Super Bright White 5mm LED (25 pack) (ADA754)

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Need some really bright LEDs? Adafruit are big fans of these clear white LEDs, in fact Adafruit use them exclusively in their kits. They are very bright and have about 25 degree … read more

I’m looking for bright Led’s for a head light on an electric bike. The pc board carries 7 Led’s and a 2K Ohm resistor. The power source is 48 volts.

Hopefully you can help me here.

Robert Baker

Cheers

Hi Robert,

I don’t really have enough details here to be able to help you out. That 48V would need to be stepped down to about 5-6v to be usable by these LEDs. Do you have a headlight already that steps down the voltage and you just need new LEDs to put in it?

Thanks for getting back to me Stephen, in the attached photos you might see how this pc board fits into a headlamp assembly. I have measured the voltage to the resistor at 53v and nothing after the first led. Individual continuity tests on each of the seven
Led’s reveal open circuits. The resistor measures 1992 ohms and that’s about right according to the color code. Can you supply a voltage reducer to run your 5v Led’s and still provide good lighting. If so, how much?

Cheers Robert Baker

From the product description:
Typical forward voltage 3V at 20mA

So the 7 diodes will use 3 X 7 = 21V

Leaving the resistor to burn the rest. At 20mA a 2K resistor will burn 40V.

So you don’t need to reduce the voltage. The circuit doesn’t need a voltage reduction, just functional LEDs.

Notice that the resistor is wasting about 2/3 of the power.

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If it is supplied from the voltage source That resistor will limit your current to 14mA as the LEDs are all in series.
Vsupply = Vleds + Vr.
Vled = 7*3
Vr = Vsupply - Vleds
CurrentLED = CurrentR = Vr/R = 27/2k = 0.0135mA
Changing the value of that 2K resistor would change the brightness of your LEDs though going too low will “let the smoke out”.Sorry if I went too maths. but Robin is right the 40V is fine as long as the current is limited.

do you sell stripboard for soldering circuits for beginners or am I completely out of date and is something else used now?

Hi Robert

You seem to have measured the LEDs on the resistance (Ω) function assuming you are using a digital multimeter. Unless all of them are actually open circuit.
This function will not output enough voltage on the leads to turn on a diode or other junction, certainly not a LED.
Use the “diode check” function. This will display open circuit in the reverse direction and display the actual forward voltage drop of the junction in the forward direction. In the case of a LED this could be anywhere between about 2V - 3V depending on colour. The LED may even light dimly. The diode check function will have a little picture of a diode symbol.

All of the diodes are in series so it would only take one to be open circuit to turn the lot off. You will have to check them individually as the diode check will not have enough voltage to turn all of them on. There would be no need to remove them for this. just remove power.

Looking at the photo I would check some of those soldered joints. A bad joint here would turn the whole thing off.
Cheers Bob

Sorry. Did not notice how old the original post was. By now the LED board in question has been fixed or in the junk heap.
Cheers Bob