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Transformer secondary winding

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solis365

New Member
I've got a small mains transformer with a single primary winding for 120Vac. It was taken from a cheap 20W stereo system.

The secondary side has 4 wires: 2 brown, 2 yellow, all the same gauge.

I hooked the transformer to the wall and there are 22V brown1 to brown2, and 22V yellow1 to yellow2. This seems like a dual-secondary winding configuration.

However, when I measure between brown1-yellow1, brown2-yellow2, brown1-yellow2, and brown2-yellow1, there are certain other voltages (dont quite remember, they were odd... 15ish and 6ish I think). This makes me think that it is a single secondary winding with 4 taps.

The way it appeared to be used was yellow1/2 for one power circuit regulated to 12V and brown1/2 for a separate power circuit regulated to 5V.

not sure which is more likely. I would like to use both in a similar manner (one low-power side for indicators and such, and the high-power side for amplification)
 

BaCaRdi

Member
Sorry if I missed it, did you have a question?
I am a bit confused. You mention power, we talking higher voltage?

(P)= (E) x (I)
Power: "Voltage in Amps" x "Current" will give you Watts.

Are you just wanting two voltages? Just tap on what you need. That doesn't mean that transformer will be able to power it. Since I don't know the Load or the Output of your transformer.

Hope this helps.
-Marc



I've got a small mains transformer with a single primary winding for 120Vac. It was taken from a cheap 20W stereo system.

The secondary side has 4 wires: 2 brown, 2 yellow, all the same gauge.

I hooked the transformer to the wall and there are 22V brown1 to brown2, and 22V yellow1 to yellow2. This seems like a dual-secondary winding configuration.

However, when I measure between brown1-yellow1, brown2-yellow2, brown1-yellow2, and brown2-yellow1, there are certain other voltages (dont quite remember, they were odd... 15ish and 6ish I think). This makes me think that it is a single secondary winding with 4 taps.

The way it appeared to be used was yellow1/2 for one power circuit regulated to 12V and brown1/2 for a separate power circuit regulated to 5V.

not sure which is more likely. I would like to use both in a similar manner (one low-power side for indicators and such, and the high-power side for amplification)
 

crutschow

Well-Known Member
Most Helpful Member
Here's a diode connection for a center-tapped secondary transformer that gives you ½V and V outputs. Although it may not be apparent, both outputs are full-wave rectified. The center tap output uses the two bridge rectifiers to ground as the return for the ½V full-wave rectification.

NOTE: The center-tap must be at the 1/2 voltage point for this configuration to work properly.
Dual Voltage Rectifier Ckt.gif
 

Hero999

Banned
You don't need a diode at the ½V tap.
 

MikeMl

Well-Known Member
Most Helpful Member
Put a 1K resistor in parallel with your Voltmeter leads, and I'll bet that the weird voltages between brown and yellow will disappear. I'll guess that you are using a voltmeter with a 10meg input impedance, and there is some capacitive coupling between two DC-isolated windings.
 

crutschow

Well-Known Member
Most Helpful Member
You don't need a diode at the ½V tap.
You are correct. Good observation. The two bridge rectifiers act like a full-wave rectifier to the center-tap but the output is taken from the center tap, rather than from the diodes, as is normally done.
 

BaCaRdi

Member
Nice spot Hero :)
 
Last edited:

crutschow

Well-Known Member
Most Helpful Member
You don't need a diode at the ½V tap.
Although with small loads the diode is not needed, further simulations showed that the diode minimizes interaction between the two output loads. At higher loads without the diode, some of the ripple from the bridge output appears at the 1/2 tap output.
 

Hero999

Banned
That will probably improve if you make C1 equal to C2 and connect them in series and connect the centre tap where they join, as you would with a bipolar power supply.
 

crutschow

Well-Known Member
Most Helpful Member
That will probably improve if you make C1 equal to C2 and connect them in series and connect the centre tap where they join, as you would with a bipolar power supply.
Where does the centre tap go other than to circuit common, which is where the capacitors are already connected?
 

kpatz

New Member
With the transformer disconnected from power, measure the resistance between the brown and yellow leads. If you get an open circuit, then you have separate windings. If you get current flow, then it's taps on a single winding.
 

Hero999

Banned
I mean like this.
 

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crutschow

Well-Known Member
Most Helpful Member
I mean like this.
That would give ripple and noise at the ½V output equal to one-half of that at the V output, so it's not clear how much that configuration helps. It also halves the filter capacitance for the V output for a given capacitor size. But it will likely be acceptable for many applications.

For applications where the ½V output is used to power sensitive, low signal circuitry it may be preferable to leave the diode on the center-tap output (and also have a larger output capacitor if desired) to minimize ripple and noise on that output.
 

solis365

New Member
sorry, I lost track of this thread for a while.

Put a 1K resistor in parallel with your Voltmeter leads, and I'll bet that the weird voltages between brown and yellow will disappear. I'll guess that you are using a voltmeter with a 10meg input impedance, and there is some capacitive coupling between two DC-isolated windings.
yes its a cheap one. mastech I believe, but it has all the features. I could try with a fluke meter I have at work which will measure resistances accurately to 60Mohms. Might this fix the problem?
In addition to a multimeter I tried looking with an oscilloscope, still weird coupling. I will try the resistor trick.

kpatz said:
With the transformer disconnected from power, measure the resistance between the brown and yellow leads. If you get an open circuit, then you have separate windings. If you get current flow, then it's taps on a single winding.
They read as open circuits, so they should be separate windings. This is why I was confused when I got voltages between two windings. Perhaps it is as MikeMI said, capacitive coupling.

I am testing with 60Hz.
 

crutschow

Well-Known Member
Most Helpful Member
They read as open circuits, so they should be separate windings. This is why I was confused when I got voltages between two windings. Perhaps it is as MikeMI said, capacitive coupling.
Type loading the transformer with a 1k resistor when you do the measurement (in parallel with the meter input). That should minimize stray capacitive coupling.
 

bountyhunter

Well-Known Member
I've got a small mains transformer with a single primary winding for 120Vac. It was taken from a cheap 20W stereo system.

The secondary side has 4 wires: 2 brown, 2 yellow, all the same gauge.

I hooked the transformer to the wall and there are 22V brown1 to brown2, and 22V yellow1 to yellow2. This seems like a dual-secondary winding configuration.

However, when I measure between brown1-yellow1, brown2-yellow2, brown1-yellow2, and brown2-yellow1, there are certain other voltages (dont quite remember, they were odd... 15ish and 6ish I think). This makes me think that it is a single secondary winding with 4 taps.

The way it appeared to be used was yellow1/2 for one power circuit regulated to 12V and brown1/2 for a separate power circuit regulated to 5V.

not sure which is more likely. I would like to use both in a similar manner (one low-power side for indicators and such, and the high-power side for amplification)
A power amp would use a pos and neg rail design, so a center tapped secondary makes sense.
 
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