Power Transformer: is this the right one?

Discussion in 'The Projects Forum' started by sintau.tayua, May 11, 2009.

  1. sintau.tayua

    Thread Starter Member

    May 11, 2009
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    Hello Everyone!

    I am working on a project that involves creating a power supply unit. This PSU needs to output 2A at between 4 - 5.5 volts (only one voltage within this range, not a variable supply). The input for the PSU is New Zealand mains supply; that is, 230V AC at 50Hz.
    I am currently trying to select the transformer to use. I have chosen this one here:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=237-1045-ND

    Would this meet my requirements? Is the manufacturer reputable? I have done as much research as I can, and it seems to be right, but I want someone knowledgeable to check my choice. This is the first time I've ever used a transformer or played with mains AC, and quite simply I'm terrified of choosing the wrong transformer.
    Plus, shipping from USA to New Zealand isn't going to be cheap - I don't want to have to reorder.

    Thanks,
    Sintau.tayua
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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  3. sintau.tayua

    Thread Starter Member

    May 11, 2009
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    That one certainly looks good - especially with the fuse included. However, what makes you think the first transformer will run hotter than the second? Shouldn't the first transformer run cooler, because has a higher VA rating (i.e. if it can withstand higher loads, shouldn't it run cooler when not operating at maximum load?). I trust your assessment; I just want to learn why you say it, so I can know for next time.

    Thanks,
    Sintau.tayua
     
  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    There are still some information missing. They affect the choice of the transformer.

    1. output voltage wanted, AC or DC ?

    2. if DC, regulated or not?

    3. is the 2A requirement continuous?
     
  5. sintau.tayua

    Thread Starter Member

    May 11, 2009
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    1: Sorry, I wasn't aware transformer could output DC. Doesn't that require diodes (rectifiers)? Anyways, I want a transformer that outputs AC; the rectifier will be separate.
    2. See above.
    3. Well, obviously AC current will be kinda back and forth, but the eventual result (after rectifiers, filter cap, etc) will be a continuous power supply.
     
  6. AchMED

    Active Member

    Aug 5, 2008
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  7. sintau.tayua

    Thread Starter Member

    May 11, 2009
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    Hey, thanks. That's a good link. The equations indicate I need a much more powerful transformer than I expected - glad someone caught that.
    The equations for a full-wave bridge with cap describe my situation quite nicely. I used their results for current and VA, but decided to still use a 5V transformer. Even with the voltage drop they describe, the Vout will still be above 4V.
    I looked on digikey, and came up with this transformer:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=237-1067-ND
    It's a lot beefier, and costs more, but I figure it's worth it. What do you think? I'm pretty sure it's fine, but then I said that about the last one . . .
     
  8. AchMED

    Active Member

    Aug 5, 2008
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    If you want to build a safe reliable power supply read through the relevant sections in this pdf.(see below)

    What you are doing now is “UNREGULATED DC POWER SUPPLY DESIGN PROCEDURE” which is covered in detail starting on page 14. You typically would already have your regulator selected so you know what the drop out voltage is. Knowing this helps to economically select the transformer and the capacitor. For a 5VAC transformer to you should use an LDO if you want 5VDC regulated out.

    If on the other hand your not putting a regulator on you should be aware that regulation of transformers isn’t tight (particularly small ones). This means that at full load you will get the rated output (or very close) voltage but under light load (small current) you could get 30% greater output voltage (depends on the transformer). Also any change in the input voltage gets translated to the secondary by the turns ratio something else to consider. The AC line voltage can vary throughout the day depending on loading other factors.

    Transformers are speced in output AC rms resistive load unless otherwise stated. That transformer should be fine for an unregulated power supply as long as you realize at light load it could be higher. If you want to regulate the output (5V) you will need an LDO.

    This is the pdf.

    http://elearning.algonquincollege.com/coursemat/saurioc/ELE-3/ELE-3-NOTES/VOLT-REG.pdf
     
    Last edited: May 12, 2009
  9. sintau.tayua

    Thread Starter Member

    May 11, 2009
    11
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    First, I want to say a big thanks to all you guys for helping me. It's very much appreciated!

    Okay, so for the regulator I chose this once:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-4252-5-ND
    I chose one the can supply 3A, because I though it would be good to have some leeway. It needs a minimum Vin of 6V. (however, some sites seem to indicate that Vout + dropout = minimum Vin. Therefore, one would think that Vin min should be around 5.5V. Is this equation only true of some regulators, or an I misunderstanding things?)
    Anyway, the last pdf you gave me indicates that any transformer I choose must produce a minimum output of 7.5V, with a current of at least 3.6A (though probably 4A simply because they're easier to get). It will need to be able to handle 15W, and supply 24VA. Is this right?

    As per the regulator manufacturer's recommendations, I will place a capacitor of 0.33uF between between Vin and GND, and a cap of 10uF between Vo and GND. I will also have a filter cap beside my transformer, of a yet-undetermined value.


    I think that's all for now. I'll work on it more tomorrow - but alternatively, considering how this discussion is going, I might just buy a 20VAC 10A transformer and be done with it! :)

    Thanks again,
    Sintau.tayua
     
  10. AchMED

    Active Member

    Aug 5, 2008
    41
    0
    You would probably get more specific answers if you provide specifics. People have already asked you some of what is required namely are you putting a three terminal regulator on the output? From one of your other responses I assumed you weren’t. When you said you wanted between 4 and 5 Volts out. This usually means unregulated. If you put a regulator on there yes you will need at least 2V headroom. Hence why they say a 7.5VAC tranny. That pdf adds margin for thermal (heating) as well.And adds margin for mass production considering tolerances so you have room in VA to go down about 10% to 20% then what you calculate from there equations.

    Now from your most recent response you seem to be putting a regulator on there, so what is it?

    If you have a 5Vac secondary transformer you will get about 7V peak out (full load). Once you rectify and filter the most you could hope for is 5VDC with ripple at full load nominal input voltage. So you couldn’t even use an LDO for reliable regulation of 5VDC.

    If you are putting a regulator on there yes follow the recommended transformer sizing (7.5VAC or better) you have some room for the VA rating but you have to insure you will have the headroom needed by your regulator usually 2 to 2.5V.

    Everything you need to build your power supply is in the second pdf. I suggest you read through it before you order anything. You will likely need other things namely heatsinks.There is a full design of a three terminal regulator thermal, fusing etc. Read it so you don't waste your time and money haveing to order multiple times.

    The closest transformer meeting those specifications (115V/230V) at Digikey is

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=TE70060-ND

    If you live in New Zealand you could order from Farnell.They have a suitable tranny for $15.

    http://nz.newark.com/jsp/search/productdetail.jsp?sku=93F8772&_requestid=685

    They will have everything you need there.


    But still go through the pdf and be sure you know everything you will need before you order so you get it all at once. When you rush things get expensive and mistakes get made.
     
    Last edited: May 13, 2009
  11. sintau.tayua

    Thread Starter Member

    May 11, 2009
    11
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    Okay, I thing I need to do more explaining. Obviously I know what I need, but I have to remember people can't read minds . . .

    (Yea, I'm still working through that pdf. It looks good, and I'll go through all of it before I ask more questions, but I'm just putting the information up here because, as you point out, my descriptions do need some more work)

    I am working on a microcontroller project that runs from two NiMH (1.2V), 2000mAh, rechargeable batteries in series. These two batteries will be charged from a charger of my own making, which will be based around the
    DS2711/DS2712 battery charger ic from Maxim. The datasheet for this ic can be found here:
    http://datasheets.maxim-ic.com/en/ds/DS2711-DS2712.pdf
    It is this charger that needs an input of 4V - 5.5VDC. The charger as a unit will run off mains power, but obviously the chip itself can't. Therefore, I need a transformer, rectifier, etc. This is the part I am trying to design at the moment.
    In regards to the voltage regulator: I initially intended not to use a regulator, as I thought I wouldn't need it, and I wan't to save money. However, after reading the various .pdfs and listening to your advice, I realised that an unregulated power supply wouldn't output a tight enough voltage. Not planing on using a volt.reg. was a mistake on my part; hopefully I have fixed that error.
    The batteries will be charged in series, so the current needed for charging is 1A. However, I wanted more amperage so I could potentially use this power supply in the future for other, more demanding applications. Hence, 2A.

    So, to summarise the circuit in reverse (because the charger ic's parameters set all other parameters):
    -Charger ic (1A, 4V - 5.5V input range).
    -Voltage regulator (probably 5V output, as this is the most common, but anything down to 4V)
    -Fuse
    -Filter cap between supply and GND
    -Full-wave rectifier
    -Transformer
    -Mains power

    The rectifier will obviously produce a voltage drop. All equations (eg from the big pdf) seem to ignore the rectifier - is this because they take it into account without stating it, or because we are mean to add that margin in too. To the best of my knowledge, rectifiers voltage drop seems to be included in the equations without stating it, because they were refering to the voltage difference between Voltage Regulator (dc) and transformer (ac), so a rectifier is necessary.

    Anything else?
     
  12. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I would say otherwise. That's why my #4 post asking you about questions on PSU, which you misinterpreted as asking you about transformer.


    Now those are the missing info I have asked in post#4 that are required to properly select a transformer for the PSU. Sadly I have to wait until now to see it.

    You need to remember a forum is for users to help others not not for one to demand help. I don't know if english is your second language but you need to be more polite in answering other people's questions.
     
  13. sintau.tayua

    Thread Starter Member

    May 11, 2009
    11
    0
    It seems I need to apologize. I am deeply sorry that I have caused you offense; it was never my intention to take you for granted or to demand answers. Indeed, you have helped me far more than I expected, and I am very thankful for that. I tried to express this thankfulness in my fourth post, but I see the wording of my posts have unintentionally communicated the opposite feeling.

    This was intended as a light joke at my own expense, about how hopeless I am at communicating and formulating my posts. Also, my comment at the end of my last post was a joke about how I forget to disclose useful information. In New Zealand, we make jokes like this to show we are embarrassed about a mistake we have made. It is only looking at these statements now that I see how they could come across as arrogant and know-it-all to foreigners. Trust me, this was not my intention.


    Yes, I realise I made a mistake, but hopefully it has been fixed. Thank you for your patience!


    What's worse, English is my first language! Perhaps, however, part of my mistake could be put down to cultural differences. Last year my sister went to America for a year, and many jokes she made about others* caused offense. I realize you come from Britain, so my theory is weakened, but perhaps something of the same is happening here.

    It's funny, really: I was so careful to write a descriptive title and not use words like 'URGENT', yet it was the body of my text that offended!

    Again, my sincerest apologies. Perhaps we could all start again, with clean slates?


    -----------
    *In New Zealand, a lot of what we say when we're hanging out with friends is actually insults - but you see, we don't intend or receive them as true insults, but merely banter. I didn't even realise I was doing it till my sister told me about the american reaction.
     
    Last edited: May 14, 2009
  14. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Thanks for your reply. I didn't realize the reply to my post#4 was meant to be a light joke.:(

    Apology accepted. No bad feelings. :):):)
     
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