running LED christmas lights with a power supply instead of batteries

[tetoncraftworks]

i have an old motorola ac power supply and i want to convert my battery powered led christmas lights to an outlet.

The power supply says

Input 120v
Output 4.0 vdc 350mA

Will this work? The batteries box uses three 1.5 double AA's.

Thanks

 

[brianmoooore]

3 x 1.5V = 4.5V and your PSU is 4.0V. Chances are they will work, but use a couple of bits of wire to try it out, before you cut anything.
Don't forget to get positive and negative the right way around.
Depending on the type of PSU you have, an electrolytic capacitor across the output might help.

 

[tetoncraftworks]

3 x 1.5V = 4.5V and your PSU is 4.0V. Chances are they will work, but use a couple of bits of wire to try it out, before you cut anything.
Don't forget to get positive and negative the right way around.
Depending on the type of PSU you have, an electrolytic capacitor across the output might help.

how do i know which wire is positive and which is negative? there are no colored wires on the PS... see pic.

 

[brianmoooore]

Positive is usually the centre pin, but no guarantees. Best to check with a multimeter.

 

[tetoncraftworks]

Positive is usually the centre pin, but no guarantees. Best to check with a multimeter.

i have a multimeter that i inherited from my son, but wouldn't know how to use it... i think i should cut the cord, and then test... otherwise once i cut the cord i still don't know which wire went to the center of the plug

 

[brianmoooore]

Put up a pic. of the front of it.

 

[tetoncraftworks]

Put up a pic. of the front of it.

here it is

 

[brianmoooore]

It's autoranging, so that's one thing that's done for you.
Leads are in the correct sockets.
Turn the knob two clicks to 'V with a solid line above a dotted one', and if the screen comes on, you're good to go.
Turn the PSU on and connect the red probe to the centre of the output plug, and the black probe to the outer metal bit. The screen should read the output voltage of the PSU.
If there is no minus sign in front of the reading, then that confirms that the centre pin is positive, and the outer bit negative.
If there is a minus sign, then the centre pin is negative, and the outer positive.
Connect the black probe to the centre pin and the red to the outer sleeve, and confirm that you DO get a minus sign, if you didn't before.

 

[tetoncraftworks]

It's autoranging, so that's one thing that's done for you.
Leads are in the correct sockets.
Turn the knob two clicks to 'V with a solid line above a dotted one', and if the screen comes on, you're good to go.
Turn the PSU on and connect the red probe to the centre of the output plug, and the black probe to the outer metal bit. The screen should read the output voltage of the PSU.
If there is no minus sign in front of the reading, then that confirms that the centre pin is positive, and the outer bit negative.
If there is a minus sign, then the centre pin is negative, and the outer positive.
Connect the black probe to the centre pin and the red to the outer sleeve, and confirm that you DO get a minus sign, if you didn't before.

OK... following your instructions, i get a minus sign... however, i have to cut the cable in order to know which wire is which... yes?

Black probe to center, no minus sign. Red probe to center, minus sign appears.

 

[brianmoooore]

Sounds like it is negative centre pin and positive outside.
Yes, you will have to cut the cable and identify the polarity the same way for the finished article, but I suggested you try with a couple of pieces of wire from the plug first to see if it works.

 

[tetoncraftworks]

Sounds like it is negative centre pin and positive outside.
Yes, you will have to cut the cable and identify the polarity the same way for the finished article, but I suggested you try with a couple of pieces of wire from the plug first to see if it works.

aha... now i understand what you are saying about the pieces of wire... good idea, however, the PS is going in the trash anyway if it doesn't. It's gotta be 15 years old. Trying now.

 

[tetoncraftworks]

cannot get any connection... going to try cutting the wire

 

[tetoncraftworks]

ok... cut the wire, how do i know which wire is positive?

 

[brianmoooore]

With the meter, same as before. What was the voltage reading?

 

[tetoncraftworks]

With the meter, same as before. What was the voltage reading?

black on left wire, red on right wire - p
black on right wire, red on left wire - n

 

[brianmoooore]

When you have positive reading, the red probe is connected to the positive wire.

 

[tetoncraftworks]

When you have positive reading, the red probe is connected to the positive wire.

Yay Brian!!!! Works! See pic of me holding the P on the left and N on the right and tree lights glowing beautiful... Thanks so much!

 

[tetoncraftworks]

PS... never like to mess with electricity... know just enough to kill myself accidentally. Thanks again.

 

[brianmoooore]

That's my bit done for anglo-US relations for today

 

[tetoncraftworks]

That's my bit done for anglo-US relations for today

lol... here is my professional soldering job... and thanks for showing me about the multimeter... was my son's, but he died a few years ago in a car crash... he was 22, and I always thought one day I would use it for something and touch his memory in a new little way using it.