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Gerald_D
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Username: Gerald_d
Post Number: 39
Registered: 11-2005
| | Posted on Tuesday, April 25, 2006 - 01:59 pm: | 
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Toroid transformers are preferred because they are quiet - both in terms of audible noise and electric interference. 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! |
Dirk Hazeleger
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Username: Dirk
Post Number: 17
Registered: 12-2005
| | Posted on Wednesday, June 21, 2006 - 08:25 pm: |
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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 |
Gerald_D
Registered
Username: Gerald_d
Post Number: 136
Registered: 11-2005
| | Posted on Wednesday, June 21, 2006 - 09:08 pm: |
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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. |
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 18
Registered: 12-2005
| | Posted on Wednesday, June 21, 2006 - 09:22 pm: |
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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 |
Gerald_D
Registered
Username: Gerald_d
Post Number: 137
Registered: 11-2005
| | Posted on Thursday, June 22, 2006 - 08:06 am: |
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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???? |
Dirk Hazeleger
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Username: Dirk
Post Number: 19
Registered: 12-2005
| | Posted on Thursday, June 22, 2006 - 02:47 pm: |
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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 |
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 20
Registered: 12-2005
| | Posted on Thursday, June 22, 2006 - 04:30 pm: |
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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? |
Gerald_D
Registered
Username: Gerald_d
Post Number: 138
Registered: 11-2005
| | Posted on Thursday, June 22, 2006 - 05:47 pm: |
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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. |
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 21
Registered: 12-2005
| | Posted on Thursday, June 22, 2006 - 06:17 pm: |
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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. |
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 22
Registered: 12-2005
| | Posted on Thursday, June 22, 2006 - 11:42 pm: |
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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. |
Gerald_D
Registered
Username: Gerald_d
Post Number: 141
Registered: 11-2005
| | Posted on Friday, June 23, 2006 - 08:04 am: |
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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. |
Gerald_D
Registered
Username: Gerald_d
Post Number: 142
Registered: 11-2005
| | Posted on Friday, June 23, 2006 - 08:09 am: |
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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. |
Dirk Hazeleger
Registered
Username: Dirk
Post Number: 23
Registered: 12-2005
| | Posted on Friday, June 23, 2006 - 04:42 pm: |
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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. |
Gerald_D
Registered
Username: Gerald_d
Post Number: 145
Registered: 11-2005
| | Posted on Friday, June 23, 2006 - 08:14 pm: |
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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. |
vadeem
Registered
Username: Vid1900
Post Number: 20
Registered: 10-2006
| | Posted on Thursday, November 30, 2006 - 06:47 am: |
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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 |
Gerald_D
Registered
Username: Gerald_d
Post Number: 553
Registered: 11-2005
| | Posted on Thursday, November 30, 2006 - 07:09 am: |
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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. |
Mike Richards
Registered
Username: Richards
Post Number: 76
Registered: 05-2006
| | Posted on Monday, March 05, 2007 - 06:30 pm: |
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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. |
Gerald_D
Registered
Username: Gerald_d
Post Number: 1109
Registered: 11-2005
| | Posted on Monday, March 05, 2007 - 07:31 pm: |
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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 enduser picking his own trip values..... |
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