12 volt power supply

[billcreller]

This may not belong here, but I have to build a 12 volt power supply to power the speakers for my computer.  These are JBL speakers with great sound, and were originally powered from an old CRT monitor.
  I can buy a 12 volt filiment trans from Antique, and a rectifier bridge, but not sure how to filter the 12 volt output.
Any recommendations ?
  They require 1 amp, which is labeled on the input.

[John]

Hi Bill, this looks like what you want. Be sure to read the spec sheet on the 12v regulator (if you're using one) since it will probably need a heat sink. Ignore the 230V PT of course.  I think Doug even has that regulator in fact.


Hope this helps!

[sluckey]

Radio Shack has all the stuff in that schematic that John posted, if that's more convenient for you. I'd do a bridge rather than individual diodes.

[EL34]

Bill,
I have the bridge or diodes, 7812 voltage regulator and caps in stock

[billcreller]

OK !! thanks for the responses !  I had a 12 volt heater trans, but gave it away after having too much hum in one of my projects.  I'll figure something out & get the bridge & regulator.   I can likely make a heat sink from aluminum if it needs one, or, I have some heavy duty types too.

BILL

[eleventeen]

Yes you can build such a thing. I would first look for a big fat wallwart at a thrift shop. A wallwart can do 1 amp if they are of the somewhat bigger variety. The 7812 can do 1 amp, but you'll have to heatsink it pretty well, more if your input voltage is much over 18+ volts. 3-terminal regulators are magnificent things to be sure; But when you use them near the top end of their current ratings, it is somewhat of an art getting enough input volts (probably needs dead minimum 15) so that they do not drop out without getting TOO many input volts, because the input volts need to be dumped through the device at the current you wish. If you need 1 real honest-to-goodness amp and supply the 7812 with 22 volts, that's 10 watts the 7812 has to dump. That implies a serious heatsink.


The real beauty of 3-terminal regulators is when you need very low power and you can just slam them into a circuit with (only) their own tab as the only heatsink. Easy as pie. Once you start to get close to their ratings, they need heatsinking.


I would also not rule out an open-frame Power One or Condor (or any of a dozen other brands) power supply from ebay. You find these for sale for $5-10-15 all the time. They are pretty bulletproof. The problem is that the cost of all the parts you need plus the assembly effort for one you build from scratch is generally a lot more than just buying one of these things. Seriously, can you go buy a suitable transformer for $10? Never mind the caps, the regulator, etc etc. Search ebay for "Power One 12 volt supply" or "condor 12 volt supply". There are loads of them.


http://www.ebay.com/itm/Condor-Power-One-HAA15-0-8-A-DC-Power-Supply-/180983770218?pt=LH_DefaultDomain_0&hash=item2a23792c6a


Finally, If you decide to build the thing yourself, suggest you get a 7812 in a TO-3 case, the type that looks like a 2N3055 transistor, versus a TO-220-cased one. Generally easier to heatsink.

[Ken Moon]

What am I missing here?

I've always used a 16-0-16/3A filament tranny, like the Hammond 167M16, to make a 12V supply using an LM7812.

Doesn't the LM7812 need an input of 14.5 to 24 volts to put out a regulated 12V?

 

[sluckey]

That Hammond 167M16 is nice but kinda pricey for a 12V power supply for computer speakers. You can get a 12.6VAC/3A transformer from Radio Shack for $12.

[PRR]

> Doesn't the LM7812 need an input of 14.5 to 24 volts to put out a regulated 12V?

Yes. Or rather, it needs 2V-3V excess to do its job; for 12V out that means well above 14V.

I shudder when I see 6VAC "regulated" to 6VDC... this often falls short. The meter may even read 6VDC but there's narrow dips at the troughs of the AC ripple. 12VAC to 12VDC is a "maybe" thing.... allowing for diode drops you maybe have 15VDC raw, 3V in excess.... but the least wall-dip or mis-compute and you fall out of regulation. Nevertheless it *often* "works"; small transformers have 20% regulation so at lighter loads a "12VAC" may run over 14VAC, and some folks don't notice that their "regulated DC" has dips in it.

16VAC seems high to me. Especially 16-0-16 which avoids one diode drop so you are past 21V DC, and have a good 9V to lose in the regulator. It will regulate good as long as you can keep it from over-heating. 9V at 1A will need a non-trivial heatsink.

[Ken Moon]

Good stuff 

A 167N12, which is 12.6-0-12.6/4A might be a better choice for 12v with a 7812, then.

I've tried a couple of wal-warts, but they must have been bad ones,because they created a low volume but irksome tick-tick-tick.

I've noticed that wall-warts are now much lighter than they used to be, and I assume that's because they're transformer-less, it would be intersting to know if there are performance differences.

A long time ago, I saw a pic of the guts of an amp, where someone had put in an entire Tube Screamer and wall wart with duct tape and connected it using speaker wire and Scotch tape.  The controls were inaccessible inside the amp. I guess that would be "factory preset for optimum tone"