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Is a resistor necessary in an l.e.d. light?

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Is a resistor necessary in an l.e.d. light?

Old 12-23-17, 12:52 PM
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Is a resistor necessary in an l.e.d. light?

I have been told a resistor is necessary but not told why.
The lighting circuits I have built without resistors seemed to work well.
As in most builds they get built better if we know what goes where and why.
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Old 12-23-17, 04:00 PM
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Depends on how well the LEDs match the power source. And what circuitry might be included in the LEDs.
Basic LEDs are mainly concerned with the current running through them, and - depending on voltage of the power source - may need a resistor to limit the current running through them.
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Old 12-23-17, 04:24 PM
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For any voltage beyond its forward drop, a diode can be thought of as having basically zero resistance. If you attach a simple power supply to an LED, having the voltage even slightly too low means it won't turn on, and having the voltage even slightly too high means "infinite" current and the LED explodes. The typical solution is, you use a power supply with more voltage than the LED needs to turn on, and then use a current-limiting mechanism of some kind to control the power/brightness/whatever. A resistor is a pretty simple and reliable option for the current limiter, although there are other ways to do it.

If you connected an LED to a power supply with no intermediate current-limiting mechanism, and it worked, then either the LED package contained some control mechanism, or the power supply was limiting its own output.
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Old 12-23-17, 05:23 PM
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Of course you realise now that I know a L.E.D. can explode................
Just joking there but my instructors told the class "If you are not breaking stuff then you are not learning as much as you could." My lab' partner could blow capacitors by looking at them. There are no electronics stores anywhere near me. Too bad,I used to do things
with electricity and simple switches that stumped the instructors.
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Old 12-23-17, 07:05 PM
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Well, pretty much. Specs for a LED should give a max current and a voltage drop. Subtract the voltage drop from the voltage of the power source. Then find a resistor that allows less than the max current with that remaining voltage, and that should work. Example DC 6v supply, simple series circuit. LED says 1.8v drop, 30ma max. So 6v - 1.8v is 4.2v. 4.2v / 0.030a = 140ohm resistor. Practical use, derate the current to say 80%, 0.8 * 30ma = 24ma, 4.2v / 0.024a = 175 ohm resistor, but in the parts bin probably find a 180ohm, call it good. If you don't have specs for an LED, use a resistor that you're sure is enough, hook it up and turn it on, then measure the voltage drop with a meter. Maybe start with a resistor that gives 5ma, and try that. Keep going up till your satisfied or you burn it up.

And everything HTupolev said above.

Last edited by grizzly59; 12-23-17 at 07:09 PM.
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Old 12-23-17, 09:33 PM
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Grizzly59? Great educated advice and an even better quote on your signature.
The one thing I do have is quite a few bright white l.e.d.s. A lady I knew used
to make those landscape photos with l.e.d.s. in strategic spots like
lamposts and bridge towers.
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Old 12-24-17, 08:14 AM
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Originally Posted by PdalPowr
There are no electronics stores anywhere near me.
Doesn't matter, eBay delivers to your home.

And for the thread topic:
Every LED needs a constant voltage and has a specified current range.
So if you know another solution to provide the LED with the needed energy, no need for resistors.
Think about using a suited power source
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Old 12-24-17, 01:35 PM
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Exploding LEDs? I've fried a few and am still around to tell. I bought a couple strings of fairy lights on eBay and requested 12V strings so I could use a battery pack I already own or a 12V DC converter scavenged from old electronics. Instead they sent strings designed to work on 5V and they fried instantly when I flipped the switch. These strings have a resistor built into the wiring. The only difference between a 12V string and a 5V string is a higher value resistor in the 12V circuit. If you are into DIY LED projects, this site allows you to determine the correct resistor size and watts: LED series parallel array wizard

Different battery types deliver different voltages. Carbon and alkaline batteries yield 1.5V, rechargeable NIMH 1.2V, and Li-ion batteries 3.7V. The resistor matches the voltage required by the LED to the battery or battery array. That's why some LED flashlights using a single alkaline battery (1.5V) will not work with rechargeable NiMH batteries. Not enough voltage to drive the LED. A string of lights designed to work on 12V DC will work on 3 Li-ion batteries in series (11.1V) but will be a bit less bright.
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Old 12-26-17, 12:50 AM
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Originally Posted by PdalPowr
I have been told a resistor is necessary but not told why.
The lighting circuits I have built without resistors seemed to work well.
As in most builds they get built better if we know what goes where and why.
Where required, in a well designed circuit, a resistor in series limits current should an LED catastrophically fail in the shorted mode.
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Old 12-26-17, 02:16 PM
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Originally Posted by angerdan
Every LED needs a constant voltage and has a specified current range.
So if you know another solution to provide the LED with the needed energy, no need for resistors.
LEDs just need a voltage in their active range, most LEDs are not driven by a constant voltage.

Generally, I would say that using a resistor as a current limiter is not the way I would do it if energy is an issue. There are plenty of current regulating chips out there that will do the job and not waste energy. Of course, in many cases using a resistor is fine. I think using a resistor in a high power LED is a bad idea.
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Old 12-26-17, 02:47 PM
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Depends on the LED etc. For what it's worth, I use a 12 Volt vehicle trailer LED for a tail light, powered directly by a 9 volt radio battery (close enough) and it's worked fine for a year, maybe two.
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Old 12-26-17, 06:29 PM
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Originally Posted by wphamilton
Depends on the LED etc. For what it's worth, I use a 12 Volt vehicle trailer LED for a tail light, powered directly by a 9 volt radio battery (close enough) and it's worked fine for a year, maybe two.
I love reutilisation and improvisation.
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Old 12-27-17, 04:42 PM
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Originally Posted by PdalPowr
I have been told a resistor is necessary but not told why.
The lighting circuits I have built without resistors seemed to work well.
As in most builds they get built better if we know what goes where and why.
as hinted by the other answers, the circuit needs to limit the amount of current that can flow through the LED.
Sometimes this is done with a resistor.
Other times, a regulator circuit is used.
Still other times, the designer used a battery that can't provide much current, or whose voltage is barely sufficient to get the LED to conduct.

The literal answer to your question is "no, a resistor is not always required", but the general idea of needing to limit the current is quite true.


Steve in Peoria
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