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Converting a Laptop Power Supply???

IIIBOOMERIII

Silver Supporting Member
Messages
1,590
I hope I am putting this in the correct section. In the past when I had technical questions I always posted here, but those were tube amp questions and mods. If this is the wrong section I do apologize.

I have a power supply for a laptop, I actually have a couple from dead laptops over the years. I have been looking at them and would like to turn a 12 volt 3.3 amp laptop power supply into a power supply for 9 volt pedals. I am hoping because of the over voltage and surplus current I can put a few capacitors to work to quiet the power and some resistors to keep everything in range for 9v pedal use, possibly around 1 amp. I am just guessing that 1 amp would be enough current to power about 6 pedals, possibly more.

Anyone have any ideas as to what values I should use for caps and resistors?

Thanks in advance.

2821390F-307B-4A56-AE79-03A3D194EBE6-13784-00001C81DA2A761C_zps893417d4.jpg
 

drbob1

Gold Supporting Member
Messages
29,552
Most laptop power supplies are pretty well filtered already, so I don't think you need caps. And adding resistors is just turning power into heat. A couple of directions you could go:

1. Build a small voltage regulator/power convertor. Basically a chip that converts the 12v into 9v.

2. Just use pedals that can tolerate 12v (most of them).

Your biggest problem is that, while 3a will likely power any number of pedals you can fit on a board (like dozens), having them all on a non-isolated power supply is just like having a big "One Spot", you get hum, noise and weird behavior from them interacting with each other. So you could do option 3:

3. Create an isolated power supply with multiple taps for 12v, 18v and 9v using individual voltage regulator chips and caps/resistors to isolate each circuit from the others (like the PP2+). Then you could run pretty much anything you wanted. Of course, it's a pain in the neck to build and you can get used PP2+ for less than $150 but if you have more time than money...
 

teemuk

Member
Messages
3,443
Those computer power supplies will also usually require some sort of "logic" input (commonly supplied by the motherboard) before you even get them operational. Something you probably have to figure out how to get across.
 

Promit

Member
Messages
2,481
Those computer power supplies will also usually require some sort of "logic" input (commonly supplied by the motherboard) before you even get them operational. Something you probably have to figure out how to get across.
You can see on the label that this one uses a plain two terminal barrel connection, not an Apple style smart connection (which I've seen almost none of outside Macbooks).

Personally I would be inclined to rig it up with a MOSFET voltage regulator but watch your power ratings.
 

Kyle B

Silver Supporting Member
Messages
5,222
I love the idea of reusing that PS instead of tossing it in the trash! I use a home-brew switching supply on my pedals as well. Have been for years and had zero issues with it thus far.

One problem with switching power supplies is that they often have a minimum load requirement to maintain their output voltage. Too small of a load, and they tend to run away (output higher voltage than listed). Like way higher sometimes, to the point of being able to fry whatever you might plug in. Moreover, with very low loads, they tend to be quite rich in high-frequency noise & harmonics of the switching frequency. I.e. 'BUUUUZZZZZZZZ' is a real possibility for you.

Your pedals will consume a tiny fraction of the possible 3 amp output. So you'll definitely be running this in the equivalent of a "no load" situation so far as the power supply is concerned.

To see if this is even feasable, you should start by sticking your voltmeter probes on the plug, and see how much DC voltage it's actually putting out. If it's 12V or very close to that, then you've passed the first test. (A voltmeter is basically a "no load" setup also)

Next, to see if you're gonna get mad buzz or not, just wire in a few diodes in series to safely drop the 12V to 9V, and plug in one of your pedals. Inconveniently, the PS you have is 'tip positive' and your pedals area almost surely 'tip negative'. So you need to flip the wires half-way through. I suggest you use 3 or 4 1N4001 diodes for this test (easily obtained... Use 1N4002, 4003, etc if you can't find 4001's...) That will take your 12V and drop it to 9 or 10V (depending how many diodes you use)

Like this:

PS shell (-) connect directly to Pedal tip. No diodes or anything else.
PS tip (+) ---|>|----|>|----|>|----|>|---- Pedal shell. (that's 4 diodes in series)

Please experiment with a pedal you don't care about just in case you mess something up!

If there's very little buzz or no buzz at all, and your pedal works OK, you've passed test #2 !!!

Post again if you get this far, and I'll give a simple schematic for a well-regulated power supply. If either test fails, post the results and we'll see about coming up with a "fix".
 

Kyle B

Silver Supporting Member
Messages
5,222
And adding resistors is just turning power into heat. ......

1. Build a small voltage regulator/power convertor. Basically a chip that converts the 12v into 9v.

A small linear voltage regulator putting out 9V will turn exactly as much power into heat as using resistors dropping to 9V. Just sayin' ;)

Of course, if you meant a buck regulator, that's a different story. But I would take the simplicity & suffer the heat of the linear in this case anyday.
 

CharlyG

Play It Forward
Silver Supporting Member
Messages
7,771
I use them for mutli effects, mixers and such that need 12v. They work great for that.
 

Keyser Soze

Member
Messages
1,472
My experience playing with with switchmode power supplies has been exactly what Kyle mentioned - a high frequency hash that is more trouble to eliminate than the value saved from buying something else.

Man I miss the old transformer based wall-warts...
 

CharlyG

Play It Forward
Silver Supporting Member
Messages
7,771
That's the first time I EVER heard anyone wishing for those ugly,heavy,BIG wall warts.
 

mark norwine

Member
Messages
17,229
Here's what I do / did.....

Simple wall wart...I think it's a 18V....feeds my board. Why an XLR? Because if I need more length, mic cables are readily available at any gig.

pedalboard-m-5_zpsa97d51f2.jpg


Under my board is a LM317, couple of caps and some buswires. It isn't very neat, I suppose.....but it works just fine.

pedalboard-m-6_zpsf389749d.jpg


Wires are soldered to the busbars and power up the pedals:

pedalboard-m-4_zps53d2860d.jpg



Easy - peasy....

pedalboard-m-3_zpsea965b8a.jpg


Simple, easy, and functional. And I have *zero* issues with my pedals sharing this common supply.

I would *think* that the OP's power supply would / could work in a system like this......The regulator + a well-placed tantalum cap should kill any hash, no?
 
Last edited:

Keyser Soze

Member
Messages
1,472
That's the first time I EVER heard anyone wishing for those ugly,heavy,BIG wall warts.

Really? My impression is that they were once quite popular in the DIY community, either for powering pedals or as a pair for running tube pre-amps (one to step down wall voltage to get 12 volts for the heaters then reverse wire another to step back up to ~250 volts SS rectified for B+).

Of course that was back when they were as common as sand on a beach. Now, for a pedal board, you are probably better off ordering a decent flat pack tranny from Mouser or similar outfits.
 

Kyle B

Silver Supporting Member
Messages
5,222
Here's what I do / did.....

Mark. I'm speechless. It is a thing of beauty :) :aok After I get severe parkinsons, you may fix my amplifiers for me!!!


I would *think* that the OP's power supply would / could work in a system like this......The regulator + a well-placed tantalum cap should kill any hash, no?
I'm thinking the same thing. Well, a tant and a ceramic IMHO... And "well placed" is certainly the key phrase, in't it? ;)

Though instead of a 317, I'm going to suggest a 7809. Two fewer components to deal with
 

IIIBOOMERIII

Silver Supporting Member
Messages
1,590
Thank you so much gentlemen. And yes I will take any and all wiring diagrams and schematics you can supply. Great ideas one and all! You guys rock!
 

mark norwine

Member
Messages
17,229
Mark. I'm speechless. It is a thing of beauty :) :aok After I get severe parkinsons, you may fix my amplifiers for me!!!

Thanks for the kind words, but save 'speechless' for when I invent something big. Or figure out how to turn lead into gold, etc. This is just a simple regulator circuit. Entry-level stuff!

Though instead of a 317, I'm going to suggest a 7809. Two fewer components to deal with

I had a couple 317 in stock so that's what got used, but I do agree with you and generally prefer to use fixed-value regulators. That said, I've yet to see a pedal that couldn't benefit from a we bit more voltage on the rails, so if I do it over again, I'll use a 7810....maybe even a 7812.
 

Kyle B

Silver Supporting Member
Messages
5,222
Thanks for the kind words, but save 'speechless' for when I invent something big. Or figure out how to turn lead into gold, etc. This is just a simple regulator circuit. Entry-level stuff!

Yes, the circuit is childs play. I was loving the workmanship. Well done ;)
 

Kyle B

Silver Supporting Member
Messages
5,222
Thank you so much gentlemen. And yes I will take any and all wiring diagrams and schematics you can supply. Great ideas one and all! You guys rock!


Schematic follows after you run those two "sanity check" tests. Make sure this supply will work for you before proceeding is wise thing to do.
 

Kyle B

Silver Supporting Member
Messages
5,222
LOL No harm. I was just too lazy to get a picture quite yet :)

Just one question... Instead of 100nF, can't he just use 0.1uF ??? (Yes I'm kidding... ;) )

Seriously though -- I didn't draw anything yet pending the results of the testing. If he found the voltage running away, a high-power low value resistor would need to be placed from VIN to GND (to load down the supply a little). And if he got massive 'BUZZZ' we'd be looking at alot more filtering, yes? :)
 




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