i need about 100amps at 12 volts
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 26th November 2009, 01:48 PM #1 anwaypasible   Banned   Join Date: Aug 2007 i need about 100amps at 12 volts i am trying to hook up a hifonics 1,000watt RMS amplifier to a subwoofer in the house. i am trying to come up with a cost effective approach at finding the amperage needed. i've thought about buying a car charger with a 200amp jump start feature. i've thought about combining computer power supplys together to equal the amperage needed. maybe someone can give me a parts list to build my own power supply with a soldering gun and PCB board. i know that combining computer power supplys is an escape goat.. but i think with the amperage that i need, this option isnt the best financial choice (probably the easiest choice though). so i started looking into building my own AC to DC convertor that would supply 12volts at 150amps. i learned that the start of the process comes from a step down transformer that will reduce the 110volts down to 12volts. although i cant find any that will pass along the amperage that i need for this massive amplifier. the amp is said to be 800watts at 4ohm bridged 900watts at 3ohm bridged 1,000watts at 2ohm bridged i intend to use the amp at 3ohms.. so: 900watts @ 12volts = 75amps if i use computer power supplys.. i would connect the 12volt wire with the 3.3volt wire to make 15.3volts (something closer to the car, and ideal for the amplifier) 900watts @ 15volts = 60amps now that would be the amount of amperage needed if the amplifier was 100% efficient.. so i need extra, which is why i said a home-built 150amp 110v AC to 12v DC convertor would be good for the amplifier's thirst do i hunt down a 110v to 12volt transformer.. get some rectifiers and use a kinetik powercell as a stabilizer? (and if so, where do i find the parts and how do the legos go together?) the output should be massive.. but i will be using an equalizer to give myself about 80-90dB @ 40-50hz and then a substantially solid boost at 10-30hz (which is where the power is going to go me thinks) i'm not an electrical engineer, but i do own a soldering iron and some solder.. i would be willing to follow instructions on how to build my own for a fraction of the price for something already pre-built / intended for something else. besides, i think the 200amp automotive jump starter/battery charger on wheels go dead after only 3 tries on the key.. so therefore, the unit would drain too fast despite being plugged into an AC outlet. anyone with some suggestions?
Steerpike
diyAudio Member

Join Date: Jan 2006
Location: Johannesburg, South Africa
Quote:
 Originally Posted by anwaypasible i intend to use the amp at 3ohms.. so: 900watts @ 12volts = 75amps
It will only draw 75 amps intermittently if operating on a music signal. The continuous average is probably 20% of that, or less.

Quote:
 if i use computer power supplys.. i would connect the 12volt wire with the 3.3volt wire to make 15.3volts
I don't think you can do that - all outputs of a PC supply share the same ground, can't series wire them. And the 12V supply has a WAY lower current than the 3.3v output.

Such a power supply is not easy to build, nor cheap. Would have to be a switched mode supply, a 100A 60Hz transformer is going to be humongous and heavy and expensive. (think of the size of old fashioned welding transformers)

An easy solution is to use an automobile battery or two, and float charge it.
That way, you use it up for a couple of hours, and recharge it the rest of the day, with a low current charger (10 amps say).
The continuous current draw of the amp is much smaller than the maximum, so the battery will not drain at the rate it would supplying 100 amps.

BETTER solution, cheaper, easier: buy a 1kW sound reinforcement / PA amplifier designed to run off 110V ac.

In the old days before switched mode power supplies, this would be done with a rotary converter: get a big old old car/truck alternator, and drive run it from a 110V AC induction motor.
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Steerpike's Toybox

 26th November 2009, 02:37 PM #3 cliffforrest   diyAudio Member   Join Date: Sep 2004 Location: South Coast, UK Car battery and mains charger! Does it all!
 26th November 2009, 02:48 PM #4 anwaypasible   Banned   Join Date: Aug 2007 i had no idea that a transformer to fit my needs would be so large/expensive. i have two power supplies.. one is 460watt and the other is 375watt. the combined amperage on the 12volt rails equals 62amps. that 1kW sound amplifier would probably cost at least \$600-\$800 i dont see why i cant wire the 12volt rail to the 3.3volt rail in parallel.. they are both active and functioning at the same time when the computer is turned on and used normally. there is an idea to use an AC motor belted up to some alternators on a piece of wood.
 26th November 2009, 02:51 PM #5 Bill Fuss   RIP   Join Date: Oct 2005 Location: Central Florida It can be done but it would be more prudent to buy the correct amp. What you will be doing is starting with 110v, lowering it with a trafo to 9v, rectifying it to 12v DC, and filtering it to a lower ripple percentage, feeding it into the amp, where the 12vdc is converted back into a higher dc voltage to run the amp internally. Off the top of my head the amp would actually run on around 60v to be capable of the rated power. You're talking about a massive power supply with a 1.5KVA 110/9v trafo and industrial rectifier diodes, and huge capacitors. \$500 easy.
 26th November 2009, 04:42 PM #6 DigitalJunkie   diyAudio Member   Join Date: Apr 2003 Location: Portland,Oregon I built a big 12V supply using an old microwave oven transformer,some 10awg wire,some 35A bridge rectifiers,and some big caps. I wound 2 secondaries on the core (after removing the original HV winding) and fed each one into it's own bridge rectifier,and paralleled the output of the rectifiers,added some caps for filtering,and called it good.
Steerpike
diyAudio Member

Join Date: Jan 2006
Location: Johannesburg, South Africa
Quote:
 Originally Posted by anwaypasible i have two power supplies.. one is 460watt and the other is 375watt. the combined amperage on the 12volt rails equals 62amps.
Ok, if you have the figures. The ones I've encountered all give big amps on the +5 and +3.3 V, but about 1/3 of that on the 12V rails.
You have to be careful though, parallelling supplies, since they will each put out slightly different voltages, and then fight each other trying to force the output to what each one wants. That will result in current circulating only through the power supplies, wasting a lot of energy.

Quote:
 i dont see why i cant wire the 12volt rail to the 3.3volt rail in parallel..
Because the (-) of the 3.3V rail and the (-) of the +12V rail are internally the same thing; you can't wire the (-) of the 12V to the (+) of the 3.3V
You could do it with the -12v and 3.3V supplies, but I think -12V rails are even lower current capability than the +12V.

A possible solution - but it's not simple if you don't have a good knowledge of electronics - is to modify a computer power supplies feedback sensing, and reconfiguring its 'donut' transformer. Doing that can get the 5V rail to go up to +12V (or more) and still deliver almost the same current that the +5V was originally specified at.

A pair of microwave oven transformers used 'backwards' might work; backwards they'd only give about 6V out I think - hence the need for 2. But the smoothing caps still have to be BIG.
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Last edited by Steerpike; 26th November 2009 at 06:04 PM.

 26th November 2009, 06:03 PM #8 anwaypasible   Banned   Join Date: Aug 2007 i'm tempted to re-wire a transformer and check the voltage output with a multimeter.. but having no money and therefore no room for any accidents.. i am passing on it. i've just started reading about building my own AC to DC convertor and therefore i doing want the trouble of any errors i might encounter. i will eventually sit down with the brain power and financial resources to play around. meanwhile.. this thing looks like one bad mama-jama. i'd hook up a kinetik powercell to that and then let the amp suck amperage off the two of 'em. but hey.. if they can do it in a relatively small package (and under \$500) i'm sure i could too. i read about first winding and secondary winding.. the ratio and such to make it a step up or a step down transformer. i just dont trust my multimeter and i dont have money for a new one. nor do i have money for little things like electrical tape. so i'm just waiting and chatting about it.
 26th November 2009, 06:14 PM #9 paulb   diyAudio Member   Join Date: Jun 2001 Location: Calgary Here's a good article on some of the considerations for such a supply: High Current 13.8V Power Supply
anwaypasible
Banned

Join Date: Aug 2007
Quote:
 Originally Posted by Steerpike Because the (-) of the 3.3V rail and the (-) of the +12V rail are internally the same thing; you can't wire the (-) of the 12V to the (+) of the 3.3V You could do it with the -12v and 3.3V supplies, but I think -12V rails are even lower current capability than the +12V. A possible solution - but it's not simple if you don't have a good knowledge of electronics - is to modify a computer power supplies feedback sensing, and reconfiguring its 'donut' transformer. Doing that can get the 5V rail to go up to +12V (or more) and still deliver almost the same current that the +5V was originally specified at. A pair of microwave oven transformers used 'backwards' might work; backwards they'd only give about 6V out I think - hence the need for 2. But the smoothing caps still have to be BIG.
that is a cute little mod to reconfigure the donut transformer to make the 5v rail go up to 12volts and deliver almost the same current that the 5v was specified at.

see.. i figured if i do one +12 and one ground combined with one +3.3 and one ground, there would be a simple addition taking place.
i understand the added current from the +3.3 would only be a fraction (also known as 'only an extra 100watts that the powersupply is helping my amp produce').

your saying if i did the one +12 and one ground combined with one +3.3 and one ground.. WITH ONE POWER SUPPLY that the two rails are going to float/bleed/fight and then shorten the life of something?
(or are you saying that the combining of two power supplys will do that?)

i thought that there would simply be more stress on the filter caps and more stress on the step down transformer.. but nothing cross-eyed unless i started to mess around with the -12 or -3.3 or -5 rails (which i have ZERO intention of doing)

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