Hello,

I recently salvaged a transformer from an old Emerson radio clock. I figured it would be nice to create my own power supply since I'm just learning about electronics and I'm at the power supply section of my book.

I've read the chapter of my electronics book, I've searched online, and I've tried my best to match the transformer to similar pictures online. For the life of me, I can't figure out the transformer.

The source of my confusion is that the transformer I am dealing with includes an AC plug. On the side where the plug's input goes in, on the top I have Blue and White wires coming from the top. I'm assuming this is the "primary" side. On the other side, I have red, black, and yellow. I believe this to be the secondary side.

I'm pretty sure that the red, yellow, and black cords are the secondary side because while I was pulling parts from the old PCB, I noticed that red, yellow, and black went into the LED display.

I believe I can figure out how to wire this, if only I knew what to do with the white and blue wire. From all the other examples I've seen, there wasn't already an AC plug attached....and if there was, there were no other wires on that side of the transformer.

Another issue I'm having is determining the step-down voltage. In other words, I know this is supposed to step down 120V, but I'm not sure what. How can this be determined? Also, how can the number of amps be determined?

Just so you know, the only markings on the transformer is on the top marked "WH41 - 19E3CE". Also, the transformer was salvaged from an Emerson model AK2735 AM/FM radio clock.

Attached is a picture.

Thanks,

Rick

 

First, measure the resistances of the secondary windings to work out the connections.
It is probably the blue /white are one winding and the red/yellow/slate are the other. Try to figure out from the resistances what is the center tap.
It is a good idea to take a photo or draw a diagram of the connections before you remove the transformer.

But, a better idea is for you to get an old plug pack to use for the power supply as it will have ratings on it and you do not have to muck around with high volts.
That transformer from the radio will be quite low power.
What sort of specs are you aiming for as a power supply?

 

First, measure the resistances of the secondary windings to work out the connections.
It is probably the blue /white are one winding and the red/yellow/slate are the other. Try to figure out from the resistances what is the center tap.
It is a good idea to take a photo or draw a diagram of the connections before you remove the transformer.

I'll measure the resistances shortly and try to deduce which is the center tap. You're correct, I should have taken a photo of the board before I removed it. Unfortunately, I didn't. I'll remember to do so next time.

But, a better idea is for you to get an old plug pack to use for the power supply as it will have ratings on it and you do not have to muck around with high volts.
That transformer from the radio will be quite low power.
What sort of specs are you aiming for as a power supply?

I agree, wall warts are the way to go on the other hand I'm just trying to learn. I'd like to at least make one power supply from a transformer while I'm learning. I'll eventually transform an old PC power supply I have to make a nicer power supply for myself.

I'm betting that the transformer will be at least 120v to 5V or 3.3V which is all I need at the moment. I haven't encountered anything that I really need to go higher. I actually have wall warts if I need to go to 12V or above. Again, this is purely for me to learn about full and half rectifiers and power supplies.

Thanks,

Rick

 

It is probably the blue /white are one winding and the red/yellow/slate are the other. Try to figure out from the resistances what is the center tap.

I just measured the resistances and it appears as though the yellow wire is the center tap. The resistance between Red and Black is around 6.5, the resistance between Red and Yellow is around 4, and the resistance between Yellow and Black is around 4.

I measured the resistance between white and blue and it appears that you're correct that Blue and White are on a separate winding from Red, Yellow, and Black.

Now that I know this, where do I go from here?

 

Make sure the wires can't touch each other and plug it in. Measure and record the secondary voltages.
There is a widely quoted guesstimate of 20W per pound - so weigh it.
Your problem is that you don't know how that total power is distributed between the two windings.

 

Now that I know this, where do I go from here?

You make a power supply!

Yeah - plug it in, measure your secondary winding voltages to get an idea of the output voltage, full wave rectify them, install LDO's for a simple quite power supply. You'll probably need to heat sync the LDO's so find some aluminum.

 

Just hook the end of a diode which does NOT have the band on it to the white wire. With your meter on a high voltage range--say 200 volts, measure the voltage between the banded end of the diode and the blue wire. If you get 0 volts, reverse the leads on the meter. If you get a voltage reading you will know about the voltage that pair puts out. If it is really low, lower the voltage setting on the meter till you get a reasonable reading. Repeat with the other pair but connect the unbanded end of the diode to red wire and measure between the yellow wire and the banded end of the diode. Find the voltage. Now move the diode to the black wire and repeat--It should produce the same voltage if the yellow wire is the center tap.

Now you know about what the outputs of your transformer and can follow the directions in your book.