200amp 12v on the cheap

I'm a complete n00b to this kind of thing, so I'm sorry if this is a common question or somethin, I searched but didn't really find any non-insanely-technical answers.

I have two infinity amps, which combined draw almost 200 amps. I'd like to hook them up inside my house for about 2 months so I can break them in until I get the car to put them in. I'd like to hook up the amps inside my house, and everything is pretty much easy for me to do, except power these two huge amps. I need a method to power these, reliably, and without breaking the bank. Any ideas?
 
I can't think of a cheap alternative since 12V 100A SMPS cost approx. 1,500 euro here (that's why I'm developing a 120A unit :D ). You could use batteries and a charger but they will probably discharge too quickly and charge too slowly, thus they will get sulfated very badly in a short period of time.

You could also use a smaller and more affordable 12V 25A PSU with a 1F or 2F capacitor connected at its output, this will improve things a lot but may not allow for the desired volume.

And finally, due to the outrageous amount of hype routinely seen on the car audio world, I doubt that these amplifiers could even draw 200A of peak current during transients since that would imply almost 2,400 watts of peak power delivered to the load.
 

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Member
2005-06-05 12:52 am
Midwest
How did you plan on powering these in a car anyway? Typical car alternator is ~ 100A, the car wouldn't even run powering one of them if they actually used 100A per.

Redefine the real power requirements. There's no point shooting for the moon whether thinking "affordable" or not. Your two main choices are huge/expensive transformer derived power or smaller transformer with a switcher- which is noisier but "could" be significantly cheaper. Either are typically multi-hundred dollar projects or even more if a ready-made supply. I suppose there's another option, a gas engine belt-driven alternator(s) hooked up to a large truck battery- poor option IMO.
 
A car audio amplifier rated at 1200W rms with a proper load may actually draw 200A peak for approx. 10ms periods during music transients. However, due to the pulsatile nature of audio signals, average current consumption is going to be much smaller, maybe as low as 30A or 40A just below clipping. Batteries, alternators and capacitors can cope quite well with such a high peak-to-average ratio but solid state power supplies doesn't.

In car audio shows, I have been able to power threee amplifiers rated at 1080wrms, 530Wrms and 320wrms respectively (I checked the ratings by myself) with just a 14.4V 72A power supply (built by me) and a 1.2 F capacitor. Even when allowing for some clipping, average current consumption was between 40A and 72A depending on the kind of music used (the PSU featured smooth limiting without shutdown). Note that theoretical maximum peak current consumtion for that system was in excess of 300A

Actually I had more trouble with the amplifiers themselves than with my PSU, since the former showed incapable of continuous operation just below clipping for 8 or 10 hours without additional forced air cooling while the latter was designed with such a though job in mind.
 

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Member
2005-06-05 12:52 am
Midwest
raintalk said:
Get's me thinking a quick short term solution might be a electric motor and an alternator bolted to a board, belt driven, with a regulator and cap.

Yes that could work but there are a lot of very significant variables. It requires a fairly hefty motor, with right drive ratio (pulley size). Regulator is unnecessary as the alternator and battery are same scenario faced in a car. You'd need a cap though, but the cap (within reason, a few hundred dollars' worth) is not a sufficent buffer, it'll need a battery.
 
It's a clever and cheap idea but it won't work, at least not without current sharing resistors. Nearly all of these AT/ATX PSUs are not designed to provide smooth current limiting, in case of overcurrent most of them will just shutdown and the rest will enter chaotic discontinuous-mode operation (not destructive but noisy).

However, there are methods to provide current sharing between several PSUs, but slight modifications to the PSUs, additional circuitry and some SMPS knowledge would be required. The easiest method is based on sensing output current of each unit through a small sense resistor and adjusting output voltage accodringly by means of an additional error amplifier.
 

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Member
2005-06-05 12:52 am
Midwest
Stocker said:
Come on guys,

Go to a good secondhand store and get a stack of AT computer power supplies and parallel them all up.

POssible, but a bit more involved than that. The one(s) with higher set-point could bear all the load and the others could potentially shut down from what was (perceived by their feedback) as a continual overvoltage on one rail while the other (5V) dipped below minimal value as it kept reducing the switching to lower 12V to what it "thought" was it's own output.

You could use isolation diodes but they'd have to be beefy and there's inherant voltage loss and heat considerations with this high-amperage goal. If possible one might just cut the 12V feedback out of the loop and regulate via 5V rail alone, but anyone really competent to do so would not need the suggestion.

Plus, typical PSU is less than 75% efficient with output from only one rail used. Suppose it's 75% efficient, running on 110V you're needing (with some margin) over 30A circuit for this.
 
Surely you didn't think I was talking about just tieing the wires together and calling it a job! :dead: A bit of google searching will yield at least one page I have seen, complete with step-by-step and photos.

Whether you use something as simple as resistors or as complicated as error amplifiers with pass MOSFETs or anything in between, it is hard to get such high-current 12V as cheaply anywhere else.

2x 15A household service circuits = 30A combined, BTW. ;)
 

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Member
2005-06-05 12:52 am
Midwest
Stocker said:
Surely you didn't think I was talking about just tieing the wires together and calling it a job! :dead: A bit of google searching will yield at least one page I have seen, complete with step-by-step and photos.

Whether you use something as simple as resistors or as complicated as error amplifiers with pass MOSFETs or anything in between, it is hard to get such high-current 12V as cheaply anywhere else.

2x 15A household service circuits = 30A combined, BTW. ;)

Well.... you DID only mention the easy part, the devil is in the details.

2 X 15A circuits are not sufficient, IF we were to assume it's actually going to be a "supply" worth 200A. There's no margin in that, it might trip the breaker simply turning it on. Such a supply really should have a dedicated 50A circuit strung for it, and of course an electrical inspection to stay within code in many 1st world countries.

It's not really all that easy to use computer power supplies though, as you may find old low-grade AT supplies (common for that era because they never actually had to output in excess of <=150W for a "PC" box of that era) aren't such a reliable way to get the power and would end up being derated. Reasonably speaking if you were lucky you could get 10A per out of *good* old AT supplies, probably less out of the average one. With a lot of hunting, cleaning, and trial-n-error it could be done but it has more of the feel of a Mad Max emergency effort rather than a plan for a continual power source.

IMO, one of the best suggestions was already made by someone, an industrial battery charger.