Linking Computer Power Supplies

[SINNISTER]

Hey Guys,

I have a Plan to power a Car amplifier with a few Computer power supplies,

I don't want to use a car battery and car charger, This is how i want to do it.

I want to link 4x 300watt computer power supplies together using there +3.5V rail which puts out roughly 25amps.

I want to link them together and have in total roughly 14V at about 100Amps,

But i know that a car amplifier can draw 100amps easily so i want to put it thru a 1 farad capacitor and then use the power coming out of the capacitor.

Can someone help me figure out a way to link all the power supplies together,

Thank you for your time

 

[(*steve*)]

You can't do this for a number of reasons

Primarily, 4 x 3.5V @ 25A does not give you 14V @ 100A (The best you'll get is 14V @ 25A or 3.5V @ 100A)

If 14V @ 25A was enough, you would have a further problem that placing these power supplies in series may be problematic.

I am also somewhat sceptical that an amplifier designed for a car would require a power supply rated at 1.4kW.

The alternative would be to use power supplies with a 12V output and parallel them, but there are problems with this approach too.

 

[SINNISTER]

OK, so if I had to put two power supplies together in parallel both 12V at 15Amps,

How would I go about doing this?

Thanks

 

[(*steve*)]

In theory, you simply connect all the +ve outputs together and all the -ve outputs together, however in practice connecting power supplies like this can result in one supply trying to supply all the load and then shutting down (leading to a cascade of power supplies shutting down).

There are other problems, and this document describes some of them. Note that the document referred to mentions some power supplies which *can* be connected in parallel, perhaps you should investigate them?

 

[SINNISTER]

Ok well that sounds like monster mission,

If I had to use two transformers in parallel would that work better>?

My plan is to rewire a Microwave transformer and then put them thru 2x 35amp Diode bridges and then into a Capacitor, now if i use two transformers and 1x 1farad capacitor, would that work better?

Thanks for all the help

 

[(*steve*)]

You would be better off with one transformer, and you will have problems with voltage regulation and ripple which the 1F capacitor won't really fix (well, it will help with ripple).

To get 12V under full load without a regulator, you may well end up with 20V off-load. The amplifier may not appreciate that.

A 12V lead acid battery has way better performance than the 1F capacitor. There's a reason the obvious solution is a lead acid battery and a charger.

As an alternative, get a power supply that's appropriatly rated. Building one is really non-trivial.

 

[SINNISTER]

So i had a genius idea, if i had to use a oild bath welder, that is able to produce 100amps, at about 20V could I be able to put some kind of reducer on it and get the voltage to about 12V and not necessarily 100amps, i know that more old oil bath Arch welders are AC not DC so would I be able to put it thru a Diode bride and convert it to DC?

I really need a power supply that will product 12 - 14.8V at anything above 30amps.

What do you suggest i do?

 

[(*steve*)]

Get a power supply that's appropriately rated. Building one is really non-trivial.

Maybe someone else can come up with a good suggestion.

 

[poor mystic]

It's hard to know what to suggest here, because as Steve says, this is a big job.
The big transformer you have isn't suitable either... if yu use it you'll end up making something (a transistor?) very hot as you dissipate something like 250W of power stepping the voltage down. Consider, many domestic kettles consume only 4 times that power.

I just looked at google and found 12V 30A power supplies at less than $50. If you were to build your own I doubt you could get close to that cost, and I have to say that on this occasion "google is your friend".

 

[SINNISTER]

Remember Im from South Africa, and things here cost alot more, thats why i opted for the "build something route" You see a 12V 30a power supply, will do the trick to power it up, but once you start putting the music louder and start drawing more current the power supply is going to go into over load mode and shut off.

I have found a person that used a microwave transformer and rewound it and it worked perfectly for up to 1000watts ofcourse it generates alot of heat but you just need a good cooling system,

He had a small capacitor than a 1 farad, but one of his friends used a 1 farad capacitor and it worked perfectly.

Why would it be so hard to build two transformers from Microwaves and put them into One 1/2 farad capacitor and then draw from the capacitor? a transformer is able to give alot more than a switching power supply, and yeah its bigger and much heavier, but its do able,

heres the link of the guy that built a custom 12V 30A power supply.
http://www.users.qwest.net/~ptaylor/Electronics/MOTpowersupply/MOTPSpage.html

 

[poor mystic]

Hi Sinnister
I suppose if you can rewind a pair of transformers for 12V 30A+ working and do it safely that's up to you.
The thing to watch with huge capacitors is huge inrush currents. If you're on 50Hz mains you'll have 100 pulses of current every second; these pulses will come as current spikes, and the bigger the smoothing capacitor the shorter and heavier these spikes will be. Huge capacitors require very high current diodes.
Exactly what currents the diodes will be called upon to handle should be investigated by simulation.
A lot of people would say that you'd be better to aim for an over-voltage from the transformer and soak up the ripple in some kind of regulator, though once again the regulator will be a high-current device. Perhaps you could run a series of simulations to find out how much a satisfactory solution would cost in the various proposed designs. (The simulators on the web nowadays are so much easier to use than the old methods of graphing and estimating. Boy that was hard work!)

 

[SINNISTER]

Haha, I can Imagine,

Awesome, I shall put together a few simulations, Do you know of any good online ones?

Also i wanted to ask, if say i had to over shoot and get say 20V out of the power supply at roughly 30amps, Are there ways to bring the voltage down?

Thanks so Much

 

[poor mystic]

Hi again! (confusion - my post disappeared)
If you can rewire a pair of transformers safely that's up to you.
I suggest you use one of the circuit simulators available free on the web (I like Falstad's one though it has some issues with components being drawn backwards) and get an idea of how the circuit will behave. You will notice that the greater the capacitance used in smoothing, the heavier and shorter will be the current spikes supplied to it by the transformer. These current spikes must be conducted through the diodes, which need to be big enough for the job.

I'm sure that for audio usage you'll need some sort of active electronic regulation.

 

[poor mystic]

About regulation:
You want to regulate your supply. That's what it means to "bring the voltage down". When you needs 12V from a 20V supply you end uo using only 12/20 of the power. That's uncomfortably like half the power gone. For audio usage that power needs to be regulated out. The outcome of these requirements is that you'll need a 300W transistor for the job. Go google.
The 20V transformer is really not an option.

 

[SINNISTER]

Thanks Poor Mystic.

Im still not to sure on what route to take,
You see Im more a mechanical guy and Electronics Kinda scare me due to it being a new and unknown area for me, thats why i went searching for your knowledge.

Ok, so If i had to scrap all ideas and aimed for a higher amperage at about 14.8V at say around 50amps,

what other alternatives do I have? surely there is a way, there must be many uses out there that need a higher amperage at 12 - 15V.

Thanks for your time, and Im learning a lot I promise

 

[poor mystic]

Hi Sinnister
I could have suggested that you go switch-mode but that's no beginners project at all. If you want to go that way I'm sure there are many good books, all of which you will need to read.
In my opinion the basic problem is this:
If you need a thousand watts say out of a 250V supply it's no drama... that's 4 Amps. Try the same with a 12V supply and you draw more than 80 Amps. That means much bigger conductors, higher costs, less consumer enthusiasm.
Generally when people find themselves considering as much current as you require they say "No". They migrate to a higher voltage, because the equipment is much cheaper to build and lighter to transport. That's why there is so little high current 12V equipment.

 

[donkey]

hi i saw this post and had to ask, what stereo are you using maybe we can figure out the exact transformer needed from there

 

[kwoolsey94]

You can't do this for a number of reasons

Primarily, 4 x 3.5V @ 25A does not give you 14V @ 100A (The best you'll get is 14V @ 25A or 3.5V @ 100A)

If 14V @ 25A was enough, you would have a further problem that placing these power supplies in series may be problematic.

I am also somewhat sceptical that an amplifier designed for a car would require a power supply rated at 1.4kW.

The alternative would be to use power supplies with a 12V output and parallel them, but there are problems with this approach too.

He is right if your wanting to power a car amplifier your best would be to get a adapter set up and power it coming out of your wall.

 

[SINNISTER]

Well the Amp has a 2x 20Amp fuses, so I take it it can Handle 40Amps, But It could Handle a few seconds of more Power.

I need roughly 40Amps. I spoke to a guy that used a Encom 12V power supply to power an 1600Watt Car amp at home with 2x 12" subs and He pumped that system Hard, and Never did the PSU go into Protection Mode,

Im not sure about what Power supply it was, just that its a Encom 12V PSU,
Does anyone know anything about this Power supply, It would really help

Thanks

 

[poor mystic]

Hey Sinnister
Here's what I think is your best course.
Get the biggest power supply you can find from at least 12V and up to about 14V, put a "12V" lead acid battery in good condition across it, and run your stereo from the combination.
Monitor the battery using a permanently connected and often-consulted analogue meter movement across the battery with limits marked on the dial.
Switch the power supply on/off manually (between songs).