building a power supply, looking for any suggestions on this schematic.

[Drunk_on_NyQuil]

input is greatly appreciated.




hammond 28V @ 0.7A, model 164H28(mouser part#: 546-164H28) is the transformer I am thinking will work to give 200mA on: 1 x 18v, 1 x 12v, 6 x +9v, 1 x -9v, and one adjustable +9v. Any ideas would be great. Thanks for looking.

    Ryan

[Seljer]

I believe you need to feed that negative voltage regulator (or is it smaller?) than -9v to have the 7909 there regulate it, you can't just turn +12V into -9V with just the 78xx/79xx series
you'd need one of those MAX1044 chips if you want to generate a negetive voltage from a positive one
see: http://www.geofex.com/circuits/+9_to_-9.htm


or just set up your transformer for a bipolar setup right from the start? (maybe pass on the 18V or get a transformer with 20-0-20 on the secondary for +-18V, or a 40V secondary)

or you could just use the 18V and 0V outputs with the 9V as the practical ground for +-9V circuits? (though you'd have to be careful when connecthing things together/using the supply for multiple things at once)

[Sir H C]

I would hang all the positive regulators right off the bridge diodes.  Otherwise the 18 volter is going to have to supply the current for all the other ones and possibly get overstressed.

I am not clear how you are hooking up the -9 volt regulator, you need some negative supply for it to run off of.

[gez]

I take it that transformer's 28V rating is rms??

[gez]

Well, apparently it is.  Better to plump for a lower voltage transformer.  The peak voltage, even accounting for voltage drops across the rectifier, is going to exceed the max input voltage rating of your 18V regulator (and all others if in parallel, which is better as has been pointed out).  When choosing a transformer, also bear in mind mains fluctuation in your neck of the woods (if I recall it's + or - 10% in the UK).  Always best to calculate for a 'worse case scenario' 

PS  Could you reduce the size of the image (scroll bar hell).

[gez]

Some ideas for -ve voltage:

http://www.kronjaeger.com/hv/hv/src/mul/

[Mark Hammer]

Yep....a little too much Nyquil.

I know I have advocated cascading regulators in past, but I think Sir H C makes a couple of good points.  You certainly want the primary regulator to be able to handle the current requirements of the second and 3rd order regulators.  If the transformer was, say, 18vAC @ 1 amp,  the 18v regulator a 1-amp type, and the others were 100ma types, then I suppose you could get away with it, but it is unclear from what you have drawn whether there are "conflicting" requirements as far as powering things.

The other thing is that while it IS possible to derive a bipolar supply from a single-ended transformer, it can't be done in the way that you have chosen to rectify prior to the 18v regulator.

Is that enough capacitance to smooth that sucker out?  Not by my standards.

Nope, I think you need to aim a little lower on the 2nd draft.  Certainly an admirable set of goals, but perhaps not as practical, in the long run, as you imagine.

[Drunk_on_NyQuil]

Cool, Im glad I got such great response. I do have a few questions, however. I was assuming a <200mA load from any 9v output(which I think is extremely conservative), could run at least 8 x 9v and one, or two, 18v outputs and still come in under 1.0A@18v(which is the limit of the regulator). So:

1.) Would I be better off using a single transformer to power the 18v and 12v outputs, and a seperate transformer to power the other 9v? I dont currently have any pedals that run off 12v, but I was trying to leave myself room for expansion.

2.) With a 7818 rated at 1.0A, assuming I didnt go overboard with the 9v outputs, should I be able to run the other voltages off that regulator? If not, what is the proper way to step voltage down from a larger transformer to a rating that the 7809 can handle? If I wanted to stick with one single transformer.

3.) If I used a diode to drive the 7909, that should give me negative power, correct? As long as my polarized cap afterward was placed + to ground.

How does everyone else power their pedals? Do you guys use one transformer or multiple? Thanks again for all the response, Im glad I didnt purchase everything to build it yet. Have a great week!

     Ryan

[Sir H C]

Okay, for a linear regulator like all of these, the power issues is the difference between the input voltage and the output voltage times the current.  If you are drawing an amp total through the regulator, you will definitely need a decent heat sink if you set the transformer up with the proper headroom to be sure the 18v output works under worst case supply conditions (probably looking at 100v input minimum and 125-130 max).  To get the regulator to work with the 100 volts in (ratty club with all the power pulled from one cruddy 20 amp circuit) you have to have some headroom - say 2 volts - input to the 18 volts out.  Now kick the input supply up 30%, and you have 26 volts in (versus 20 in the worst case).  8 volts across the regulator with 1 amp out will need a *very* good heat sink to keep it happy.  I could see using 2 18 volt regulators, 1 for the 18 and one to knock down the voltage for the 12 and 9 volt outputs, that might make sense.

To gett he 7909 to work,you have to do the 1/2 wave on the +- where one side of the transformer secondary is tied as ground, and the other goes to a single diode for the + and one for the -.  Since you are charging the storage caps 1/2 at half the rate of the full wave rectifier, you have to really increase the storage cap size to get the same ripple.

[gez]

Following on from Mark's comments, a smoothing cap of at least 1000uF would be needed before your regulator(s).  My little Boss uses a 2200uF cap.  There's an excellent R Penfold book on all this (takes you thru the design process) and it gives a rule of thumb for caps size Vs current, but can't remember the figures off the top of my head.

If you're interested in the book let me know and I'll post the details later (again, can't remember what the damn thing is called).

[Drunk_on_NyQuil]

Yes, I would be extremely interested in the book.  Im thinking maybe I will just use two transformers, one for the 18v and 12v, and one for the 9v side. Is there a difference between rectifying voltage using 4 diodes, or just two? Thanks again to everyone. I will try to come up with a new schematic tommorrow if I have time. Have a nice evening.

     Ryan

[gez]

Is there a difference between rectifying voltage using 4 diodes, or just two?

Two diodes are often used with a centre tap to obtain a full wave rectified output (less ripple).  4 diodes in a bridge configuration gives you the same result but means you don't need a centre tap, although there's an extra diode drop to account for.  Two diodes can also be used to provide a negative and positive supply, as shown in that link in one of my posts above, but this gives you half wave rectification (more ripple) - though with a regulator, a large smoothing cap and modest current consumption you're not going to experience audible ripple on the output.

Book (recommended reading, plus it's dirt cheap):

Power Supply Projects by R.A. Penfold (Babani Publishing)

You should be able to get a cheap copy via Amazon or Ebay

[gez]

Almost forgot.  From aforementioned book:

"The smoothing capacitor needs a value of about 1uF per mA of output current, and so a 470 uF component is the smallest that is likely to give satisfactory results.  If the powered circuit is something like an audio amplifier which is not likely to be tolerant of ripple on the supply lines, a higher value of 1,000 uF or 2,200 uF would be advisable"

[Drunk_on_NyQuil]

alright, so i just wanted to say thank you to everyone who gave me input. i think i have it figured out. its going to wind up being two seperate transformers. a 24v tran. for the 18v and 12v outputs; a 12(or 14)v tran. for the 9v outputs, with two 7809 regulators, 5408(3amp) diodes, 1000uf caps(because i have some lying around) the lm317 variable, etc.

my only question left is: in the r.a. penfold book, all of the grounds for the circuit diagrams goto earth. when i was reading the info on the small wart supply from small bear, it says that to use metal jacks in a metal enclosure, the jacks need to be isolated. why do they need to be isolated if the center pin is pos, and the other is earth. for the positive ground, i was thinking i could just use the center pin for negative voltage, and switch the cable going to the pedal if needed. thanks again for everyones help, i will post pictures once it is completed. have a great weekend.

[gez]

when i was reading the info on the small wart supply from small bear, it says that to use metal jacks in a metal enclosure, the jacks need to be isolated. why do they need to be isolated if the center pin is pos, and the other is earth.

The vast majority of pedals use PSUs with a negative tip.  A positive barrel connected to a negative ground is a short.  Not good.

[Drunk_on_NyQuil]

i understand that. i was thinking if i use a metal jack, with a positive tip and negative shell, grounded to the chassis, and color coded the input and output dc plugs and jacks. just switched the wires within the cables going to the pedals.

[gez]

i understand that. i was thinking if i use a metal jack, with a positive tip and negative shell, grounded to the chassis, and color coded the input and output dc plugs and jacks. just switched the wires within the cables going to the pedals.

I don't see a problem with that, other than stuffing up one day and putting the wrong lead in, which may not be a big deal (all depends on the protection - if any - in your pedals).