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  #1  
Old Tue 25 April 2006, 06:59
Gerald_D
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Transformers

Toroid transformers are preferred because they are quiet ( both in terms of audible noise and electric interference) and they give off little heat. Popular USA source is www.plitron.com
Good data source from another supplier link

When mounting a toroid transformer with metal brackets, under no circumstances should both ends of the central fixing bolt contact a metal chassis or frame as this would create a 'shorted turn' causing irreparable damage!
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  #2  
Old Wed 21 June 2006, 13:25
Dirk Hazeleger
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I'm having space problems for a special box I'm building using a single large toroid transformer. I was planning to use 2 smaller units and wire together. Would this work?

Dirk
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  #3  
Old Wed 21 June 2006, 14:08
Gerald_D
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The toroid only gets slightly smaller when you halve the power output. Maybe mount it on edge as I did?

Wiring transformers together is fairly standard practice, but check this with someone like Plitron.
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  #4  
Old Wed 21 June 2006, 14:22
Dirk Hazeleger
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I only need an extra inch or so. The smaller wire size is also much more manageable using the smaller units.
Dirk
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  #5  
Old Thu 22 June 2006, 01:06
Gerald_D
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Different suppliers will have different "shapes". I can get flatter/wider versions here. From the Plitron site:

"As long as the cross sectional area of the toroidal core is held constant, the height and diameter may be varied to meet the customer's requirement.

The functional optimum diameter to height ratio is 2:1. For low profile equipment requirements, a 3:1 ratio may be used. In cases where a minimum footprint is required, a 1.5:1 ratio could be considered. Aspect ratios of between 1:1.5 to 1:4 can be achieved without performance degradation.

Examples include: 1) The customer wanted to replace a laminated transformer in the same physical space. A 1: 1 diameter to height ratio toroid was designed, at twice the output power, and with vastly improved noise specifications. 2) A custom 500 VA toroid engineered to fit into a single rack height package. The transformer is less than 38mm high, but still exhibits good electrical and magnetic efficiency. The only physical restrictions are the limitations of winding machinery. A minimum center hole must be maintained in order to permit the insertion of the winding shuttle, for application of wire and insulation."


If you do connect two transformers in parallel, be very careful of the polarity - mistakes can be spectacular.

Have you considered two rectifiers and then paralleling the DC over multiple caps? If your two tranformers are then "out of phase", you could get a smooth DC,......I think????
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  #6  
Old Thu 22 June 2006, 07:47
Dirk Hazeleger
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Hi Gerald
I found different sizes available from different manufactures. They were less expensive also. The diameter was causing me troubles. Interesting on using 2 recitifiers. I'm wondering how much difference it would make.
Dirk
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  #7  
Old Thu 22 June 2006, 09:30
Dirk Hazeleger
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Couldn't you hook up 2 recitifiers on a transformer with 2 secondary coils and cross phase the output and have the same results as 2 transformers?
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  #8  
Old Thu 22 June 2006, 10:47
Gerald_D
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You are tangling my brain after a hard day, but I think your reasoning is valid. Anyway, would you be so bold as to reduce the recommended capacitor sizes because you have less ripple? If not, the point is moot. (The cap size is also to give oomf to a starting motor or to store energy from a braking motor.)

I would personally be happier to parallel after the rectifier because it is much easier to get the polarity right.
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  #9  
Old Thu 22 June 2006, 11:17
Dirk Hazeleger
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I was thinking crossing it between the transformer and recitifiers. The DC side would hook up parallel and go on to the caps.
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  #10  
Old Thu 22 June 2006, 16:42
Dirk Hazeleger
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I don't know what I've been thinking, but doesn't a full bridge recitifier pick up all the positive and negative pulses already? I can't see where more than one recitifier would help.
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  #11  
Old Fri 23 June 2006, 01:04
Gerald_D
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You have a pair of wires coming out of each transformer. Connecting a rectifier to each of them is a no-brainer.

But, if you want to connect those 2 pairs of wires to a single rectifier, you will have have 2 wires going to each input pin of the rectifier. If one wire is going positive while the other is going negative, then the magic smoke appears. You have a 50/50 chance of getting it right, or wrong.
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  #12  
Old Fri 23 June 2006, 01:09
Gerald_D
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I just read this little bit:

"Parallel connection of transformer windings is permitted in one case only - the windings must have exactly the same voltage output, and must be connected in phase. Different current capacities are not a problem, but it is rare to find a transformer with two windings of the same voltage but different current ratings.

Even a 1V difference between winding voltages will cause big problems. A typical winding resistance for a 5A winding might be 0.25 ohm. Should two such windings be connected in parallel, having a voltage difference of 1V, there will be a circulating current limited only by the resistances of the windings. For our example, the total winding resistance is 0.5 ohm, so a circulating current of 2A will flow between the windings, and this is completely wasted power. The transformer will get unexpectedly hot, and the maximum current available is reduced by the value of the circulating current.

Should the windings be connected out of phase, the circulating current will be possibly 100A or more, until the transformer melts or the fuse blows. The latter is generally to be preferred.

The transformer manufacturer's specifications will indicate if parallel operation is permitted. If you are unsure, measure the voltages carefully, and avoid parallel connection if the voltages differ by more than a couple of hundred millivolts. There will always be a difference, and only the manufacturer's winding tolerances can predict what it will be. With toroidal transformers, the windings are often bifilar, meaning that the two windings are wound onto the transformer core simultaneously. The tolerance of such windings is normally very good, and should cause no problems."


I am starting to get nervous........

The solution to the nervousness may be to connect the two smaller transformers (of half the voltage) in series, if you want a single rectifier....?

Dual rectifiers are looking even more appealing.
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  #13  
Old Fri 23 June 2006, 09:42
Dirk Hazeleger
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I would definatley use 2 recitifiers on 2 transformers. I was questioning whether 2 recitifiers on 1 transformer with 2 secondary coils had any advantage.
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  #14  
Old Fri 23 June 2006, 13:14
Gerald_D
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After some pondering during supper, I now believe there would be no advantage in terms of ripple. After each rectifier you will see the negative (bottom) part of the sine wave flipped up to the top. Comparing the outputs of the two rectifiers, the peaks of those waves will be at the same time, so the ripple isn't smoothed.
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  #15  
Old Wed 29 November 2006, 22:47
vadeem
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Would it work to use both secondary windings in series of this transformer?:

http://www.toroid-transformer.com/AN-3225.pdf
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  #16  
Old Wed 29 November 2006, 23:09
Gerald_D
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Yes, if you wanted 50V AC. (70VDC, 4.3Amp continuous, (peaks can be higher) after the rectifier). This output spec is practically exactly what I have on the MechMate now:


You can see a yellow and purple wire joined to each other in the red crimp sleeve - the other yellow and purple go to the rectifier.

With that AnTek transformer you would join a blue to a green and send the remaining blue & green to the rectifier.

Difference between my transformer and that Antek is that I only have a single input coil, and this one cannot go to a 110/115V country. Your dual input coils will be paralled for 110/115V countries and serie'd for 220/230V countries.
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  #17  
Old Mon 05 March 2007, 10:30
Mike Richards
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I just read this when I visited the Avel transformer site:

"It is recommended that slow-blow (type T) fuses be used in the primary circuits of all transformers over 100 VA. For larger toroids, either NTC thermistors or circuit breakers designed for motors and transformers (with type D delay characteristics, for example) should be incorporated. Simple relay-switched resistor soft start circuits can also be used effectively, and a delay of between about 30 to 300 mS will usually work effectively (some relays themselves have pull-in delays of approximately this time). Soft start circuits of this kind should be implemented with, and never replace, the proper circuit protection provided by fuses or circuit breakers."

The required fuse might be important for us do-it-yourselfers.
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  #18  
Old Mon 05 March 2007, 11:31
Gerald_D
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Mike, I think that Avel is addressing the issue where large magnetics have high inrush currents, causing nuisance tripping and blowing of "quick" fuses. They are pleading for users to modify their circuit breaker or fuse characteristics instead of dispensing with the protection offered.

Another difference of approach on this side of the pond is to use circuit breakers instead of fuses. We won't design a fuse into a circuit if there is a circuit breaker that will do the job. More expensive to build, but cheaper to maintain. Less risk of the end-user picking his own trip values.....
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  #19  
Old Thu 17 July 2008, 06:51
bob
Just call me: Frank
 
us
US Minor Outlying Islands
Hi,

I got 25-0-25 V, 500VA transformer. I want to use Unipolar Vexta pk296sg7.2, but now I find there are only 3 output wires Blue - Brown-Blue.
How to use parallel winding ? Should I peel middle one and cut centre tapping. Toroidal I think is double wound ((Secondary)
(I should have purchased 2x25V)
Thanks,
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  #20  
Old Thu 17 July 2008, 08:47
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Frank,
Sounds like your secondary is a center tapped winding. This would mean that the brown is your common and the blue wires are your 25V outputs. You would need to separate the center tapping as you mentioned to be able to parallel the secondaries to get one 25V output. You cannot connect the two blue wire together, as the windings are out of phase with each other.
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  #21  
Old Thu 17 July 2008, 10:55
bob
Just call me: Frank
 
us
US Minor Outlying Islands
Thanks Heath,

That's what I was suspecting. I've aleady found center common tap and now seperating that.
I also have to check voltage of both secondary coils as mentioned by Gerald.
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  #22  
Old Thu 17 July 2008, 14:51
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Frank,

Be careful! Spending another $75 for the proper transformer is a lot less expensive than frying everything.

A center-tapped transformer is usually just one coil with a tap. The kind of transformer that you need is one that has two independent coils. They are entirely different in construction.
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  #23  
Old Thu 17 July 2008, 14:55
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Frank,
Mike is right on there. I may have over simplified my response. Dont do anything unless you are sure you can gain access to both secondary windings and can identify phasing to properly hook up the windings as you require. As Mike stated, sometimes you cannot separate the secondary windings to do what you intend.
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  #24  
Old Mon 21 July 2008, 18:48
bob
Just call me: Frank
 
us
US Minor Outlying Islands
Thnks for care guys,

You are right, mine is center-tapped one coil.
Either single coil or two independent coil in both cases these should be on same core in multiple windings and in same direction. While center tapped is continuous, 2x coils should be side by side wingings.
The question is whether these coils have same voltage or not?
I cut coil from center tap, have checked voltage, thankfully these are having same voltage and output is constant with no heating or humming of transformer.
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  #25  
Old Tue 22 July 2008, 00:47
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Sounds like you are a good surgeon Frank
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  #26  
Old Sat 15 November 2008, 09:43
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Build your own toroid transformer from a partial kit . . . .

Here is the source:
http://www.toroid.com/standard_trans...ormer_kits.htm

It looks quite legit and straightforward. Get exactly the voltage you want. Put on 2 or 3 output voltages if you wish. I think this is a cleaner option than stock transformers where you need to series/parallel coils. Rather expensive though, especially considering you still need to buy some wire yourself. (the primary coil is already wound and insulated).
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  #27  
Old Tue 10 March 2009, 12:38
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Supplier in Romania

http://www.petra-toroid.ro
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  #28  
Old Fri 03 July 2009, 12:42
caneckel
Just call me: Carlos
 
Florianópolis, SC (South of Brazil)
Brazil
Hi Gerald

I would like to order a Toroidal Transformer with different outputs, 27VAC (400VA), 9VAC (1A) and 12VAC (3A). My question if this can cause some interference in the system.
Tk's
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  #29  
Old Fri 03 July 2009, 13:04
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
It is okay to have many different outputs - it has no effect on interference.

Quote:
Originally Posted by caneckel View Post
. .My name is Carlos, I am Electro-electronic eng. . .
Hey, you should be able to tell me if it is okay - I am only a mechanical eng.!
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  #30  
Old Fri 03 July 2009, 13:19
caneckel
Just call me: Carlos
 
Florianópolis, SC (South of Brazil)
Brazil
You are right!
I had the same opinion but I wanted to confirm with the master, I still didn't dominate the whole system of MM command this is new for me, but I will.
Tank's
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