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How to decrease power, voltage, and amps coming from a wall socket in your house? Answered

I am making a table top and would like to put LEDs in it. I want to be able to connect them to a variable resistor so it acts like a dimmer switch. I want to be able to run this off of wall power. I do not know how to drop the power, voltage, and amps coming from the wall to the ratings of a variable resistor?

I know P=VI and V=IR, but I can't find a resistor with a high enough power rating to decrease wall power. what I would need help with is find a componet or something that greatly decreases the Power; thus decreasing voltage and amprage; to a level that I can run through a variable resistor and to the LEDs.
Yes I know LEDs are supposed to run off of DC, but they do work with AC; they just dont get as bright, which is fine by me.



8 years ago

For such high voltages you probally want to get a Wall wart (it looks like a cellphone chardger). Just go down to your local eletronics store aand look for what they have. You can get all diffrent types and voltages and AC/DC converters. They are not very expensave and you don't run the risk of blowing somthing up.

ac-to-dc-adaptor-834.jpgwall wart.jpg
Quercus austrina

8 years ago

What you are looking for is a driver for the LED's that is controlled by Pulse Width Modulation, PWM for short. There are a number of 'ibles to look at if you search for "555 timer PWM LED". Find one where you can vary the duty cycle. This will allow you to dim the LED's. You will also need to make sure the driver includes a power transistor as you are looking at least 400 mA, a TIP31 would be a good choice.

If you want to look outside Instructables, type "555 timer calculator" or "555 timer tutorial" into your favorite search engine. There are loads out there to choose from.



8 years ago

How many LEDs are you trying to run? What LEDs are they?

If you tell us that we can find you a suitable power supply. As Steve said... you need a transformer to drop the voltage and a rectifier to turn it from AC to DC. If you tell us what you're trying to run it's likely we can find you some circuitry to do this SAFELY for <£10, a standard mains powered power supply.


Answer 8 years ago

I am trying to run about 20 LEDs. I was thinkin the small 5mm white ones, with a forward votage about 3.3V and 20mA current

Thanks for the help guys


8 years ago

This site will help you sort out the resistor values. BUT please do some research and use the information with CAUTION.

DONT run these direct from the mains - ever.

This is a solution from the wizard for a 50 volt power source, allowing for 200 LEDs 3 volts forward voltage each and 10 mA current through each LED.


Answer 8 years ago

Probably - it works better for single LEDS hence my caution.

Jack A Lopez

8 years ago

Two topics: 

The first:  AC adapters:

I am sure you have seen these little box, or brick-shaped, devices.  What they do is convert the higher voltage AC power found at your wall outlets (so called mains power) into lower voltage DC or AC.  Usually it's DC.   The way to tell which it is, and approximately what voltage, is to read the markings on the back of the adapter itself, and/or test it with your multimeter.

Used AC adapters are cheap and easy to find, on the used market, e.g. dumpsters, thrift stores.  The dumpsters are cheaper, but the thrift stores have better selection.  You might even have some used AC adapters already in your home, provided the things they used to power are unwanted or broken.

The second trick:

is understanding how LEDs work.  The best summary I can give you is that LEDs should be driven by constant current rather than constant voltage.  Moreover the brightness of an LED is proportional to the current flowing through it.  Since you want to make your LEDs dim-able, current is the parameter you want to control.

I was looking for a good 'ible on how to build a nice switchmode contstant current regulator for driving LEDs, but there don't seem to be any good 'ibles on this topic... If anyone notices one, be sure to correct me.

However, I assure you, controllable constant current is what you, and your LEDs, want.


8 years ago

You need to use a transformer. The LEDs are at mains potential and lethal otherwise.