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 Post subject: Power Transformer Wiring for Amplifier Kit
PostPosted: Dec Tue 05, 2017 1:35 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 162
Good evening,

I believe I’m in the correct category for this subject and I apologize if I’m not.

I recently purchased a 100 watt (50 x 2) stereo amplifier kit from an overseas supplier. The kit seems to have potential. The manufacturer recommends a 56 volt, center tapped (28v-0-28v) secondary transformer with a minimum of 3 amps per side, if I am stating this correctly. The primary voltage is 115 volts. (My house typically runs around 121 volts.)

I found a toroidal power transformer at a decent price that meets the above requirements but I am not sure how to wire it. In the pictures below, I have shown how I think it should be done. Please let me know if this is correct and if not, what I should do.

Thank you.

Jamie


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 Post subject: Re: Power Transformer Wiring for Amplifier Kit
PostPosted: Dec Tue 05, 2017 2:33 am 
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Joined: Sep Mon 16, 2013 2:42 am
Posts: 2414
Location: Tucson, Arizona U.S.A.
I wonder about the company that makes that transformer. The first problem is that they didn't provide a current rating for the 28 volt windings. They appear to be OK for your use but they didn't outright tell us.

The second problem is with the wires. Having windings, both primary and secondary, that can be connected together, all the wires should be different colors. Chances are your connection for the primary is correct, but what if you wanted to connect it for 240 volts? Which red wire do you connect to which black wire? If you guess wrong, you connect 240 volts to a 120 volt winding and short circuit the other primary coil.

The same situation occurs with the secondaries, connect the wrong blue and green together and there is a short. And there are no polarity dots on the drawing. How do you know that you are supposed to connect the blue to the green? Perhaps you are supposed to connect the two blues together.

A reasonable manufacturer would color all the wires differently and provide instructions on how to connect it. This isn't an insurmountable problem but you should proceed with caution and use a dim bulb tester while trying to determine the correct connections.

They also don't know how to measure core loss. They multiplied the primary voltage times the current and stated the result in watts. It isn't; it's volt amps. A transformer with no load is highly inductive so the actual loss is less than this number.

It's nice(?) that the amplifier manufacturer left out the transformer; that's the most expensive and heaviest part!

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Jim Mueller


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 Post subject: Re: Power Transformer Wiring for Amplifier Kit
PostPosted: Dec Tue 05, 2017 2:34 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3143
Looks right to me. I'm assuming you'll check the voltages before wiring it up. I'd also check it with some resistance for current and voltage drop.


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 Post subject: Re: Power Transformer Wiring for Amplifier Kit
PostPosted: Dec Tue 05, 2017 4:56 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 162
Jim, I tend to agree with you about the transformer possibly being suspect. It’s from an eBay seller and is the cheapest price ($39+shipping) I have found—by far. I have looked at several other transformer manufacturers and most prices are in the $60-$70 range.

I will probably end up with this one from Mouser. Its voltage isn’t quite as high as I would like, but I know Triad Magnetics has a good reputation and so does Mouser. http://www.mouser.com/ds/2/410/media-1067311.pdf

The amp kit was from a seller in Sri Lanka of all places and was a little over $20 including shipping. It didn’t even come with a schematic and has nothing but the components that go on the PC board. I simply look at this as an interesting little winter project. In addition to the transformer, I will have to purchase a 6”-7” long heat sink. This may turn out to be a big waste of money but I guess I’m just optimistic it will work out fine. Once it’s all said and done, I should have a little less than $100 in it.

Macrohenry, thank you for validating my understanding of how to wire the transformer. I will definitely check voltages and I do have a dim bulb tester.

Here’s a few pictures of the kit. I’m about 70% done.

Jamie


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 Post subject: Re: Power Transformer Wiring for Amplifier Kit
PostPosted: Dec Tue 05, 2017 8:41 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 9423
Location: Powell River BC Canada
Fuse the primary and secondary of the toroidal transformer.

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Power Transformer Wiring for Amplifier Kit
PostPosted: Dec Tue 05, 2017 11:14 pm 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 162
Radiotechnician,

I had planned to fuse the primary side but hadn’t considered the secondaries. Do I use two fuses—one for each 28v side?

Jamie


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 Post subject: Re: Power Transformer Wiring for Amplifier Kit
PostPosted: Dec Wed 06, 2017 9:25 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 9423
Location: Powell River BC Canada
Yes. One spec that transformer has shows full primary load current flows
when the secondary is shorted. (About 7 % impedance) Also mentioned is
heavy wire used in winding.

It is the secondary providing the amps, if the diodes short out in a certain way.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Power Transformer Wiring for Amplifier Kit
PostPosted: Dec Sun 10, 2017 2:50 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 162
Here’s a quick follow-up to an earlier post. I completed the little Sri Lankan amplifier kit and began testing it this afternoon. Honestly, it sounded quite good. I only ran it for about two minutes as I don’t have the heat sink for it, yet.

As the photos show, I had absolutely no trouble hooking up the toroidal power transformer. I did NOT get the suspect AnTek transformer from eBay as mentioned in my earlier posts. Instead, I got a Triad Magnetics transformer from Mouser.

I sincerely appreciate the help I got from each of you in my earlier posts.

Regards,

Jamie


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