Large transformer, amp project..need advice

Discussion in 'General Electronics Chat' started by chunkmartinez, May 9, 2013.

  1. chunkmartinez

    Thread Starter Senior Member

    Jan 6, 2007
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    Originally I wanted to work on a large power amplifier in a few weeks...It was going to be 4x50wrms class A/B IC amps. And a single subwoofer channel at 300W with a class D IC amp. This all spawned from a large transformer I found...

    I found the large transformer from a motor home power supply(AC to DC powersupply I guess) and it had 4 breakers in it that totaled upto 85A! Not only that but I read places on the box that suggested it may be able to supply about 85A...I assumed it was at 12v ofcoarse especially at that amount of current and not much more also because it was supposed to be a DC converter... Well, I measured the secondary with my oscope in RMS measurement mode and measured 15V per side(center tap) for a total of 30V. Assuming that the rating is for dual 12v anyway, that comes out to around 2kw. Asumming that I can get even 1kw or maybe 1500w out of it makes me excited.

    So, I figure I will be about 40 or so Volts rectified/filtered, But....I cannot even source 80A from a standard room that typically has a 25A breaker so I wanted to boost the voltage higher while my current sourability requirement drops and I don't require as much current anyway. What simple voltage boosting methods should I try out? Voltage trippler? I hneard it reduces current sourceability but that is obvious due to coservation of energy and that's what we want, the "trade-off" for higher voltage but what I want to know is if voltage multipliers are impractical, give bad trade-offs, not the best idea? etc...

    EDIT: oops I wasn't thinking, the primary will draw less current not the 80A...But I am getting confused maybe it is a steup up transformer for AC I didnt see any large bypass capacitors anywhere...But for a lower guage stepup secondary to handle 85A, the primary would be able to handle even more then 85 right? It dosn't seem like it would but I don't know. What would you guys think? The smaller wire side is not exactly small by any means but idk about 85A. I need to look it over again..

    Does the voltage multiplier sound feasable? And I know people always suggest things like why don't I get a different transformer to work with etc..Well, It was free...
     
    Last edited: May 9, 2013
  2. chunkmartinez

    Thread Starter Senior Member

    Jan 6, 2007
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    Okay it appears to be a dc to AC converter that converts 12V to the std 120V and the Max current rating says 30A at 120V....Doing math is this legit? Sounds too good to be true around 3kw? Can I use it backwards as I intended, and multiply the voltage? The largest breaker was 30A, are breakers rated in RMS?
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Yes your original analysis is correct. You can use that transformer in reverse to obtain a lower voltage at higher current.

    Static inverter transformers have different saturation characteristics from linear ones since they are designed for bursts of high current. So I would derate the transformer to 75% at best. Remember that the 15-0-15 you measured was off load.

    Again yes, a voltage multiplier would work to obtain a higher DC voltage, but you will probably have trouble sourcing the necessary high current/capacity condensers. Ex computer grade may be the way to go.
     
  4. chunkmartinez

    Thread Starter Senior Member

    Jan 6, 2007
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    By condensers, do you mean capacitors? I have two large rubycon 80v 8200uF caps, would those work?
     
    Last edited: May 9, 2013
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Yes condenser is an old name for capacitor.

    You will need more than two capacitors if you are using a voltage multiplier, more again if you are opting for split (±) supplies.

    Further you are talking about high amperage supplies. Capacitors for these need to be rated for for the working current as well as the capacitance.

    Specify your target requirements more fully.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    A ±40V supply would give a maximum power at clipping of near 100W into an 8Ω load.

    The subwoofer could deliver near 200W into a 4Ω speaker with that voltage which would require each power supply voltage to deliver about 3.2A average DC current. That would require some huge capacitors for the doubler. I measured about 40,000μF required in my LTspice simulation to give a ripple voltage of about 1V as shown below.

    Voltage Doubler.gif
     
  7. chunkmartinez

    Thread Starter Senior Member

    Jan 6, 2007
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    Is the insanely large capacitance only an issue with the voltage multiplier idea? Can I go another route? I bought a switch mode voltage booster(simple to-220 case with a few outside components)before whcih I think handles 3 amps, and could boost upto about 70V. Could I use two of them per IC to boost each rail(each 15v rail to 50- or more volts)? This would give me a total of 6 amps capacity I believe...and the parts count is nothing pratically and small. I cannot find another one of those simple switch regulating boosters anymore though..Maybe I can go the inefficient route of a normal voltage booster? Since my transformer may be more then I need.

    If the transformer is truely as powerfull as I think it is I want to build a 6 channel amp, 1 class D 900w channel(subwoofer), and 5 100w channels...It sounds crazy but I have been thinking about it.
     
  8. chunkmartinez

    Thread Starter Senior Member

    Jan 6, 2007
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    After thinking about it I am confused on what I should consider my limits of the transformer is? Is 120V for AC mains an RMS number? How should I figure this out? 120v X 30A *.75(% derate)....Or Do I divide 120 by 2 and multiply by .707 then multiply 30A and then take assume 75%?
     
  9. crutschow

    Expert

    Mar 14, 2008
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    120VAC is RMS.
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    The RMS values are the values of DC that would have the same heating effect.

    So to a pretty good approximation the RMS current available from a transformer equals the max Direct Current available from a simple rectified supply based on that transformer.

    Let us say you have a guaranteed 1500 watts from the transformer at 24 volts after stepping down. This translates to 62.5 amps RMS.

    I don't know how many amps you intend to supply, but it would be safer to supply each from its own bridge rectifier/ capacitor filter.
    This way any failure would not be passed down to destroy other amps connected to the same supply.
    Further it is much easier to avoid interaction via supply line modulation that way.

    If you like you can regard the 24 volts as a 24 volt AC main.
     
  11. chunkmartinez

    Thread Starter Senior Member

    Jan 6, 2007
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    Oh so that's why schematics show different filter/bypass components per ic/amp.

    I guess I just didn't want to under utilize this large of a transformer since a new one runs aloot of money.

    What do you all think? What other methods could I use to boost voltage? Are "simple switchers" impractical? If you don't know what I mean I can explain...basically a simple smps in a to-220 case that can supply 3 amps a peice and boost upto 70v I believe(it requires maybe 5 components aswell externally)...I figured maybe I could use multiples...Man I really need to reed into smps design lol...
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Certainly a switcher is an efficient way to boost the voltage if you can find one with a sufficient voltage and current rating.
     
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