Help on transformers

Discussion in 'General Electronics Chat' started by Adamf001, Aug 28, 2012.

Sep 5, 2011
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I'm building a bench power supply. I'm going to use two transformers one for the positive supply and one for the negative supply.

my question is how does one get a negative supply from a transformer connected up to the mains. I've never really used transformers so I'm not sure the theory behind them... thanks

Sep 5, 2011
67
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using two transformers how would I wire them to create a center tapped transformer like this one

I think I remember someone saying it is possible but I'm not sure how it would work, any ideas?

3. t06afre AAC Fanatic!

May 11, 2009
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Have you given the need for current any thought

Sep 5, 2011
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I was just going to measure the amp value after I connect the transformers and make the alteration to my circuit depending on the current being supplied

5. t06afre AAC Fanatic!

May 11, 2009
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Take a look at these PDFs. Use the "dual complementary rectifier circuit

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6. crutschow Expert

Mar 14, 2008
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The DC output polarity is determined simply by the direction of the rectifier diodes.

You can't easily "measure" the maximum rated output current of a transformer. It's determined by the design and its temperature rise due to current. The easiest way to determine the approximate current rating is to find a comparable size and voltage transformer in a manufacturer's catalog.

7. vk6zgo Active Member

Jul 21, 2012
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As he said,it just depends on which way round the diodes are connected.

You are using one transformer for the positive supply,& one for the negative supply,so they are really two independent supplies.

Use a bridge rectifier for each supply,& if you want them to have a common point ,connect the -ve output of the "positive" supply to this "common",& the +ve output of the "negative" supply to "common" also.

Bridge rectifiers are commonly available in packages with the AC inputs marked with ,& the outputs with + & -.
Google for "bridge rectifiers",and/or have a look in the "E-Book" associated with this forum.

You also have to filter the output of each rectifier,to remove AC "hum",& probably also need to regulate the output voltages.

Sep 5, 2011
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Yes you did thanks, using two transformers for me is much easier than using a centre tap transformer.

I'll have to look out for the rectifier packs and search on my friend Google, my main goal was to have two independent power supplies, Thanks guys for your help

9. t06afre AAC Fanatic!

May 11, 2009
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If the transformers are equal. You may connect the transformers secondary output in series. To create a center tap function

Sep 5, 2011
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I was going to have a higher +v than -V so the transformers won't be equal and I can't seem to find anyone selling a decent centre tap transformer

11. vk6zgo Active Member

Jul 21, 2012
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He doesn't need a centre tap,as he has two separate transformers,& can use a bridge rectifier on each one.

12. t06afre AAC Fanatic!

May 11, 2009
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Dude I know my way then it comes to this topic. The problem in this case as in many others. Is that the OP present information in small drip. Slightly changing the terms on the way.
So to Adamf001. Let us cut to the chase. Just tell us something like this.
I want to build a power supply. The voltage needed is plus xx volt, and minus xx volt. My estimated need of current will be xx ampere for the positive supply, and xx ampere for the negative supply.
After this you write some words about what kind of equipment you want to connect to the power supply. Good questions draw good answers

Last edited: Aug 29, 2012

Sep 5, 2011
67
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ok ,

I'm building a bench power supply, using LM337 and LM317.
I'm aiming to get about +20 to +25V for the positive output
and up to -15 to -20V on the negative output
with no more than 1A from either output

my original question was how using two transformers would I get a negative supply from one of the transformers... I've never gone into depth on the workings of transformers and bridge rectifiers

I'm probably use two septate bridge rectifiers for each supply, I'd didn't know that the config of the diodes dictated the polarity of the supply. thanks to crutschow I now know, and thanks to t06afre PDF's I have a rectifier circuit that can give me negative supply from transformer.

14. t06afre AAC Fanatic!

May 11, 2009
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This is something we can work with. Maybe in your case it will be better to build two separate LM317 based devices. Then put them together to create the +/- voltage. Like shown in the picture below.

You will also need some cooling fins/ heat sinks on the regulator ICs. Your project is doable for a beginner. Given that you have some solder experience. How do you plan to mount the electronics. Will you be making a PCB. Or some sort of prototype board.

Sep 5, 2011
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I think I know what you mean, I was going to build two separate supplies in one housing just like the pic one negative, the other positive, each connected to a analogue voltmeter
I have a PCB development kit that I'm dying to use, and a box full of heat sinks

Thanks

16. t06afre AAC Fanatic!

May 11, 2009
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Just build two lm317 based units. Then you put them together to create the +/- voltage. Just as shown in the picture. Also feel free to post your PCB here for auditing.
If you are a beginner in this field I would recommend it

17. vk6zgo Active Member

Jul 21, 2012
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The OPs original question was clear.
Unfortunately,the second poster got the wrong slant on things & confused him with the comment about centre tapped transformers.

Sep 5, 2011
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This is my initial design for the positive voltage regulator, I hope there are no major problems or I hope I haven't missed anything important, Thanks

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19. t06afre AAC Fanatic!

May 11, 2009
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As a tip draw the bridge rectifier as shown here http://en.wikipedia.org/wiki/Diode_bridge
Also you need filter caps. Also mentioned in the wiki paper. The rule of thumb is 1000uF each of DC ampere output. However in your case I would have used the standard size 4700uF. And once again. Since you will not be using a centertap transformer. You will need to build two positive supply that you put together to a +/- supply