### Author Topic: Toroidal transformer, Help?  (Read 1452 times)

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#### mictas

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##### Toroidal transformer, Help?
« on: July 30, 2017, 08:59:00 am »
Okey dokey, so I am looking at possibly reusing one of two toroidal transformers, I recovered during an ewaste phase, I wish I could continue, but what are you going to do, anyway.

So please see the transformer, below.

I think I am missing something here, see after doing some math, that thing is able to out put 21A of force

So how did I get to that, see this toroidal has three voltage outputs on its secondary widening, while its primary winding has two. Primary winding has two 110v, while the secondary has x2 24v and a 10v widening.

So testing with two multimeters, I got 1.8 ? and 1.1 ? on both the 24v widenings, while the 10v was giving me a nice 0.7 ? over all, am I wrong or?

Anywho, please take a look at those other photos of you want a bit more of a look to help me out.

#### Zero999

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##### Re: Toroidal transformer, Help?
« Reply #1 on: July 30, 2017, 09:53:23 am »
The DC resistance of the secondary windings only partly contributes to the expected short circuit current. The DC resistance of the primary and to a lesser extent the leakage inductance will also contribute to the short circuit current.

Note, that I'm talking about the short circuit current here which will be much more than the rated current. If you put 21A through a 1.8 Ohm winding , the power dissipated would be 212×1.8 = 793.8W which would cook a transformer that size in no time.

To get a more realistic idea of the power rating, weigh the transformer and compare its mass, to that found on the data sheets of similar sized toroidal transformers.

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#### Ian.M

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##### Re: Toroidal transformer, Help?
« Reply #2 on: July 30, 2017, 10:20:04 am »
The manufacturer has a custom transformer sizing page with weights: http://www.avellindberg.com/transformers/custom_sizes.htm
You should be able to get a very good estimate of its total VA rating.  Then its just a matter of ACCURATELY (3 sig.figs. on a low-ohms meter) measuring the secondary resistances so you can calculate the I2R losses and proportion them between the windings to get an equal dissipation per turn* to get the individual secondary ratings.

* Turns ratios from the ratio of the voltages - if you want an absolute count, wind on 20 turns of hookup wire, measure the voltage from that and scale everything accordingly.
« Last Edit: July 30, 2017, 08:11:50 pm by Ian.M »

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#### mictas

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##### Re: Toroidal transformer, Help?
« Reply #3 on: July 30, 2017, 11:45:03 am »
The manufacturer has a custom transformer sizing page with weights: http://www.avellindberg.com/transformers/custom_sizes.htm
You should be able to get a very good estimate of its total VA rating.  Then its just a matter of ACCURATELY (3 d.p. on a low-ohms meter) measuring the secondary resistances so you can calculate the I2R losses and proportion them between the windings to get an equal dissipation per turn* to get the individual secondary ratings.

Okay, so going with your advice, I tried that page you sent me to and I got 30VA 1.25A, which to me sounds WAY better.

So, if I am guessing correctly, by bridging both 24v secondary's I should end up with 48V 2.50A/60VA. Which doesn't make it any good for my lab power supply project thingy.

O wait a miny, if I am guessing correctly, if I take the 10v secondary winding, and join it with one of the 24v secondary, that would give me 34v at 4.25A.
The project I am going for is the elektor 1982 precision power supply, that needs two transformers x2 12v 400mA and one 33v 4A. I'll have to ask later when it comes to reworking the whole thing.

And yes, I do plan to reinsulator the transformer, I was thinking of more 3D printing a box a can it into.

#### Ian.M

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##### Re: Toroidal transformer, Help?
« Reply #4 on: July 30, 2017, 12:06:12 pm »
Your resistance measurements are too uncertain to use to calculate the ratings - you need to measure the resistances accurately e.g. put 100mA DC through the winding (e.g. using a LM317 wired as a current source) and measure the mV across it then calculate the resistance.  N.B. when using a high current to measure transformer winding resistance, short-circuit the highest voltage winding that you aren't measuring to damp the inductive kickback when you connect/disconnect the winding you are testing.

Okay, so going with your advice, I tried that page you sent me to and I got 30VA 1.25A, which to me sounds WAY better.
I don't know where you are getting 1.25A from.  30VA sounds reasonable if the size & weight match.

It would be unusual to have two secondaries of the same voltage but different current ratings.  Lets *GUESS* that all the secondaries are wound with the same wire size so have the same current rating.  If all the secondaries were in series, it would have an total output voltage of 58V.   Assuming the input voltage is nominal, that would give you a max secondary current for any secondary of 30VA/58V which is ONLY 0.5A.

It gets more complex if the secondaries don't have the same wire size but its absolutely certain that you aren't going to get 4A from *ANY* winding on this transformer.
« Last Edit: July 30, 2017, 02:09:18 pm by Ian.M »

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##### Re: Toroidal transformer, Help?
« Reply #5 on: July 30, 2017, 01:19:30 pm »
Notice that the fuse on the primary side (with the two primaries in series) is 250mA, i.e. the transformer is fused at 60VA and long term is most likely only capable of delivering quite a bit less than 60VA.

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#### Zero999

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##### Re: Toroidal transformer, Help?
« Reply #6 on: July 30, 2017, 07:48:32 pm »
Notice that the fuse on the primary side (with the two primaries in series) is 250mA, i.e. the transformer is fused at 60VA and long term is most likely only capable of delivering quite a bit less than 60VA.
The fuse is quite often rated to around double the expected current, so 250mA sounds reasonable for a 30VA transformer.

#### mictas

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##### Re: Toroidal transformer, Help?
« Reply #7 on: July 31, 2017, 05:15:50 am »
Your resistance measurements are too uncertain to use to calculate the ratings - you need to measure the resistances accurately e.g. put 100mA DC through the winding (e.g. using a LM317 wired as a current source) and measure the mV across it then calculate the resistance.  N.B. when using a high current to measure transformer winding resistance, short-circuit the highest voltage winding that you aren't measuring to damp the inductive kickback when you connect/disconnect the winding you are testing.

I ended up winging it, but I ended up with 2.84mA when it was stable and 113. something at its biggest spike.
I should add that I went with a 300mA supply.

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So, I have been reading this http://engineering.electrical-equipment.org/forum/general-discussion/difference-between-series-parallel-transformer very interesting.
« Last Edit: August 01, 2017, 01:26:11 pm by mictas »

Smf