How to design a 12V 2A power supply

I bought a stereo amplifier on ebay for $7. It's a complete amp except for a housing. I'll make my own housing. It's made to be used in a car. It has output of 15W per channel.

Instead of buying those cheap computer speakers which seem to never last more than a year, and dont have very good sound anyhow, I want to connect this amp to some quality bookshelf speakers that I already have.

But I want to use this in the house. It requires 12VDC at 2A. I'll have to find a 12V (Or?) power transformer, and a bridge rectifier, plus a capacitor and of course a switch and power cord. Pretty basic....

But do I need a transformer with higher voltage? I know there is some voltage drop from the rectifier. And what size cap should I use? Do I need anything else?

HOWEVER, I'm wondering if I already have what I need, if I take the power supply out of an old (early pentium) computer and just use the 12 volts from that? I have several old computers that are too slow for modern use anyhow, so if I can recycle the power supply, I will, and it wont cost anything. But do they output 2amps or more?

Lastly, do they sell 12VDC wall transformers that are rated at 2A? That too would work and be easy to connect. I have seen them rated at 12VDC

1A, but never seen 2amp ones.
Reply to
boomer#6877250
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Or pick one up at goodwill, or an old laptop supply or a router power supply or a garage sale or..or..or. If you want to learn how to build a power supply, go spend a lot more on parts. If you wanna listen to music, buy a wall wart. Stop by and I'll give you one or five...

Reply to
mike

If you're brave and can wait a while:

And yes, you can buy 12VDC 2amp wall warts off the shelf, or on eBay.

Reply to
bitrex

that will get you an unregulated psu with ripple, probably good enough but no guarantees.

You'll want a 9v transformer & lots of capacitance.

Quick look at a random computer psu... 12v at 17A. If you're using a desktop pc you can just plug it into that.

Yes, 2A and more. Cheapo Chinese ones have a habit of being capable of a fraction of their claimed rating.

NT

Reply to
tabbypurr

I don't remember what would be a typical Amp rating for those early Pentium power supplies but they're certainly more than 2A. I occasionally use old computer PSUs for other purposes myself.

Reply to
Pimpom

Then it will probably operate on ~9-15VDC -- though whether or not it will actually deliver 15W into a load over that range is questionable.

The power supply out of a PC is typically:

- physically large/bulky

- noisey (i.e., has an internal fan)

- contains multiple outputs (i.e., a large cable harness)

- *might* require a load on some other voltage

Find a "brick" for an LCD monitor (many LCD monitors are DC powered -- no "AC power cord" into the monitor itself). These are typically 12VDC @ ~3-5A. They are regulated power supplies (not just a xformer and rectifier) so their output is more stable with load variations. One of your neighbors probably has a "dead" monitor lying around that likely has a viable "brick".

Either find a mating connector for the DC output to be fed into your amplifier *or* just cut the connector off and wire directly to the amplifier. Verify proper polarity with a VOM :>

If you object to a "second box" (in addition to the one you will be creating for your amplifier), consider packaging the "brick" inside the amplifier's enclosure.

The one complication will be adding a power switch. Ideally, that would be located in the AC mains before power is fed to the "brick". But, that means modifying a (modular) power cord as it enters the brick.

Or, plug the power cord into a switched outlet strip alongside your computer (so the computer, brick-powered-amplifier and any other peripherals all power down with a single switch).

Reply to
Don Y

A car battery on charge with the alternator running is typically ~14v so almost any laptop power supply of the 15v nominal output ought to do. Most are good for 3-5A. Newer ones are 19v which is pushing it and the amplifier might not like that much extra voltage.

You won't get 15W out if it is on a true 12v supply more like 11W and distortion will rise rapidly at about half of that rated power. (assuming it does genuinely output 15W when on a car battery)

Worth looking up the chip used in the amplifier and capacitor ratings since with a bit of care the amplifier might operate with much lower distortion if powered with a higher voltage than in automotive use. You might need extra heatsinking or just be gentle with the volume control.

Yes. But buying or scrounging a redundant ex laptop PSU with about the right output voltage would be my choice. Just get the polarity right!

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Regards, 
Martin Brown
Reply to
Martin Brown

I have found most of the newer ones to be 19 volts. This is quite a differe nce. If you put 6 more volts on a 12 volt line whatever it is feeding dissi pates twice as uch. If you double the voltage it dissipates four times as m uch, not twice.

Of course the bbrick can power a 7812 regulator, I think they still make a high current type, and if not there is always the old pass transistor.

But getting to a wall wart, I get the ompression that ratings are not what they once were. If you see 12 volt at 2,000 mA that might not really mean i t will put out 12 volts at 2 amps. It may just mean that the maximum curren t is 2 amps, at whateever volotage, not 12.

However, there are alot of reasons manufaturers go with wall warts, nott th e least of which is that it gets the UL and EC out of their hair. All prote ction is supposed to be done in that box so you don't have to worry much ab out even fusing, let alone ground faults and if the thing falls into the oc ean how much plankton it might kill. ASll of that shit is completely out of your ballpark, already taken care of. A scheap as they are it is no wonder that so many manufacturers like to use them.

But mindoing the UL etc., which is not actually a regulatory agency, it is an outgrowth of the insurance companies, thus the name "Underwriter's Labs" . If your device is not UL listed, if it causes a fire that could give the insurance company an excuse not to pay off. If the power cord comes from th e wall directly into your device, you have to think about what happens if t his thing is run over by a truck, or dumped into a hot tub or somehow the c ase gets cracked and it causes an insidious ground fault which may be fatal . With a walll wart all this is out of your hands. And actualy that does no t mean they are al safe, not long ago there were these really cheap USB cha rgers that plugged right into the wall. They were made in a slave labor cou ntry (they really aren't but...) and pretty much there is nobody to sue, bu t the distributors have something to lose. But that is the exception, I hav e had these things apart and usually they seem pretty safe, what's more fro m what I can glean from the circuitry, it would be a very strangee and rare failure mode to make them put ut more than they say, which is of course a nono.

Now if you ust go get an old brick from a printer or something, it might be too much voltage but after the brick you are covered except in realy wierd situations, it will probably be 18 or maybe 24 volts. Do your calculations on the disspiation of your regulator or pass transistor and give it enough heatsink.

Fifteen watts per channel, if measured right (please not again with that) m eans it is running BTL which yields 9 watts into eight ohms and double that (ideally) into four. If you have old vintage speakers, that extra currrent capability might come in handy because some of them are a bit on the induc tive side. If it is not class D, 2 amperes just barely makes it really. But then you are not stting up for a rick concert... I think.

Reply to
jurb6006

Yes, you can buy a safe one from Digikey for about $15 qty 1. Eg. Phihong PSAC24A-120L6. Higher currents too- up to 3A from several vendors, for maybe $5-8 more.

Or roll the dice and get one offshore sans (non-counterfeit) approvals for about $3.50 including shipping. Risky, y'know $20 is $20 and everything we do has a certain amount of risk.

--sp

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Best regards,  
Spehro Pefhany
Reply to
Spehro Pefhany

Power amps almost never have a regulated supply. The amp itself can be seen as essentially an AC regulated supply, tracking an input.

By definition, an audio power supply drives a load which might at any time consist predominantly of any audio frequency.

Unspecified regulated switch-mode supplies that feed such a load will quite likely behave inappropriately when the load frequency interferes with its control loop.

Much better to use a simple unregulated supply, and let the amp "regulate" it.

Clifford Heath.

Reply to
Clifford Heath

Old computer power supplies have 12V that is not regulated. It's a ratio, more or less off the regulated 5V supply. If the 5V is unloaded, there isn't enough flux in the core to supply the required 12V current. One solution is to load the 5V to the point that the 12V doesn't droop during peak loading. Get a wall wart and be done with it.

Reply to
mike

Yeah, prices start at $3 online, but the cheap ones may be junk.

eg:

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Reply to
Jasen Betts

15+15W out from 13.5V @ 2A in is most unlikely, but ISTR car audio Watts aren't the same as engineering watts.
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Reply to
Jasen Betts

They are "engineering watts" but because you don't understand the issues invloved, they may not measured as _you_think_ they should be measured. The purpose of the amplifier isn't to heat the interior of the car.

Reply to
krw

Best to stay away from wall warts off eBay. Some of them are seriously dangerous, and you won't know until and unless it gives your a bad shock (if it kills you, you also won't know, except perhaps briefly).

Sylvia.

Reply to
Sylvia Else

No problem at all. Just connect the DC outputs of two 12 Volt, 1 Amp to a common bus, and drsw poert from that common bus. Add a reasonable capacitor bank for output voltage stabilization. We have designed, built and sold 12 Volt, 5 Amp power supplies, based on this concept. Hope that helps.

Reply to
dakupoto

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