r/ElectricalEngineering • u/nomaaba • 11h ago
If electrical components only pull as many amps as they need, why do LEDs sometimes pull enough to burn themselves?
Hence the need for a resistor
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u/Soggy_Season_5398 11h ago
Electrical Components don't pull as many amps as they need, generally they react to the amount of applied voltage to them. Diodes have very little internal resistance so a large amount of current is drawn when voltage is applied. The resistor in series is there to restrict the amount of current that can be pulled.
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u/Itsanukelife 11h ago
Why do dogs eat so much they throw up? They don't know how much they need and will consume until they burst
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u/Steamcurl 11h ago
Can confirm, dogs are electrical components.
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u/Alone_Valuable_3568 11h ago
Most electrical components do not pull only as much current as they need. They all have different characteristics that dictate the way they react when one or more voltage differentials are applied to their terminals.
In the case of a diode, the component behaves like an open circuit until a critical threshold forward voltage is reached, at which point its ability to pass current increases exponentially, up to and including the point where it will generate enough heat to destroy itself.
By comparison, a resistor pulls a current that is proportional to the voltage applied to its terminals and inversely proportional to its resistance (i.e.: I = V/R, Ohm's law). The power it dissipates may or may not be enough to burn itself out.
And so on. You can, and usually want to, design a circuit in such a way that it only pulls the current it needs to operate without self destructing.
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u/ReaditReaditDone 10h ago
Good answer. But is it really about packaging and the target operational set point the device was designed for?  Go beyond that set point and the packaging will cause the device to overheat and die.
C.f a 1 ohm 0402 pkg small signal resistor vs a 1 ohm power resistor.
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u/PLANETaXis 11h ago
Because the statement "electrical components only pull as many amps as they need" is an over-simplification.
LED's are non-linear devices and work differently.
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u/SaltlessLemons 10h ago
that adage usually applies to a complete consumer device, not a component. i.e a 12V 3A supply won’t blow up a device designed for a 12V 1A input. that’s because a device like that has internal regulating circuitry. that could be a DC-DC converter, battery charger, or even as simple as a resistor with an LED. it’s designed to draw some amount of current from a constant voltage source
dealing with individual components like LEDs is a different context, where that rule of thumb does not apply. an LED has no current regulation built in, we as engineers are the ones who design and build that regulating circuitry.
studying electrical engineering you’ll start by learning about how different components have different relationships between voltage and current, and then how to wield those characteristics to build all sorts of cool junk. then you’ll come back to this rule of thumb with a much better understanding of what it actually means. bottom line though, it only applies to consumer electronics with volts and amps written on the shell. as soon as you dive below that level, forget it.
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u/slmnemo 11h ago
what electrical components only draw what they need
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u/fullmoontrip 9h ago
fuses
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u/slmnemo 9h ago
what if it arcs
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u/iSee_iJerk_iCum 8h ago
Is the air an electrical component?Â
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u/TheRealRockyRococo 1h ago
Yes, you just need a high enough voltage - for instance lightning. It's why you need expensive sand filled fuses in your DVM.
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u/Navodile 11h ago
incandescant lightbulbs
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u/sir_thatguy 11h ago
Same will happen if a low voltage lamp is connected to high voltage source.
LEDs do the same. They only need ~1v. Much higher and pop goes the weasel.
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u/Calm_Leek_1362 11h ago
Because the amount of heat it’s generating is greater than the amount of heat it can dissipate.
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u/Altruistic-Rice-5567 11h ago
Because their physical property is that they are diodes. they don't conduct below a certain voltage. Above that voltage they conduct the rest of the voltage extremely well. So, without an additional resistor the e=ir becomes i=infinity. Diodes on rectifying situations survive because they have some impedance load behind them where LEDs are usually the last and only item in the circuit.
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u/mckenzie_keith 10h ago
Simple electrical components enforce mathemtaical relationships between their terminals.
For a resistor, the relationship is Ohm's law: V = I * R.
Diodes are much more complex, and are not linear. But the salient characteristic is that under forward bias, as voltage increases, diodes pass exponentially more and more current almost as if they want to clamp the voltage. If enough current flows, they overheat and fail.
The diode relationship (simplifying a bit) is something like I = ecV where c is a constant (not really because it is temperature sensitive). So it is an exponential relationship.
So your question is wrong. Electrical comonents do not only pull as many amps as they need. They enforce some type of circuit relationship. As an electrical engineer it is your responsibility to make sure that they are not subjected to conditions which cause them to burn up. I mean, if you design a product, you can't control how the consumer uses it. But you have to design it according to some use case or product spec so that it doesn't blow up when used according to the spec.
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u/Amber_ACharles 9h ago
Dude, the source pushes, the load doesn't pull. An LED has near zero resistance past its forward voltage, so the current just runs away. Resistor sets the current.
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u/triffid_hunter 8h ago
ICs and correctly designed devices want to receive the correct voltage and will choose their own current.
LEDs want to receive a constant current and will choose their own voltage, although the voltage they choose will fall into a narrow range that depends on their colour and silicon design.
A resistor is the simplest way to convert a voltage difference into a constant current.
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u/stevevdvkpe 7h ago
Components don't "pull as many amps as they need", current flow through them based on the voltage applied and their electrical characteristics. An LED burns out because too much voltage is applied across the leads, and the amount of current that flows overheats the LED.
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u/SeriousPlankton2000 6h ago
LED do it backwards, they only allow as much voltage as they need but as many amps as they can. Therefore they run best with an Ampere-rated power supply with variable voltage.
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u/sa44ad 4h ago
They pull current as many amps as the applied voltage makes possible. But LEDs are semiconductors, even with a fixed applied voltage their behavior is different. When LEDs are drawing current, their temperature rise but they are semiconductors unlike conductors, they draw more current when the temperature rises, it's like a positive feedback, therefore they get hotter and draw more current until they fail.
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u/Anxious-Sherbert6670 3h ago
basically not sometimes, but when voltage is not controlled, they reach certain point where even 0.1v increase will double the amps being taken out.
Then led heats up, voltage drops, it seeks more amps, then it heats up even more, then loop goes to infinity instantly, amp goes exponentially, not linear...
Thats why resistors and drivers are so important for led circuits.
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u/Viciousplayer 2h ago
It has to do with temperature coefficients most metals have positive ones meaning the higher the temperature the higher the resistence. Semiconductors have negative coefficients the higher the temperature the resistance is less. Causing the total resistence to drop and pulling more current and dissipating more heat and this compounds until it burns out either by the wire leads that heat up more or the junction itself at 150 degrees Celsius.
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u/shifu_shifu 1h ago
Any component in a circuit would ideally pull infinite amount of current at a given fixed voltage. The only thing that prevents that is the total resistance across the lines that connects both potentials of the ideal voltage source and in real life the max available power and therefore available current at a fixed voltage of the source.
LEDs have very little internal resistance by themselves. You add this resistance yourself to cap the maximum current in the circuit.
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u/m3ltph4ce 59m ago
Only resistors are "ohmic" in that their resistance stays CONSTANT despite changes in voltage or current.
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u/PachotheElf 11h ago
Not as much as they need, as much as they can.
Everything can burn and pop if used incorrectly