Each winding is a phase unto itself, not the fabricated center tap. By measuring from the center tap, you confused yourself by measuring the lower winding backwards.

 

Each winding is a phase unto itself, not the fabricated center tap. By measuring from the center tap, you confused yourself by measuring the lower winding backwards.

I believe I understand...

Please confirm if the following is correct:

Attached is a diagram from a transformer manufacturer with the dots labeled on the primary and secondary.

The dot on the upper primary and upper secondary show that the secondary is in phase with the primary. The same for the lower primary and secondary. If one wants to connect the secondary coils in series, they would need to ensure that they are connected in phase relative to the primary. In the attached picture this would mean connecting 6-7 on the secondary. This would maintain the phase relationship between the primary and secondary, provide the maximum voltage at the extremes, and allow for a center tap to be created.

Is this correct?

Also, if you don't have any dots on your schematic, then do you do what you suggested earlier, connect in series, measure the extremes, and the configuration with the larger voltage means that it is connected correctly in phase? Is there any other way to find out?

 

The primary and secondary are isolated so there is no concern about phasing relationship between them, only with windings on either primary OR secondary.
6 & 7 and 2 & 3 retain the in phase connection of the respective windings.
The easiest way to find out is a simple voltage measurement on outer ends with one end of each winding connected.
Max.

 

Is there any other way to find out?

As far as I know, "read the label" (the dots) or "measure the voltage" are your choices. After all, you just tried to do it with an oscilloscope and got confused. I say lower your standards and just measure the voltage, but then, I'm a very pragmatic person. I only care if it works. What you call it is your business. "In phase", "out of phase"...meh. Just measure the voltage.

 

As far as I know, "read the label" (the dots) or "measure the voltage" are your choices. After all, you just tried to do it with an oscilloscope and got confused. I say lower your standards and just measure the voltage, but then, I'm a very pragmatic person. I only care if it works. What you call it is your business. "In phase", "out of phase"...meh. Just measure the voltage.

Thank you, going forward when confronted with a transformer without the dots indicated I will measure as indicated as this seems to be the easiest method practically.

 

The primary and secondary are isolated so there is no concern about phasing relationship between them, only with windings on either primary OR secondary.

I having trouble understanding this. I read the following:

Then the construction of a transformer can be such that the secondary voltage may be either "in-phase" or "out-of-phase" with respect to the primary voltage. In transformers which have a number of different secondary windings, each of which is electrically isolated from each other it is important to know the dot polarity of the secondary windings so that they can be connected together in series-aiding (secondary voltage is summed) or series-opposing (the secondary voltage is the difference) configurations.

http://www.electronics-tutorials.ws/transformer/transformer-construction.html

Then I read the following at this site:

Notice in the drawing that the phasing dots on the secondaries are at leads I and L. If the primaries of the two transformers are configured and the input voltage applied identically, then the voltage at the secondaries will be identical hence "in phase" at leads I and L.

http://www.radioworld.com/article/working-with-transformers-keep-an-eye-on-phase/1402

The above leads me to the notion that there is a concern about the phasing relationship between the primary and secondary. I would think when we talk about phase, such as the upper secondary coil, we would be talking about it relative to something else such as the phase of the upper primary. Therefore if we have two separate secondary coils, and want to connect them in series (secondary voltage is summed), wouldn't we want to have them connected so that the entire secondary is in phase relative to the primary?

 

The only time the primary phasing matters is when you have (2) transformers. Then, it is nice to have everything orderly. When you only have one transformer, the phase of a secondary winding only matters to which other winding you connect it to. There are situations when you attach a secondary to the primary, and then primary phase matters. Otherwise, ignore it.

 

The only time the primary phasing matters is when you have (2) transformers. Then, it is nice to have everything orderly. When you only have one transformer, the phase of a secondary winding only matters to which other winding you connect it to. There are situations when you attach a secondary to the primary, and then primary phase matters. Otherwise, ignore it.

I see, so even though the dots indicate "which end of each winding is which, relative to the other winding(s)" 1 [secondary winding end relative to the primary winding end], we don't need to worry about the primary when connecting the secondary's in series-aiding. We would therefore use the dots on the secondary to aid us when we connect the secondary's in a series-aiding configuration.

When looking at the attached diagram, if I am to understand correctly, if you wish to have the secondary connected in series-aiding (secondary voltage is summed) then one can either connect points 6-7[as shown] or 8-5, correct?

 

Yes. A dot and "not a dot". Like connecting 9V batteries in series, positive of one battery to negative of the other gets the higher voltage.

 

And if connecting identical windings in parallel, it is dot to dot.
Max.

 

Referring to the attached transformer diagram:

I was reading other material on-line which makes reference to this transformer where it indicates 100V for the WHITE-RED or 120V for the WHITE-BLACK contacts on the primary.

It appears the 100V was meant for usage in Japan and the 120V is for countries such as the United States or Canada.

My question is, if I apply 120V to the WHITE-RED on the primary will it cause any long term damage to the transformer? (I noticed it mentions +/-10% on input so I would take this to mean that the transformer was designed to handle up to 110V on the WHITE-RED input.)

 

Transformers on the whole are pretty rugged devices, but is this an academic question? as if you have 120v available, why not use the correct winding?
Using 120 on the 100v winding will also mean you would not be able to use the TFMR at the rated VA.
Max.

 

Transformers on the whole are pretty rugged devices, but is this an academic question? as if you have 120v available, why not use the correct winding?
Using 120 on the 100v winding will also mean you would not be able to use the TFMR at the rated VA.
Max.

This would be for practical reasons. The power supply that I am building will have +/-12V output. When the transformer is at 120V [Black-White] the output on the smaller secondary is 10.58V at no load. I think after full wave rectification, regulator drop out, and transformer ESR it will be too low to get 12V regulated [or I will be cutting it very close].

When the transformer is at 120V [Red-White] the output on the smaller secondary is 12.29V [instead of 10.58V] so after load regulation of the transformer, rectification, and regulator drop out I would not be cutting it too close.

I do not know what the rated VA is of this transformer. One post earlier indicated that I might be able to get 4 to 5 amps from it. (Perhaps this was referring to when it was run at 120V [Black-White]

Looking at the attached picture, when run at 120V [Red-White] how many amps do you think I would be able to get from the blue-blue coil, and the orange-orange coil?

 

If you have the room, you could always wind a few turns on?
You could reckon ~2turns/volt.
Placing 5 or 6 turns on and measuring will tell you exactly.
The rated VA will apply what ever you do.
Max.

 

One of the hardest things to do with a transformer is guess how much current can come out of each winding. You can't see what size wire was used for the winding and you can't know the magnetic limit of the core without the original information.

I once specified a transformer with one winding rated for 200 watts and a second winding rated for 2/10 of a watt. How ya' gonna know if you can't see inside it???

I guess that's what you're asking us. The answer is: "I don't know".

 

I do not know what the rated VA is of this transformer. One post earlier indicated that I might be able to get 4 to 5 amps from it. (Perhaps this was referring to when it was run at 120V [Black-White]

Looking at the attached picture, when run at 120V [Red-White] how many amps do you think I would be able to get from the blue-blue coil, and the orange-orange coil?

You can get a reasonable idea of the VA rating of the transformer from its weight. See post #36 in this thread:

http://forum.allaboutcircuits.com/showthread.php?t=38273

Determining the rating of the individual secondaries will take a little more work!

 

I decided to try using the white-red primary input @120V discussed earlier and see what the result would be on the blue-blue secondary under load.

With no load on the scope it shows 12.5V RMS.
After attaching a 6ohm 50W resistor, I get 11V RMS.

Using 4x 1N5401 diodes [Vf 1V@3A] setup as a full wave bridge rectifier and a 12,000uF 30V capacitor, the output with no load is 16V with 400mV-PK-PK ripple (note: on my Rigol DS1052E with digital filtering on, with the upper limit set to 300Hz and averaging on, at 5V/div, with a 9V battery connected it indicates a 200-400mV PK-PK ripple so I am assuming this ripple can be ignored and it is noise from the scope?).

With the above configuration, I connected a 6ohm load. PK-PK ripple was 1.8V-2.08V. VMax 12.1V and VMin 10V.

When reading the following article:

http://www.thebackshed.com/windmill/Docs/2009-09-06_181738_LINSW_REG.pdf

It mentions this formula to calculate capacitance needed for a given current and ripple:

Capacitance = ILoad/VRipple(pk-pk) x .006

I used the following numbers and the following is the result:
2A/1V * .006 = 12,000uF

Question1:

I used the above rated capacitor (it measured 14,000uF actually) and my PK-PK ripple was double (i.e. approx. 2V at approx 2A instead of 1V as calculated) why is this? Is this kind of ripple normal for this configuration?


Question 2:

To my understanding, a standard voltage regulator can drop between 2-3 volts. Therefore VMin which is the the bottom/valley of the ripple would need to be at minimum approximately 12V+2-3V for a 12V regulator to regulate, is this correct?

Assuming the above diodes, capacitor, and a standard regulator, the 10V VMin would be too low for such a regulator when 2A are drawn, correct?

Question 3:

When a 68Ohm load was used, VPk-Pk is 600mV with a VMax of 14.4V and VMin of 13.8V. Even with this load, using a standard regulator and the above diodes, the voltage would not be enough for a 12V standard regulator?

I am thinking I should purchase a 28VCT 3A transformer. This way I would have 14VRMS which would be enough for the diode drop, regulator drop, and any ripple. Any thoughts? If I go with a 24VCT transformer then I am guessing I would need more capacitance to smooth out the ripple at 2A, correct?

Thanks for all your help!

 

I have the formula somewhere, the desired % ripple for cap selection depends on what % ripple you are want for a given load current.
If you aim for to low a %, using a large capacitance, it will have an effect on your transformer VA requirement.
The higher the voltage for a given % ripple, the capacitance is lower.
Have you considered the overwind option?
All you need to do is obtain magnet or enamel wire, it is a little more tedious with E I transformers, this is one reason that Toroidal types are very popular, they can be modified very easily by either adding or subtracting turns.
Max.

 

I have the formula somewhere, the desired % ripple for cap selection depends on what % ripple you are want for a given load current.
If you aim for to low a %, using a large capacitance, it will have an effect on your transformer VA requirement.
The higher the voltage for a given % ripple, the capacitance is lower.
Have you considered the overwind option?
All you need to do is obtain magnet or enamel wire, it is a little more tedious with E I transformers, this is one reason that Toroidal types are very popular, they can be modified very easily by either adding or subtracting turns.
Max.

The transformer I have is the E I type. As I am not familiar with the process of modifying the winding I decided not to go that route at this time.

Regarding my first question I found that the alligator leads that I was using in my test setup appears to have been the issue. When I used 14ga wire to the 12,000uF capacitor the peak-peak ripple was 800mV with a 6ohm load (instead of the previous 2Vpk-pk ripple with the alligator leads). Using two 12,000 capacitors in parallel resulted in 400mV peak to peak ripple with a 6ohm load. (My scope normally indicates 200mV peak-peak when idle so the above ripple values are likely smaller).

I am uncertain of the following: When there is no load on the full wave bridge rectifier Vmax is 16.2V on the output and with a 12,000uF capacitor at no load Vmax is 15.6V. A 600mV difference.

Then when I place a 6ohm load after rectification (no capacitor) Vmax across the load is 13.6V. With a 12,000uF cap and a 6ohm load Vmax is 11.2V. A 2.4V difference.

Why is there a 0.6V difference in the first instance and a 2.4V difference is the second case? Does adding a capacitor after rectification reduce the maximum voltage in some way? Why is the difference larger in the second case?

 

You are looking at a couple of things causing a volt drop, the natural impedance of the winding and the recharge pulse of the capacitor when a load is placed on the output.
Looking at your measured size of the transformer and the estimated core size, I think a drop of the kind you are seeing is possibly inevitable.
Like I pointed out in the previous post, the larger the cap for a given load, the energy of the recharge pulse (current) is larger, hence you will see a drop on a transformer used around its maximum Va.
BTW, if you have done all this so far, adding a couple of windings is as simple as feeding the wire around the existing winding, if there is room.
Probably 1 layer would do it.
Max.