# What value of resistor gives how much voltage, formula?

1. Nov 18, 2009

### idrees.pk

In my home I am getting 220 AC Volts of electricity. I want to connect white color Light Emitting Diod (LED) that requires 3 volts output with the resistor which will be connected to 220 Volts power. So can any body please tell me what is the formula to find that how much voltage is output from a resistor when 220 volts of AC voltage is applied. I want to use this so that I'll be able to have only this much light in the room where I sleep. I'll be very thankful for anyone who'll answer.

Also if anyone knows some relevant useful links then do please let me know. I searched and found a very interesting windows application in which if you enter color codes' information of the resistor, it'll tell you what the resistance value of this resistor is. This is the link of the website http://www.doctronics.co.uk/download.htm [Broken] you can download this application from here.

But I want an application such that if I tell it to tell me resistor's value of a resistor which gives output of 3 volts i.e I'll input 3 volts in the edit box of the application and then on pressing the calculate button it'll tell me the resistor's value and all the color codes as well. I don't know whether anyone has developed this application, if not then it should be as it is something very useful.

Once again anyone's help will be very much appreciated.

Last edited by a moderator: May 4, 2017
2. Nov 18, 2009

### Integral

Staff Emeritus
A diode requires DC, so there is no resistor that will do what you are asking. You need some kind of power supply which will convert the AC to DC. You may be able to find a wall wart that does what you need.

3. Nov 18, 2009

### mgb_phys

Welcome to PF
First a warning, 220V AC can easily KILL you
Second warning - the resistor will be dumping a lot of power (5-10W) so will get hot, this could start a fire

in theory you can run an LED from AC it will simply turn on and off 50times/second but it's not really something you want to do.

4. Nov 18, 2009

### idrees.pk

But I've seen this i.e in a device I saw that a red color light emitting LED was connected to a resistor and it was working fine. Then why can't a white color emitting LED do that same. I have a resistor and I tried this experiment but the LED was giving very small amount of light as compared to when I connected it with wall wart's 3 volts outtput.

5. Nov 18, 2009

### idrees.pk

6. Nov 18, 2009

### mgb_phys

You can connect an LED to an AC supply it will simply only turn on for half the cycle.
At 50Hz it will turn on and off 50times per second which probably isn't noticable it may effect the life of the LED and coul cause interference so you wouldn't do this in a commercial product.

To calculate the current limiting resistor you need to know the forward current in the diode, then you can use any of the online resistor calculators (http://led.linear1.org/1led.wiz) or Ohm's law.

Remember you also need to calculate the power in the resistor, if you can't do this you probably shoudln't be playing with lethal voltages

7. Nov 18, 2009

### chroot

Staff Emeritus
Your posts make it clear that you are not experienced enough with electricity to be attempting this project. 220 VAC is deadly stuff, and you should not be anywhere near it. It can take months to learn the techniques necessary to safely work with high voltages. It is against our policy to provide advice which may lead to harm. I suggest that you find another way to light your room. Thread closed.

- Warren