Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

120VAC/6VDC adaptor

Status
Not open for further replies.

doggai

New Member
Let me start by saying I get confused screwing in light bulbs. I am in the final stages of a project and I am stymied.

I want to use a LED out of a novelty flashlight that runs off two 3V 2016 watch type batteries. I want to use a 120VAC/6VDC adaptor to eliminate the (replacement) batteries. I am assuming that the LED is a 6VDC. (2 X 3VDC =6VDC) The output of the 6VDC adaptor is actually 9.3VDC. (Likewise, a 4.5VDC adaptor output is 7.3VDC.)

I am soldering wires from the 120VAC/6VDC adaptor directly to the legs of the LED. I have burned out 3 LED so far and have stopped.

How can I make the LED work? I was informed locally to install a 120 ohm resistor to one of the LED legs, did so and there is no change in the adaptor output. Is the 6VDC 400ma adaptor too hot and is that burning out the LED?
 
You may have to regulate the output of the AC/DC adaptor by adding a LM7806 fixed positive 6V voltage regulator. Then you will have exactly 6 volts output to use as a replacement for the batteries. Another problem might be that the two x 3V batteries (cells) could be in parallel for more capacity @ 3V. This you'll have to verify before connecting 6V to the circuit. Also verify whether the LEDs are connected in series or not.
 
Tiny watch batteries have internal resistance that limits the current. When the LED loads them then the voltage drops down to maybe only 5V at 30mA. Maybe the light has an additional current-limiting resistor to limit the current to 25mA, then the voltage across the LED will be lower. More than 30mA will burn out most LEDs.

Your adapters don't have much internal resistance so their current is way to high for your LED. POOF!
You need to find out the exact voltage of your LED then calculate the value of a current-limiting resistor to connect in series with it, using a measured voltage of the LED and of the adapter when it has a load.
 
If you do not want to do the math. Get a 10K pot and open it up to 10K and solder it in and turn it down, when you see what you want on the LED. Stop and check the ohms on the pot?

Not the right way. But when you get the ohms, then you can do the math. :)
 
doggai said:
How can I make the LED work? I was informed locally to install a 120 ohm resistor to one of the LED legs, did so and there is no change in the adaptor output. Is the 6VDC 400ma adaptor too hot and is that burning out the LED?
Go with the 120:eek:hm: resistor.

The idea isn't to cange the output from the adapto bt to restrict the current though the LED to a safe level.

The two watch batteries have a high impedance; the 120:eek:hm: resistor is effectively buit in, so you didn't need an to add an extra one, your adptor has a very lod impedance so without the resistor a huge current flows burning out the LED.
 
You said you tried a 120 ohm resistor in series with the LED. Did it work OK? Then measure the voltage across the LED so you know what it is, and measure the voltage across the resistor so you can calculate the current to see if it is low enough.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top