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LED and Resistor Help

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NGBEnt

New Member
Hi guys,

I am looking to use a switch with an LED and a resistor. Is my wiring and placement of the resistor correct?
 

Torben

Well-Known Member
Hi guys,

I am looking to use a switch with an LED and a resistor. Is my wiring and placement of the resistor correct?
Hi there, and welcome to the forum.

Way more information is needed. What kind of power cell are you using? What is the value of the resistor? What is the forward voltage and current draw of the LED?


Regards,

Torben

[Edit: The wiring is OK. To know whether the circuit would actually work, though, we'd need to know the answers to the questions I asked above.]
 
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audioguru

Well-Known Member
Most Helpful Member
The LED will not light.
The lithium coin cell is 3.0V which is too low to light the blue LED that needs at least 3.3V plus some voltage for the resistor.
the battery cell will light a 2.0V red LED with a current-limiting resistor.

The resistor can be anywhere as long as it is in series.
 

Sceadwian

Banned
audioguru, I have at least a half dozen button lights around the house that say you're wrong =) They all use 3 volt lithiums. No current limit resistor is required because the internal resistance of coin cells is so high it self limits. Anything larger than a 2032 may require a current limit resistor. I've had one on my keychain that I use frequently.
 

audioguru

Well-Known Member
Most Helpful Member
Hi Sceadwian,
My son gave me a keychain light with a white LED, three 1.5V alkaline button cells and no current-limiting resistor. It burned out after lighting for no more than 1 hour.
I have 6 white LEDs and at least 10 blue LEDs and they all light with more than 3.0V.

My newest white LED measures 3.22V at 16mA.

You are correct, it lights with a 3.0V supply. But it is dim at only 1.4mA. I can barely see it in the dark when its supply is 2.7V.
 

Sceadwian

Banned
Keep in mind the 3.0 volts listed on teh cell is nominal, they're much higher than that fresh, and stay that way through a large portion of their discharge.
 

Hero999

Banned
Hi Sceadwian,
My son gave me a keychain light with a white LED, three 1.5V alkaline button cells and no current-limiting resistor. It burned out after lighting for no more than 1 hour.
I doubt they are designed for continuous operation. They are probably only designed to work for long enough for you to find your keys.
 
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