I have a broken Philips alarm clock FM/AM radio broken, so I stripped some parts out of it.

One of it seems like an AC converter, but I cannot find any reference/datasheet about it. Do you have any information about this? (or what the meaning of the yellow/green wires are, I assume the black is GND and Red is VCC).

The dimensions of the yellow 'block' are 35.3mm x 29.7mm x 11.6mm.

The text is:


So all wires going out of this transformer and I would like to know the meaning of the yellow and green wires:

  • Blue wire (on the top back), going into the black cable you can see on the right page, it's going to the AC mains power plug (220V).
  • Brown wire (on the top back), going into the black cable you can see on the right page, it's going to the AC mains power plug (220V).
  • Green wire: What is the meaning of this wire?

  • Yellow wire: What is the meaning of this wire?

  • Red wire: I assume this is output? (AC/DC?)
  • Black wire: I assume this is ground

enter image description here

  • 12
    \$\begingroup\$ Identify it? Yep, it looks like a transformer. \$\endgroup\$
    – Voltage Spike
    Jul 15, 2019 at 20:33
  • \$\begingroup\$ @laptop2d I mean, is there any datasheet, or if not, what is the meaning of the yellow/green wire? \$\endgroup\$ Jul 15, 2019 at 20:34
  • 7
    \$\begingroup\$ AC to DC transformers don't exist, only AC to AC. \$\endgroup\$
    – Huisman
    Jul 15, 2019 at 20:50
  • \$\begingroup\$ @Huisman Ok ... than it's probably an AC to AC transformer (since the input is an (220V) AC. \$\endgroup\$ Jul 15, 2019 at 20:53
  • 4
    \$\begingroup\$ You probably won't find a datasheet as the transformer itself quite likely is not a consumer part. \$\endgroup\$
    – Huisman
    Jul 15, 2019 at 21:00

3 Answers 3


It's an AC/AC transformer, the rectifier was probably on the red/black lines. Usually Yellow\Green lines are for AC mains (according to international standards, but it also depends on the age of the device).

Your best bet is to hook up a signal generator, and see what the step down ratio is if you want to use it.

The other thing that will be of use is any markings on the outside of the clock that specify the current and voltage (you need the current) because that will give you a good idea of the saturation point of the transformer and how much power can be run through it.


I didn't see the blue black line in the picture which is not clearly indicated: The primary is most likely blue brown and there are two secondaries, in this case red/black are one secondary and the other secondary is yellow/green

enter image description here
Source: https://airlinktransformers.com/post/chassis-mounting-toroidal-transformers-technical-notes

  • 1
    \$\begingroup\$ Thanks ... There are two other wires going to AC mains (so there are 4 wires (green/yellow/white/red) PLUS the main cable (on the right of the picture). To be honest, I never used a transformer, and I don't know what a step down ration is, or the saturation point ... but I will check it (I also could get a clue on the plastic box of the clock probably). \$\endgroup\$ Jul 15, 2019 at 21:07
  • \$\begingroup\$ Btw ... I do not have a signal generator, but it seems there are too many variables/properties anyway that I can make use of it probably. \$\endgroup\$ Jul 15, 2019 at 21:14
  • 1
    \$\begingroup\$ Or, if your careful, you could use AC mains itself, the transformer should be isolated also, and you could use a DMM or scope to see what the voltage is. If measuring from primary to secondary, you should get close to an open circuit with a DMM \$\endgroup\$
    – Voltage Spike
    Jul 15, 2019 at 21:40
  • 1
    \$\begingroup\$ The primary is going to be yellow/green. The secondary is red/black. Every transformer has a primary and a secondary, they do not pass (or should not) pass DC \$\endgroup\$
    – Voltage Spike
    Jul 15, 2019 at 22:05
  • 1
    \$\begingroup\$ @SamGibson indeed, the mains cable come from two wires from the top/back and are blue/brown. The mains plug is connected on the other side of the cable. I will update my question. \$\endgroup\$ Jul 15, 2019 at 23:52

It's a transformer. It outputs lower-voltage AC on the secondary wires.

As you surmised, the red/yellow/green/black are secondary windings and the blue/brown are primary going to the mains voltage. You have that figured out so far. Now the next step is to figure out how the secondary windings are connected and what their outputs are to see if they're useful (they very probably are.)

The 'safe' way: Use an ohmmeter to determine which coils connect to which, and how they relate to each other. Determine this by seeing the relative series resistance between each wire. You may find that there are two separate coils, or that they're a single coil with multiple taps. More about this in a bit.

The 'fun' way: test it. Connect AC up to the primary and use the voltmeter to measure the secondary AC voltages, wire-to-wire, to figure this out. Do this with care obviously, though the secondaries are low enough voltage that you're not at significant risk for shock.

"More about this". If this unit came from a VF (vacuum fluorescent) type clock, my guess is that it has two separate windings: one for 3VAC for VF tube filament, and the other 7~9VAC for logic power which would have fed to a full-wave bridge on the board (maybe you can check this if you still have the board.)

If I had to wager further, the logic power pair would be the black and red pair.

  • \$\begingroup\$ Thanks for this answer, and the details how to test it ... I will probably have time for it in the weekend. \$\endgroup\$ Jul 16, 2019 at 8:43
  • \$\begingroup\$ How can one know this is a step down transformer just by looking at it? \$\endgroup\$
    – lucasgcb
    Jul 16, 2019 at 10:34
  • \$\begingroup\$ It came from a clock radio. That information alone says that it’s step-down. \$\endgroup\$ Jul 16, 2019 at 16:18

You probably won't find a datasheet as the transformer itself quite likely is not a consumer part but a Philips internal design, tailered to this specific product (alarm clock FM/AM radio) and to mass production.

I wouldn't bother investigating it, but just buy a transformer that fits your need, made by a reliable manufacturer with a well defined datasheet. They're not that expensive...

  • \$\begingroup\$ Thanks ... I was just 'hoping' it would be some very generic item that I always could use when I would need some AC transformation. \$\endgroup\$ Jul 15, 2019 at 21:08
  • \$\begingroup\$ That transformer would be fine for most any small project with the addition of a full-wave bridge, regulator and a couple of filter caps. As a teenager I harvested parts from a clock radio and made just such a thing to power my proto board. I used it for years. \$\endgroup\$ Jul 17, 2019 at 5:45

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