Info on custom power supplies, please

[swinginguitar]

Been mulling over putting together my own power supply for a pedalboard, drawing some inspiration from the Spyder et al.

What I'd like to have is a couple of 9v outlets (maybe one with enough current to daisy chain several pedals, and then a coupl of isolated ones), a 12v, a 15v, and an 18v.

Can I do something like this: a power xformer that takes in 115v and puts out >20v, then parallel the secondaries of that xformer to several smaller xformers to get the various output voltages that i want, then in turn send the outputs of those secondaries to an appropriate rectifier/regulator/filter circuit?

Will a regulator such as an LM317 suffice for each of these branches?

Anyone done something like this? Any recommendations as far as other voltages i might want, such as 9VAC or a +9/-9 scenario? Any specific rectifiers or regulators?

[R.G.]

1. Go read "Power Supplies Basics" at http://www.geofex.com.
2. Never parallel transformer secondaries unless the manufacturer recommends it specifically.
3. You need to figure out how many and what kind of outputs you want. Then figure out which ones have to be isolated - and why.
4. Be sure the pedals you're inevitably going to hook up to those higher voltages won't be damaged by them.

[smallbearelec]

Many people like this Weber transformer for Spyder-like power supplies:

http://www.smallbearelec.com/Detail.bok?no=597

There are a couple of others in this section:

http://www.smallbearelec.com/Categories.bok?category=Transformers+and+Inductors

that are suitable if you want to work with small, individual transformers rather than one big piece of iron.

Regards
SD

[swinginguitar]

Great responses everyone! Thanks!

2. Never parallel transformer secondaries unless the manufacturer recommends it specifically.
3. You need to figure out how many and what kind of outputs you want. Then figure out which ones have to be isolated - and why.
4. Be sure the pedals you're inevitably going to hook up to those higher voltages won't be damaged by them.

2. Why? Just curious? Sounds like I will need a transformer for each run...paralleling the mains across howevermany outs i need?
3. i run the whole board on 1 powerall (2 amps IIRC) right now, except for a 15v wallwart. i need to calculate how many amps are being drawn and determine how many 9v taps i need. Some of the pedals pull a little more amperage than others and i'd like to have them on their own circuit, and a couple of digital pedals were interacting and making noise (short term i removed the offending pedal) i also wanted 12v and 15v taps to get rid of a coupla power supplies.

Edit: which rectifiers and regulators would you use?

[petemoore]

  Heres' one way.
  Get the regulators you need, and decide on the Weber or other AC voltage conversion transformers [larger box most likely].
   Use separate converter circuit and output for the pesky digital pedals and hope that works out, experience is that they're wierd about what power input they'll 'accept' to work, not whine, etc.
   Choose the box according to what goes in, for the higher voltages, starting with an AC secondary tap which is in the voltage ballpark [regulator takes a couple volts, and the AC to DC conversion equation applies].
  Larger voltage drops across a regulator = greater heat output /..?need for heatsinking.

[R.G.]

2. Why? Just curious? Sounds like I will need a transformer for each run...paralleling the mains across howevermany outs i need?

Paralleling primaries is OK. Paralleling secondaries, which is what you said, is really hard to do with out smoking things. I may have misunderstood what you meant.

If you're set on needing transformer isolated outputs, the Weber multi-output transformer is really good for this application, and much simpler to wire than many small transformers.

3. i run the whole board on 1 powerall (2 amps IIRC) right now, except for a 15v wallwart.

Are there problems with that?

i need to calculate how many amps are being drawn and determine how many 9v taps i need. Some of the pedals pull a little more amperage than others and i'd like to have them on their own circuit,

I always think tinkering for its own sake is a good thing, but is there some issue where it's not working well now? That's important, because not knowing what you're fixing may make it hard to tell if it's fixed.

and a couple of digital pedals were interacting and making noise (short term i removed the offending pedal)

Yep. The makers of digital pedals don't in general do a good job of designing power supplies for the insides of those pedals. Digital designers in general have little understanding of analog and power issues.

i also wanted 12v and 15v taps to get rid of a coupla power supplies.

Makes sense.

Edit: which rectifiers and regulators would you use?

Read "Power supplies basics". In general, any rectifier with enough volts and amps works at mains frequency. I like the small integrated rectifier bridges, the size of a 6 pin DIP and good for 100V and 1.5A.  Regulators depend on what you want. If you want 9V, 12V, 15V, etc. fixed and unchanging, use the 78xx series. If you want variable outputs, use the LM317.

[swinginguitar]

Paralleling primaries is OK. Paralleling secondaries, which is what you said, is really hard to do with out smoking things. I may have misunderstood what you meant.

No, you heard right, I was just clarifying why it couldn't be done....

Are there problems with that?

No, the Powerall works well, except for the aformentioned digital pedal issues. But, I wanted to rid myself of the extra wallwarts for my 15v run, and thought while i'm at it i could have a few isolated outputs and futureproof against some 12v stuff i wanna get.

I always think tinkering for its own sake is a good thing, but is there some issue where it's not working well now? That's important, because not knowing what you're fixing may make it hard to tell if it's fixed.

You said it - I compulsively tinker. I would prefer to spend 2x the money and 100x the time but learn something in the process than go buy a PP2...

Read "Power supplies basics". In general, any rectifier with enough volts and amps works at mains frequency. I like the small integrated rectifier bridges, the size of a 6 pin DIP and good for 100V and 1.5A.  Regulators depend on what you want. If you want 9V, 12V, 15V, etc. fixed and unchanging, use the 78xx series. If you want variable outputs, use the LM317.

I did - thanks. The data sheets for the 78xx imply that they extract upwards of 4-5v for their trouble - is it really so? I would need to supply ~20v to get 15?

In general, how many amps do I need for 6-8 9v pedals? (the powerall is 2 amps IIRC)

[swinginguitar]

R.G. - was it you who put a custom supply or something in a 6" steel stud?

How well does that work and where could I get a steel stud that doesn't have patterns stamped into it?

[swinginguitar]

Another musing:

Could you nit have, say, a 24v xformer to a rectifier, then hit an 18v regulator for your 18v tap, then do a dropping resistor and hit a 15v regulator, and so on for 12v and 9v? Similar to a tube amp power supply but with regulators for various voltages...

something like

+24v => rectifier => 18v reg => dropping resistor => 15v reg => dropping resistor => 12v reg => dropping resistor => 9v reg


One potential problem I see is with increasing downstream loads the upstream current may suffer (unless the regs help with that)....

[mattthegamer463]

Another musing:

Could you nit have, say, a 24v xformer to a rectifier, then hit an 18v regulator for your 18v tap, then do a dropping resistor and hit a 15v regulator, and so on for 12v and 9v? Similar to a tube amp power supply but with regulators for various voltages...

something like

+24v => rectifier => 18v reg => dropping resistor => 15v reg => dropping resistor => 12v reg => dropping resistor => 9v reg


One potential problem I see is with increasing downstream loads the upstream current may suffer (unless the regs help with that)....

You don't need "dropping resistors", and I wouldn't use a LM317 for anything except voltages that you can't find a fixed regulator for.  Use a transformer that has about 18VAC secondary output, rectify, use capacitors to smooth and you'll have something north of 18V.  Then just connect a 18V regulator, 15V regulator, 12v regulator, and 9V regulator to that >18VDC line. Bolt all the regulator tabs to a heatsink for heat peace of mind and you're all set.

[swinginguitar]

And just to heap another question on...

If I wanted say a 24V xformer, will a center tapped model do? Do you ground the CT or leave it non-connected?

[mattthegamer463]

Leaving it unconnected makes it as if it isn't there at all.  Just make sure to tape it off or something so it doesn't touch anything.

[swinginguitar]

You don't need "dropping resistors", and I wouldn't use a LM317 for anything except voltages that you can't find a fixed regulator for.  Use a transformer that has about 18VAC secondary output, rectify, use capacitors to smooth and you'll have something north of 18V.  Then just connect a 18V regulator, 15V regulator, 12v regulator, and 9V regulator to that >18VDC line. Bolt all the regulator tabs to a heatsink for heat peace of mind and you're all set.

You wouldn't get a full 18v out of an 18v xformer though would you (after you rectify and regulate...the 18v reg would need a few extra volts to extract for it's "stud fee")?

[mattthegamer463]

You don't need "dropping resistors", and I wouldn't use a LM317 for anything except voltages that you can't find a fixed regulator for.  Use a transformer that has about 18VAC secondary output, rectify, use capacitors to smooth and you'll have something north of 18V.  Then just connect a 18V regulator, 15V regulator, 12v regulator, and 9V regulator to that >18VDC line. Bolt all the regulator tabs to a heatsink for heat peace of mind and you're all set.

You wouldn't get a full 18v out of an 18v xformer though would you (after you rectify and regulate...the 18v reg would need a few extra volts to extract for it's "stud fee")?

It depends on the load.  Transformers deliver an AC voltage at a particular current load.  Transformers often say "18V at 300mA" or something, meaning that you'll get 18V if you had a resistive load that draws 300mA.  Ohms law says 18V/0.3A = 60 ohms of load.  Also, the voltage rating is the RMS rating, not the peak voltage rating, so when not being loaded to the rated current the voltage can be quite a bit higher than the rated voltage, a guess would be 24V. 

To figure out how much current you're likely to use, just think of however many pedals, and their approximate current draw.  Typical 9V pedals probably do a maximum of 100mA and larger/tube pedals might do 250mA.  Higher voltage pedals probably do the same, since they have more power from the additional volts.  Since linear regulators are wasteful, the current in is the current out, just with less voltage.  So if you are using say 1A on your 9V regulator, and 500mA on your 18V regulator, you'll need 1.5A of whatever voltage from the transformer.

Also, I've never heard of a stud-fee.  Googling the term doesn't come up wtih anything, can you explain what you mean?

[swinginguitar]

LOL...it's what a dog breeder pays to have a male dog come in for a conjugal visit....

...I was just saying that the regulator extracts a "fee" for it's service i.e. a couple of volts, but making a really bad analogy....

[mattthegamer463]

Pretty bad analogy.  Basically, the regulator has a resistor inside it and a zener diode whose voltage is the same as the regulators output voltage rating; it is the component which sets this value.  So X voltage comes in, and Y voltage goes out.  The only difference is that the current going out is the same as the current going in.  Lets do some math;

say you have a 7812 12V regulator.  To draw 1A from it you would need:

12V / 1A = 12 ohms

a 12 ohm resistor (ohms law)

The power being output is V * I, so 12V * 1A = 12 watts of power.

The input voltage is 18V.  Due to the inefficient nature of linear regulators, the input current will need to be 1A as well. How much power is that?

18V * 1A = 18 watts.  Now where is that extra 6 watts going? 

Its being dissipated by a resistor.  6 watts of thermal energy is a lot, which is why regulators come in a case with a metal tab, so you can bolt it to a heatsink and suck all that heat out of there.  Its an important thing to consider how much heat they will generate.  If you can't cool it enough, you won't have as much current available on the output before the regulator shuts down due to overheating, which I believe is 75 degrees C.

I think the rule of thumb is that the input voltage needs to be at least 1V higher than the expected output voltage.  So for a 12V regulator your supply voltage must be at least 13V.  It can be up to 36V for the 78XX series I believe.  Check datasheets.

[swinginguitar]

I'll look at the data sheet again, but I thought i saw that it was several volts for the 78xx...

And btw, thanks for the extra info!!!

[mattthegamer463]

No problem.  I hope you get yours figured out. 

Take a look at some pedal supply schematics.  They're very straightforward and there really isn't any room to improve.

[Gordo]

You don't need "dropping resistors", and I wouldn't use a LM317 for anything except voltages that you can't find a fixed regulator for.  Use a transformer that has about 18VAC secondary output, rectify, use capacitors to smooth and you'll have something north of 18V.  Then just connect a 18V regulator, 15V regulator, 12v regulator, and 9V regulator to that >18VDC line. Bolt all the regulator tabs to a heatsink for heat peace of mind and you're all set.

You wouldn't get a full 18v out of an 18v xformer though would you (after you rectify and regulate...the 18v reg would need a few extra volts to extract for it's "stud fee")?

I run two boards.  One has a bunch of mostly DIY analog pedals that don't mind sharing a supply and the One Spot works great.  Don't mean to stroke RG but it's small, cheap and reliable.  My other board has all kinds of commercial stuff on it and I've had terrific success with AMZFX's power supply pcb: http://www.muzique.com/pcb.htm.  I'm using the recommended Jameco wall wart (which keeps the AC away from the board). Gives me 18, 15, multiple 12s, and multiple 9's.  I ended up deleting the adjustable 9 and -9 rails because I don't really need them.

[R.G.]

R.G. - was it you who put a custom supply or something in a 6" steel stud?
How well does that work and where could I get a steel stud that doesn't have patterns stamped into it?

Yes. It works well. I used steel "2x8" stock from a steel framing place. It had holes and stuff cut into it but I hacksawed the 20" section between holes to use.

Any really egregious surface problems can be solved with sandpaper and auto body putty. You have banged out a fender and puttied it before haven't you? If not, get acquainted with Bondo. It's pretty close to a wonder material for anything you're going to later paint. Fill in stampings, etc. Works great.