What Value Resistor?
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This any good? https://www.geeksforgeeks.org/ohms-law/
What sort of LED is this?
Discrete LEDs are current dependent devices, usually running at between 10 and 20mA.
If this is marketed as a '5V' LED, it may already have something like a 250 to 500 Ohm resistor (at least) mounted in series to limit current.
To run the same LED at 7.5V would require a resistor of between 375 to 700 Ohms.
Adding a couple of hundred Ohms extra should be fine, but equally the LED would probably run with few issues on 7.5V, without any extra resistance.
Although there will be no harm adding more; the LED won't be driven as hard, will last longer and won't be overly bright.
Edit:
As Eric says below, I forgot to take into account the voltage drop over the LED.
But the values still don't change dramatically.
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Each colour has a diffrent voltage red around 1.2 volt so 7.5-1.2=6.3/current rating say 30 mA = 210 ohm. Replace with real values to get real size.
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..and some LED's rated for a higher voltage, include a built in voltage regulator, so not a straight forward question to answer.
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Assuming you have the equipment, connect your LED to a 5v supply so it illuminates. Set your multimeter to read mA then connect it in series to measure the current that it is designed to operate at.
Then divide the voltage that you need to drop by the current which will give you the required resistance.
Eg, your meter gives a reading of 15mA (0.015A), you need to drop 2.5v, 2.5/0.015=166.6 ohms. Closest value will be 150 ohms or 180 ohms, best to go for the higher value.
Then divide the voltage that you need to drop by the current which will give you the required resistance.
Eg, your meter gives a reading of 15mA (0.015A), you need to drop 2.5v, 2.5/0.015=166.6 ohms. Closest value will be 150 ohms or 180 ohms, best to go for the higher value.
Assuming your supply is DC not AC.
If you have an AC supply it would be best to consider one of these options:-
A/ run a second similar LED and resistor as an inverse to the first one.
B/ place a diode in reverse across the LED (say 1N4001 to 4 for example).
An LED run not to exceed its max current (including running it well below its max current) and protecting against reverse voltage can be expected to live long.
Indeed, 30 to 40 years ago I ran some burglar alarm bellboxes with tell tale LED on the front cover and they still going strong today.
Back then most bellboxes were the truncated pyramid type an a Self Activation unit was a separate circuit board, usually the maker had included a Red LED on the board but off course when the bellbox lid was fitted then no one would see it.
So we might desolder the LED from the board and solder in some long leads.
Alternatively put an extra LED (long leads) on the lid and run it from the 12V supply (actually 13.6 to 13.8v ideally).
A 1k resistor would be added in series and that LED would shine brightly in the dark to advertise the presence of a powered bellbox at all times.
They last "forever".
If you have an AC supply it would be best to consider one of these options:-
A/ run a second similar LED and resistor as an inverse to the first one.
B/ place a diode in reverse across the LED (say 1N4001 to 4 for example).
An LED run not to exceed its max current (including running it well below its max current) and protecting against reverse voltage can be expected to live long.
Indeed, 30 to 40 years ago I ran some burglar alarm bellboxes with tell tale LED on the front cover and they still going strong today.
Back then most bellboxes were the truncated pyramid type an a Self Activation unit was a separate circuit board, usually the maker had included a Red LED on the board but off course when the bellbox lid was fitted then no one would see it.
So we might desolder the LED from the board and solder in some long leads.
Alternatively put an extra LED (long leads) on the lid and run it from the 12V supply (actually 13.6 to 13.8v ideally).
A 1k resistor would be added in series and that LED would shine brightly in the dark to advertise the presence of a powered bellbox at all times.
They last "forever".
I've not found it to be the problem others have warned about.
I fitted some dummy bell boxes to my house but added red LEDs to make them look like the proper thing. I used the mains transformer from a scrap cassette recorder - so about 6V or so and in the bell boxes a simple series resistor and 3mm red led superglued in a hole. I happened to drive past last week while visiting a friend in hospital and see there is still life, I sold and moved in 1993. Judging by the appearence of the house I suspect they would not have bothered with a repair.
I use these:
The 24V versions that I've opened (I often repurpose to use on lower voltage or add additional components etc) consist of a pair of resisters as a potential divider and mostly work on 12-30V without modification, they will happily work on AC or DC but polarity sensitive
As a potential divider it would have that effect, however from memmory I think it was something in the order of 1.5kΩ series and 5KΩ shunt so I don't know to what extent it would protect the LED.
Not wanting to damage one I have just measured the current on 24V DC as 36mA/2mA which doesn't quite tally with those figures but not hugely different.
A diode would be simpler, or two LED's back to back plus a single series resistor.
Yes Harry with two back to back for AC one series resistor to them is ok for both and indeed if you using one LED and a reverse diode then again you one resistor would be ok too.
The only time you`d need one resistor per LED is for multiple parallel LEDs .
I know at the time, those years ago, I calculated the cost of that single 24/7/365 LED to be in the region of a farthing a lifetime or some such low amount.
Even at todays fuel prices it would not break the bank. Although things move on and nowadays two or more twinkling LEDs viwed through the strobe lens cover seems to be the order of the day.
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