistance. So, given a total 5V, and a 3.2V forward voltage, the resistor should have the remainder (5V - 3.2V = 1.8V) across it, with 24mA through it, so 1.8V/24mA = 75 ohms.

However - any LED drawing 24mA at 3.2V will be bright as fuck. Are you sure you want to run 24mA through them? Is this the desired current, or just the devices' maximum?

[mikerj]

yes, one resistor per LED. Or, you can use a current source instead of a voltage source. I saw this from one of Dave's videos about modifying a magnifier scope or something.

Even with a single current source you will get uneven current sharing between the LEDs unless resistors are added.

[Zero999]

yes, one resistor per LED. Or, you can use a current source instead of a voltage source. I saw this from one of Dave's videos about modifying a magnifier scope or something.

Even with a single current source you will get uneven current sharing between the LEDs unless resistors are added.

Unless they're connected in series.

[dannyf]

Or do i need one resistor for each led if so then i will have to redo the design.

It is a lot easier to use one resistor per led. A more complex approach is to use an led driver and put those leds in serial.

[VEGETA]

yes, one resistor per LED. Or, you can use a current source instead of a voltage source. I saw this from one of Dave's videos about modifying a magnifier scope or something.

Even with a single current source you will get uneven current sharing between the LEDs unless resistors are added.

can you explain why? current is what's important for LEDs, and with a precise current source... what can go wrong?

thanks

[hans maree]

I would advice against driving 25 leds in series, as that would require around 80 volts, not the safest thing to have in a Christmas tree

You could do several strings of leds in parallel but that would require more wiring so i would go for the one resistor per led option.

[c4757p]

yes, one resistor per LED. Or, you can use a current source instead of a voltage source. I saw this from one of Dave's videos about modifying a magnifier scope or something.

Even with a single current source you will get uneven current sharing between the LEDs unless resistors are added.

We're discussing things connected in parallel and driven by a voltage source, and when things switch from voltage to current they usually switch between parallel and series as well, so....

[Richard Crowley]

can you explain why? current is what's important for LEDs, and with a precise current source... what can go wrong?

Jelly-bean mass-production LEDs are NOT exact. They each have slightly different characteristics that get "swamped out" with individual current-limiting resistors.
But if you expect to run several of them at exactly the same voltage, you will reveal the small variations in the individual diodes.
That is why it is prefereable to operate from a higher voltage, so you can connect several LEDs in series if you want to save on limiting resistors.

Note that there are many LED resistor calculators available online.
For example, here is one where you can enter the number of LEDS, their operating parameters, and the supply voltage...
http://led.linear1.org/led.wiz

If you were operating your circuit from an 18V wall wart (for example, you would have a very simple 5x5 array of LEDs.
Each string of 5 leds with a 100 ohm limiting resistor.

[Totalsolutions]

 Running a constant voltage source 5v 5w about £5 cost no resistor and all fine. Was a set of outdoor lights.

[c4757p]

Lucky you.

[IanB]

I would advice against driving 25 leds in series, as that would require around 80 volts, not the safest thing to have in a Christmas tree

Except the standard solution for light strings is to put many bulbs in series and plug them directly into the mains. For example in the UK you might have a string of twenty 12 V bulbs wired in series to match the 240 V mains voltage. The lamp holders are carefully insulated so you can't touch any metal parts and there is a low current fuse in the loop to prevent fires in the event of a fault.

By comparison, an isolated, current limited 80 V supply for a string of LEDs would be rather safer. Especially with the current limited to 10 mA or so.

[hans maree]

By comparison, an isolated, current limited 80 V supply for a string of LEDs would be rather safer. Especially with the current limited to 10 mA or so.

Good point. But assuming the OP is a beginner I would still advise sticking to low voltage (out of an of the shelve wall wart). Otherwise one mistake could easily result in a dangerous situation. Besides, how many houses burned down due to faulty Christmas tree lights?
But if you know what you are doing, there is indeed nothing wrong with putting then all in parallel.

[mikerj]

yes, one resistor per LED. Or, you can use a current source instead of a voltage source. I saw this from one of Dave's videos about modifying a magnifier scope or something.

Even with a single current source you will get uneven current sharing between the LEDs unless resistors are added.

We're discussing things connected in parallel and driven by a voltage source, and when things switch from voltage to current they usually switch between parallel and series as well, so....

What a bizarre argument 
Things do not suddenly switch themselves from parallel to series connected because you are using a current source instead of a voltage source.  You can run a bunch of parallel connected LEDs perfectly well from a current source, provided current sharing resistors are used, just as you can drive a series connected string of LEDs from a voltage source with a single current limiting resistor.

[Richard Crowley]

...Besides, how many houses burned down due to faulty Christmas tree lights?

In 2007-2011, U.S. fire departments responded to an estimated annual average of 230 home structure fires that began with Christmas trees. Home Christmas tree fires caused an average of six civilian deaths, 22 civilian injuries, and $18.3 million in direct property damage per year.
http://www.nfpa.org/research/reports-and-statistics/fire-causes/holiday/christmas-tree-and-holiday-lights

During the five-year-period of 2007-2011, NFPA estimates that decorations were the item first ignited in an estimated average of 920 reported home structure fires per year. These fires caused an estimated average of six civilian deaths, 47 civilian injuries and $12.9 million in direct property damage per year.
http://www.nfpa.org/research/reports-and-statistics/fire-causes/holiday/decorations