Help building/choosing power supply

[shawsofhell]

I plan to build a power supply to power around 8 effects. My prefered option is to have eight regulated and isolated outputs. This is probably the best option since it is going to have no ground loop problems and no problems supplying enough current.

Questions:
How many volts should the transformers be rated at 12V or 9V?

How many amps should each secondary need to supply?

Are PCBs safe enough to carry 240V (is it simply a matter of making the AC traces wider then normal)?

If I simply go the cheapo option and buy two 9V 1.3A regulated supplies and daisy chain the pedals together will this create any problems? (I know this will create ground loops but that is easy to fix, I have read that some pedals don't like sharing their power supply)


Just for reference the pedals I will be powering are:
Blue Magic
Big Muff
Small Stone
Tremulus Lune
Wah
Boss Giga Delay

[niftydog]

How many volts should the transformers be rated at 12V or 9V?

R.G. shows you the calcs if you look at his biblical articles at geofex.com... but 9VAC will be plenty.

How many amps should each secondary need to supply?

50mA DC would be more than adequate, but most go with 100mA because the size difference is fairly insignificant.

Are PCBs safe enough to carry 240V (is it simply a matter of making the AC traces wider then normal)?

Absolutely. Only worry about making them wider if you're planning on drawing significant current. 5mm wide would be plenty for most applications. Be aware that the traces must be well protected from shorting to the chassis as 240V can be pretty dangerous to both yourself and to electronic components.

daisy chain the pedals together will this create any problems?

In theory, no. in practice, maybe, as you've alluded to. Only one way to really tell! Suck it and see!

[ethrbunny]

I recently finished a PS. Used a 12v 1A wall wart from Mouser and split it to 8 jacks using 7809s. Lots of fun LEDs too, 'natch.

[shawsofhell]

ethrbunny wouldnt all the outputs be sharing common grounds?

[niftydog]

yes

[shawsofhell]

What benefit does having 8 regulators give? Isnt it simpler just to have one? Im guessing its for current purposes but most current regulators can supply 1A and with a simple transistor circuit it can be boosted to about 3A.

[corbs]

i have an el-cheapo 6x 9v power supply from maplins and i was quite surprised when i multimetered it - it was giving out something like 8.4v with a current of >300ma. Although it does have a crack down the pcb thats held together with gravity (must fix that sometime)...

but is it usual to get unstable currents + voltage not matching 9v from unregulated power supplies?

[ethrbunny]

Yes there is a common ground. I use 4 7809s on the premise that if one craps out I still have 6 working jacks. They are heat-sunk to the chassis so I can pull the whole 1A through one if I want. (wonder what kind of pedal would need 1A? yikes!)

[niftydog]

multiple regulators assist with current handling. One regulator running mutliple pedals may get hot and go into thermal shutdown.

To draw anything near 1A through a regulator requires serious heatsinking and to do a current bypass transistor and drive 3A is even harder to keep under control.

[shawsofhell]

Ok I think I've found the transformers I'll use. They have two 12V .416A secondaries. If I'm only running one pedal per secondary this sounds like plenty of current right?

[niftydog]

plenty of current and plenty of voltage too! You could get away with less voltage... but that'll be fine.

[shawsofhell]

Ok what about a fuse. What value would I need for four transformers with the above specs?

Also the regulation circuit I'm using is just the two cap and one regulator version. How big a cap do I need before the regulator (I have seen values from 200uf to 2200uf) and after the regulator (Again I have seen 0.1uf to 100uf)?

[niftydog]

what about a fuse.

put a multimeter in it's place and measure the normal operating current at maximum load (all your effects). Replace the multimeter with a fuse that's maybe 25% over-rated for that current.

The bigger the filter cap, the less the ripple. Use as big as you can fit/afford/buy.

The other cap, stick to the datasheet. Usually it's a 1uF tantalum or something like that.

[keninverse]

sharing grounds = possible crosstalk and more noise.

[keninverse]

Anybody use PCBs/kits  like this to power pedals?

http://www.jlmaudio.com/JLM%20Power%20Supply.htm

[smallbearelec]

You really need very little current from the transformer if you are only running one pedal from it. Unless those dual-secondary transformers are VERY cheep, I would suggest the something like the Mouser 41PG006, 12 volts 60 ma. R. G. Keen sets up a bunch of these in a housing made from electrical conduit in one of his articles. If you can find a small, low-current unit in Oz that has a 220 volt primary, that would be a good way to go. Or I offer a "flat-pack" 12-volt 200 ma. transformer for US $5.50 each plus postage.

For complete schems and some construction  ideas, check out my Small Warts:

http://www.smallbearelec.com/Projects/SmWart/SmWart.htm

http://www.smallbearelec.com/Projects/SmWart2/SmallWart2.htm

Kits and individual parts are available, and my store will be open again Monday. You asked about a fuse; note the use of the Raychem resettable fuse in the Small Warts. They are designed for exactly this kind of job, and they are cheap and tiny.

Regards
Steve Daniels
Smallbearelec.com

[ethrbunny]

There just aren't enough parts in a PS to merit getting a PCB built (IMHO).. is that enough acronyms?

I have been going with wall wart -> filter cap (1000uF) -> 33uF -> 7809 -> .1 uF -> effect.

You can probably omit the 1000 at the beginning if you use a good wall wart. The other two are what the voltage regulator data sheet recommends. If you start with a transformer then just add a full wave rectifier to the beginning and you are ready for action.

[keninverse]

There just aren't enough parts in a PS to merit getting a PCB built (IMHO).. is that enough acronyms?

agreed.  if you're using 7X09 regs.  I have several pedals that run on +/- 15V rails as well as +9V.  7X09 regs are great for supplying voltage to several pedals but when you start placing more and more regs behind a transformer the end result would make you wish you went with better regulator with lower noise and better supply rejection characteristics.  coupled with all the filter caps and voltage taps, I think it warrants the use of a PCB....but then again it's always been easier for me to build on a nice PCB than having to build on a vero.