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Thread: Pc psu question

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    Pc psu question

    Hi everyone. Can you help me understand how can the pc psu which is 550 watts can give about 30 amps at 12 volts and the transformers inside are so small. How can it deliver current that high. Can we build a high current psu without using huge transformers.
    Thank you

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by Manowar1985 View Post
    Hi everyone. Can you help me understand how can the pc psu which is 550 watts can give about 30 amps at 12 volts and the transformers inside are so small. How can it deliver current that high. Can we build a high current psu without using huge transformers.
    Thank you
    Short, non-technical answer: the size of the transformer is inversely related to the frequency at which it operates. Computer PSUs (almost exclusively SMPS) operate at tens or hundreds of kHz. Why they're not more popular with amp designers? No definitive answer on that one... other than it's not 'vintage technology' and cannot be trusted

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by eschertron View Post
    Short, non-technical answer: the size of the transformer is inversely related to the frequency at which it operates. Computer PSUs (almost exclusively SMPS) operate at tens or hundreds of kHz. Why they're not more popular with amp designers? No definitive answer on that one... other than it's not 'vintage technology' and cannot be trusted
    Couple years ago I was called on to repair a Crate tube amp made somewhere around 2005-2010 I guess, that had an SMPS power supply. Sure, it was a little lighter weight than an equivalent amp that had a "normal" power supply. Strange feature: it had a spring reverb, not a digital one. Go figure... Lucky for the owner, and me, the problem was a dodgy output tube. Easy enough to fix, and within my abilities.

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    Old Timer J M Fahey's Avatar
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    As said above, working at 100/200/500 kHz frequency allows you to save a ton of iron and copper, capacitors are smaller and to boot voltage can be tightly regulated.

    One price paid is complexity.

    You already know the standard supply, :transformer>diode bridge>capacitor(s) ... period.

    This is the schematic of a PC ATX power supply.
    Somewhat old, only 200W:


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    Juan Manuel Fahey

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    So the high frequency of the switching psu provides more efficiency and we can get higher currents with small transformers.Right?So what I want to do is build a simple power supply that can output 12 volts dc and 100 amps(if possible) but based on the simple topology (transformer - rectifier - filter-regulator) I will need huge transformer with huge secondary coil. Can you help me please?

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Manowar1985 View Post
    So the high frequency of the switching psu provides more efficiency and we can get higher currents with small transformers.Right?So what I want to do is build a simple power supply that can output 12 volts dc and 100 amps(if possible) but based on the simple topology (transformer - rectifier - filter-regulator) I will need huge transformer with huge secondary coil. Can you help me please?
    Sounds like a car battery quick-charger. You could probably buy one of those cheaper than building from scratch, and add as many farads of smoothing caps as you care to. Plenty of those caps available reasonably cheap too - those guys with big boom boom car stereos made a market for them. Don't forget to stock up on fat cable too, available at your local welding supply company. Here's a handy guide:



    Just curious, what's the purpose of your huge 12V supply?

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    Last edited by Leo_Gnardo; 07-11-2019 at 10:08 AM.
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    Supporting Member Jazz P Bass's Avatar
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    12 volts at 100 amps!

    Can you say 'welder'?

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    OK. Listen. I would like to design and build any unusual project. I have very low Rds mosfets and can deliver very high currents at about 70 amps each at low voltage. So I considered building an 100 watt rms stereo amp at 12 volts . I will be using 1 pair of these mosfets in push pull mode per channel. By using output transformer 1 ohm drain to drain to 8 ohms I will force them to output many watts. This is the purpose of the power supply

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    I would rethink the design. There are many reasons why SS amplifiers use higher voltages. 12v at 100A to provide 2x100W output suggests a very inefficient path to take that will require a lot of design considerations. My first thought is that the output voltage swing will be so low with 12V DC input that you'll need to think about what speakers you'd be driving as the output load will need to be a fraction of an ohm to develop 100W.

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    Mick Bailey let's assume the worst case scenario. Let's make a random assumption that the maximum undistorted signal will have an amplitude of 2 volts. To get 100 watts rms at these circumstances we will need to get 200 watts of peak power. The maximum current that we will need will be 200/2=100 amps. In this worst case scenario let's assume that the output stage has many paralleled mosfets that can provide the 100 amps maximum current. The output transformer is such a transformer that can convert the fraction of an ohm load to 8 ohms. I think that it will work. Plus you won't need that many turns because the load impedance is so low that the inductance required to get the lower end of frequencies is also very low. The problem in this case is that the wire needed will be very thick or many thinner together

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    Old Timer J M Fahey's Avatar
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    Heīs using an output transformer which will take care of impedance and double available swing at each Drain (idle 12V , swing 0 to 24V).

    Looks like a 12V PA amplifier like those used on carts or pickups to sell "straight from the farm" produce.

    It can be done and has been done countless times, but itīs complex, expensive, an d sensible only if only supply available is a 12V car battery.

    IF you have mains available, and need to make 12V at high current first to make it work at all, save yourself A LOT of trouble and design amp for any standard supply voltage, including split supplies, and a suitable power supply as needed to give you those voltages.

    I have commercially made 12V battery powered 40W and 60W amplifiers with output transformers and MosFET output, but nowadays just build a switching supply, 12V to whatever is needed and feed conventional amps, which to boot sound way better.

    You seem to be aiming at 800W RMS or so ... believe me you will have a lot less trouble doing it the regular way, specially since you donīt seem to have a large battery as power source, so, why bother?

    Either use a conventional supply and higher voltage rails or use a 12V inverter.

    besides: who would design your 12V 800W Output Transformer? ... and then build it.
    Itīs definitely NOT an over the counter product.

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    Thank you Mr J M Fahey. But trouble is what I am looking for. I also plan too build the output transformer on my own. I have done this before and worked many times. My goal is not only the result but the process too, especially when it comes to unusual projects. Tube amps with non lethal voltages if you can recall. I also don't care if my way of doing it is expensive or troublesome. As long as it is NOT TOO expensive I am fine with it. The only thing I want to know is if it can be done. Thank you very much.

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    My apologies - I mentally skipped the bit about the output transformer. Doing things the hard way certainly has a novelty appeal.

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    Mick Bailey I totally agree. I always try to get my hands into everything. I always think like:"What If I do this or that will It work".

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    I built a hi-fi amp using this approach, but in the opposite direction. I chose a 1KV plate voltage and push-pull input driving a single-end output - the reverse of how everyone else does it. Lots of challenges, but it sounded spectacular. I had a problem with sourcing transformers and in the end stripped turns off a microwave transformer secondary to power it.

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