How to change 5v into 3v using only resistors? Answered

I need to change 5 volts into 3v, using only resistors, to power a LED, I can't use a voltage regulator, because the LED is to see if the current voltage regulator is working, I am happy to reply to questions that would help you answer it.
Thanks

Tags:LED voltage regulator resistor electronic power

Comments

The forums are retiring in 2021 and are now closed for new topics and comments.

paulrockwell11141119

11 months ago

can i take 4 digit clack face and hook it up to be voltage read out for power supply

paulrockwell11141119

11 months ago

i need to take 5vdc 700mA to hook to my drill it is 3.6vdc 180min, rpm

RohitKumar86

11 months ago

220 k resistor

Vijimol

1 year ago

I want to use this tht lcd screen
Can anyone tell me how to?
It has a dock connector
It is from a sony experia.

Vijimol

Reply 1 year ago

If I can't then help me getting at least the back
Light

AndyGadget

9 years ago

Unless you're doing something strange, there's no need to convert to 3V for the LED - Just do the limiting resistor calculation for 5V.
Can you give more details of the actual circuit you're building.

alexw2150

Answer 9 years ago

I'm doing this (http://windisch.co.uk/robots/howto/voltage_regulator.htm) for a battlebot. The battery I'm going to use is a 7.4v lipo I've used a L7805 to get it to 5v but I just want to add a LED so that I can quickly check to see that I has power.

AndyGadget

Answer 9 years ago

Ah, I see - You've fallen into the old trap. You have a led which says something like Vf = 3V.
What it actually means is that you are going to lose 3V across the LED. Vf is the forward voltage drop. The correct way to do this is to drive the LED with a higher voltage than that and use a series resistor to set the current to a sensible value. Go to the site I've linked and use the first scenario. (I would actually set the current to 15mA which will be very nearly as bright as the 20mA LEDs are usually rated for but will save you 5mA, so slightly longer battery life.) For this you'd use a 150R resistor.

alexw2150

Answer 9 years ago

Okay, thanks i ended up with a 150ohm resistor.

AndyGadget

Answer 9 years ago

That's the one.
The actual calculation is the supply voltage minus the LED Vf divided by the desired LED current, which is (5 - 3)/0.018 = 111.111 ohms.
Round this up to the next preferred resistor value and you have 150R (although 110R would do fine).