Step down transformer for DC power supply

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Hi, I am trying to make my own dc power supply. I have this transformer on hand( http://www.signaltransformer.com...) and the spec sheet shows two different voltages for it being wired in parallel or series. I would like to know how to wire it both ways if anyone can help me, Thanks.

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Another question going along with the dc power supply, what size and type of capacitor should be used on the output. Thanks.

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The link you gave seems to go to a dead page, so I can't look at the transformer specs.

As for the capacitor value, you might want to look at this page...
http://www.changpuak.ch/electronics/power_supply_design.php

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I hope that you mean AC to DC power supply!

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Your post indicates a transformer with multiple primaries, or multiple secondaries, or both.

To obtain any meaningful help from anybody, you will need to disclose...

The characteristics of the AC mains you are deriving power from (110, 220, 440 volts??? single phase, three phase ? 50 Hz, 60 Hz?)

The characteristics of the transformer you are considering ( winding ratios, power rating)

The desired final output (volts, amperes?)

Once you have disclosed the above, I am certain you will obtain much good advice.

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Thanks chris for the capacitor link and I will put up a better link this time and yes a ac-dc suppy and mike my input is just a basic wall plug, and the transformer I have has a output of 36V in series and thats the issue, I just dont know how to hook up a transformer with as many leads as it has. Here is the link again to the transformer specs. http://www.signaltransformer.com... (yes same link but it works for me)and this one from digi-key http://www.digikey.com/product-d...

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Quote:
I just dont know how to hook up a transformer with as many leads as it has.
Even by looking at the diagram and pin numbers given on the last page for the DST type (D as in dual 115V primaries)?

If you don't understand "how to hook it up" may I suggest you just DON'T please? We want you around this forum for a long time still. :)

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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I figured out why that first link didn't work. There's an extra ) on the end of the URL.

Looking on the transformer, it can be connected in several ways depending on what you need. If your power line voltage is 230V (The European standard, I think) then connect pins 2 and 3 together, and apply power to pins 1 and 4. If your power line voltage is 115V ( North American standard) then connect pin 2 to pin 4, and pin 1 to pin 3. Apply power to pins 1 and 4.

On the secondary side, you can connect pins 6 and 7 together to get a 36V secondary across pins 5 and 8 with a maximum current available of only 65mA (0.065A)
Alternatively, you can connect pins 5 and 7together, and pins 6and 8 together. This gives you 18V between pins 5 and 8, with a maximum available current of 130mA.

No matter how you wire it, you will need a fuse in series between the transformer and the line cord, and insulate everything well. Power line voltages are something you need to treat with respect. Mistakes are painful at best, and potentially lethal.

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subieghost:

Like JS says, if you have to ask such an elementary question, you should buy one of many inexpensive commercial power supplies available for less than $100. Google is a great tool, use it.

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Well thank you JS for the concern( and hope to be on this forum for a long time too, you guys are very helpful) and that is why I am here to learn more about it before I use it, I actually know one way to hook it up but that's only because I use it on assembly of ours. Thank you Chris that is what I wanted to know and yes I know about the fuse for sure. And haha that ) is totally my fault in the link

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Well RickB i'm just asking to make sure and because I've only gotten to use simple transformers in school and I'm building one because I can obtain all the parts from where I work so I can spend way less than $100

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Designing and building a power supply is a great learning experience (just don't mess up with mains voltage).

But I do have to wonder how useful your power supply will be, if you use that transformer.

For example if you parallel the secondaries to make 18VAC at 130mA, what are you going to do with it? You get around 24VDC peak unregulated when you rectify it, and as a rule of thumb, about 100mA.

If that voltage is regulated to something like 5V or 12V with a linear regulator, you still have about 100mA available, and a lot of power wasted as heat, when you draw the maximum of 100mA.

It does sound nice to make some kind of dual supply, either two isolated 12V or just simply +/- 12V supply. Or 12V and 5V.

Personally if I had such a transformer, I'd use the other secondary to power an AVR, and the other secondary make the AVR to monitor mains quality, like voltage, frequency and time drift etc.

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Since just about everyone uses wall warts these days, perhaps one more warning would be worth mentioning, in addition to Chris-Mouse's recommendation to not forget a fuse on the primary:

If you use a transformer then either the project should be in a plastic case, with no exposed metal, or in a metal case, with the case grounded, using a three wire power cord from the Mains.

Since you appear to be maknig a power supply, you may want a true ground available on the front panel, (earth ground, the third wire on the power outlet), in which case the case should definitely be metal, with the case grounded.

Note also that transformers can get warm, are are/were often mounted to the metal case which also served as a heat sink for the transformer.

JC

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Hmm. In HS, an electronic project almost always started with a 6.3v transformer, a line cord, diode bridge and capacitor. Since we were playing with 4000 series "cosmos" (cmos), the resulting 9v served nicely. We built our circuits by gluing the chips to a piece of wood with the pins up and soldering to the pins.

These days, I often use little coin cell batteries.

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