I'm a total beginner when it comes to electrical engineering and I'm getting slightly confused with circuits and LEDs in particular (or probably my whole 'understanding' of electronics but I hope it's just LEDs).

Side note: I'm not intending to play with mains voltage for now, I'm just trying to get the theory down.

So, my question is: Do LEDs care about getting (way) too much volt?

For example, take this circuit I just sketched out: circuit sketch

From my understanding, this circuit should feed 20 mA to each LED (240V / 12000 Ohm = 0,02A)

However I'm not using any kind of voltage dropper or anything, the LEDs are still getting a pretty high current ( 240 V / 6 components = 40V per LED, minus the x Volt they drop(?)).

So my question is: Do LEDs actually care if they have that high amounts of current passing through them, as long as the Ampere value is fairly low?

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    \$\begingroup\$ "However I'm not using any kind of voltage dropper or anything". In fact you are using one which is the 12k resistor \$\endgroup\$ – Elbehery Jan 27 '17 at 11:02
  • \$\begingroup\$ "Do LEDs actually care if they have that high amounts of current passing through them, as long as the Ampere value is fairly low? " - this sentence contradicts itself. If the ampere value is low the current is low. \$\endgroup\$ – pjc50 Jan 27 '17 at 12:24
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    \$\begingroup\$ Voltage is a potential over a component and can be measured in volts. Current runs through a component and can be measured in amperes. \$\endgroup\$ – Morten Jensen Jan 27 '17 at 13:02

To answer the question you asked: Yes, assuming you count turning into a puff of smoke as caring then they care about getting too much voltage (or technically too much current).

However you've made a couple of fundamental errors in your circuit analysis.

You have 240 V over 12k ohms plus 5 LEDs. As a first approximation LEDs can be considered a fixed voltage drop of their forward voltage. Assuming the LEDs forward voltage is 2 V that means 10 V over the LEDs which leaves 230 V over the 12k resistor. Using ohms law on the resistor gives a current of 19 mA.

So you have 2 V over each LED and 230 V over the resistor not an equal voltage over each device.

The other fundamental error you have is that 240 V AC is not 240 V peak, it's 240 V RMS. 240 V mains peaks at about 340 V (RMS * sqrt(2)). At 340 V you have a current of 27.5 mA (330/12k), probably not enough to overheat and blow up your LEDs assuming they are standard 20 mA max parts since the 20 mA is a sustained current limit rather than a peak current. However it is more than you were intending in your design.

  • \$\begingroup\$ Right, I should've been more careful with the numbers. Assuming the 340V peak, if I for example use some sort of 16- or 18kOhm resistor to stay below the 20mA, would it then work? \$\endgroup\$ – Bryde Jan 27 '17 at 11:18
  • \$\begingroup\$ Generally you don't want to run things at their limit, aim for around 10 mA but yes after a resistor change to account for the peak voltage and your actual LED voltage drops the basic idea will work. You should check things like the maximum reverse bias voltage on the LEDs but that shouldn't be an issue since it's limited by the LEDs going the other direction. You also need want to keep an eye on the maximum voltage and power ratings of the resistor, use peak for the voltage and RMS for the power. 20mA*230V = 4.6W of power in that resistor, far too much for a "normal" part. \$\endgroup\$ – Andrew Jan 27 '17 at 11:36
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    \$\begingroup\$ Awesome, thanks for taking the time to give such a detailed explanation. Like I said, I'm not planning to do anything with mains in the near future anyway as I'm obviously not nearly ready for it, I just try to get a general idea of how all the stuff works and what to look out for and such. Thanks again and have a great weekend! \$\endgroup\$ – Bryde Jan 27 '17 at 11:55
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    \$\begingroup\$ @Andrew 27.5 mA is probably not enough to kill a standard 20 mA LED within the normal sine cycle. Just picking a random low power LED (20mA) we find, that for an approximate duty cycle of 0.5 and overvoltage pulse length of 5 ms the maximum permissible peak current is ~40 mA. This strongly depends on the type but your assumption is way too conservative. osram-os.com/Graphics/XPic3/00203071_0.pdf/… \$\endgroup\$ – Ariser Jan 27 '17 at 12:07
  • \$\begingroup\$ @Ariser First you say that it's probably not going to blow it up and then you say that I'm being way too conservative. So when does it stop being way too conservative? 50% chance of going bang? ;-) But your point is taken, I'd not factored in that the 20 mA limit is for a constant current, for short periods you can indeed safely exceed it. \$\endgroup\$ – Andrew Jan 27 '17 at 12:20

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