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Yellow 2V Led problem

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Lesterarrester

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I recently boughta bag of 50,000mcd yellow leds for lighting an outdoor eating area. I picked yellow because bugs don't get attracted to yellow lights. I have already been stung by some kind of wasp that fell on me.
There 2V so I hooked 6 of them up to a 12V battery. Too dull to eat under.
The 4 white 12,000mcd 3V lights did the job but attracted heaps of bugs.
Did I do something wrong wiring up the LEDS. I didn't use a resistor.
 

tcmtech

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I would guess that the actual running volts of the LED's are not exactly 2 volts. LED's are rather fussy about how much current you GIVE them though! Strong emphasis onthe give!
They dont take what they need like a regular incandesent bulb. You may be best off running the LED's 5 in a series and then limiting the current with a pair of resistors.
Given an assumed voltage drop of 2 volts while maintaining a running current of 20 - 30 Ma you would need a resistance of around 67 - 100 ohms on each set.
Most of the ultra bright LED's are rated to take around 30 ma. But if they are not you would just have to use a larger value resistor to drop the curent to there maximum rated level.
 
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audioguru

Well-Known Member
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LEDs do not have exact voltages. Your "2V" lEDs might be 2.0V or 2.4V or anything in between. Yours are probably 2.4V so six in series need 14.4V but you fed them only 12.6V.

You must use a current-limiting resistor. Connect four in series and in series with a 160 ohm resistor. If the LEDs are 2.0V then the current will be 29mA, if they are 2.2V the current will be 24mA and if they are 2.4V the current will be 19mA.
 

Sceadwian

Banned
The main problem with using a static voltage with an LED is the voltage changes with current, with temperature, and it also changes very slowly as the PN junction ages. It doesn't take much of a change in the junction to get a massive change in the current at a fixed voltage.
 

Willbe

New Member
LEDs are current operated, not voltage operated.
 

Sceadwian

Banned
Willbe, that post was pointless, you can't have current without a voltage.
 
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Willbe

New Member
Willbe, that post was pointless, you can't have current without a voltage.
The LED current is the parameter controlled by the manufacturer; the LED voltage you get kind of falls out, but is probably not controlled or screened for.
The current controls the brightness, as was demonstrated by the OP.

Not to start a philosophical discussion, but a current source
Current source - Wikipedia, the free encyclopedia
feeding a short circuit gives you current without voltage, across the short.

Relay coils also depend on amps x turns, so they are current controlled. The voltage across them falls out depending on the wire resistance. Since most sources are voltage sources, though, we spec' the coil voltage.
 
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Sceadwian

Banned
There's still a very slight voltage across a dead short from a current source because any real world devices will have some resistance (any real current source at least) ideal current sources do not exist. The very specific example where this doesn't apply is in super conductors but that's something I know very little about. If you want to respond to this post please do so in a private message as I don't want to muddy up the posters thread.
 

Mr RB

Well-Known Member
Willbe, that post was pointless, you can't have current without a voltage.
Huh?? :confused:

Leds are current operated devices. They achieve the correct illumination based on the current, see any led datasheet. The led voltage varies from device to device, varies with heat, and varies greatly from colour to colour (if you are using R G B leds specced to operate from the same current).
 

Sceadwian

Banned
So how do you expect to get the required current without a voltage?
 

Hayato

Member
Huh?? :confused:

Leds are current operated devices. They achieve the correct illumination based on the current, see any led datasheet. The led voltage varies from device to device, varies with heat, and varies greatly from colour to colour (if you are using R G B leds specced to operate from the same current).
Indeed.

But there is no Current without a Difference of Potential.

How are you supposed to get 10mA without difference of potential?
 

colin55

Well-Known Member
A LED is neither a voltage nor current controlled device. (If you want to be absolutely specific, they are limited to a mW dissipation).
A yellow LED has a “characteristic voltage” that appear across it, according to the current flowing. For less than 1mA, the characteristic voltage will be as low as 1.7v and for 20mA it will be 2.2v to 2.4v No current will flow when the supply is 1.6v (so this is where some consider it to be a voltage controlled device).
The “characteristic voltage” will change very little from about 15mA to 35mA for an individual LED but may be 200mV different for another LED in the same batch. There can also be differences between manufacturers, size of LED, brightness and types of LED such as high efficiency (low current) etc.
The only way to approach the problem of LEDs in series is to place 4 LEDs in series and add a 470R resistor. Measure the current. Reduce the resistance until the current-flow is 25mA.
Most white LEDs are 3.4v to 3.6v so I don’t know how the poster got 4 white LEDs to operate in series on 12v.
 
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Sceadwian

Banned
A white LED will light if dimly on 3 volts. Likely his 12 volt supply at that current was actually higher. The voltage rating of a power supply at it's rated load current is given, NOT it's voltage at lower loads unless it's a regulated supply, even those have some load variability to their voltage.
 
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MrAl

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Most Helpful Member
Hello there,

Seasoned engineers and electronic workers know that when something is described
as "current operated" or "current controlled" that means that the MAIN parameter
is current, not voltage. They also know that this doesnt usually mean that there
will be no voltage present across the device, just that the main parameter to
pay the most attention to is the current rather than the voltage.

In other words, you can work well with a 'current controlled' device by setting
the current at a particular level and paying little or no attention to voltage,
rather than setting the voltage to some preset value and forgetting about the
current level.
In the case of an LED, you do better to set the current to 20ma and not worry too
much about the voltage (voltage is of secondary importance) than to try to set
the voltage. Of course voltage has to be considered to some degree, because
of course you have to have enough voltage to feed the LED too, but it will
be less important. For example, we may set the current to 20ma and if the
voltage is 2.0v or 2.1v or 2.2v or even 2.4v we dont really care that much, but
we would never be able to set the voltage to 2.2v (say) and forget about it
because we wouldnt be sure the current is right, and the current being right
is the most important aspect of running the LED: too little and it is too dim,
too much and it burns up.
If this was a bulb however, we would be more concerned about what the
voltage is because that affects the operation the most.

So in the end it's mostly a matter of priority: current or voltage, depending
on the device. We call a device who's operation depends mostly on current
a 'current controlled' device, and a device who's operation depends mostly
on voltage a 'voltage operated' device. There are variants too, like before
'current operated' or even 'current mode', to name a couple.
 

Mr RB

Well-Known Member
So how do you expect to get the required current without a voltage?

At what point in my post did I say there was NO voltage?? :eek:

Anyway MrAl answered it nicely so I can just shut up now.
 
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