12v, 9v and variable 9v power supply layout?

[momo]

I Used to power my neovibe with an 18v DC torrodial transfo. I thought I would use this power in a better way and so I want to use it to build a power supply. Been looking around here, but its all seperated.
Can any one propose some build with possible multiple layouts?
I would like to get the 18DC transfo to supply 12v, 9v, and a variable 9v.
thanks

[Jdansti]

Here:

[momo]

Right on!
I will just add the rectifier bridge and will do that,
thanks.

[Jdansti]

Your OP said you have 18V DC, but if you're coming in with AC, then yes, you'll need to rectify.

[karbomusic]

I think you should breadboard it and work out any kinks first as an FYI. I built a 9/12 supply not long ago and love it but it was on a breadboard for a couple weeks in order to get it confirmed clean and steady...

http://www.diystompboxes.com/smfforum/index.php?topic=106755.0

I need to go back in and do one last small filtering addition but otherwise fine, biggest thing was testing before building.

[GibsonGM]

I think you should breadboard it and work out any kinks first as an FYI. I built a 9/12 supply not long ago and love it but it was on a breadboard for a couple weeks in order to get it confirmed clean and steady...

http://www.diystompboxes.com/smfforum/index.php?topic=106755.0

I need to go back in and do one last small filtering addition but otherwise fine, biggest thing was testing before building.

Right.  You might need to add some more caps at the regulators. Download the sheets for them ("LM78XX series") and see how they lay them out.   They are to minimize ripple (noise, to us) prior to regulation.   But otherwise sounds like a good project, and that you're on the right track!

What is the current rating of your 18V power supply?   Be aware that you WILL have limits on the current you can draw form this/these....the 18V will set that limit.  It's not quite as linear as "if the 18V unit says 1A, I can draw .333A from each".   It will be less than that.      But if you're using this for pedal-type use, and not pulling much, you're probably fine.  Draw too much, you'll probably just drop out of regulation.

[davent]

I think you could run the two 9v regulators in series with the 12v for even lower noise, and as long as the current demand downstream of the 12v regulator doesn't exceed that one's limits. Rather then getting three different regulators just get a stash of LM317's and you can use them everywhere (they'll physically fit.)

[momo]

Thanks everyone,
here is my transformer :http://www.digchip.com/datasheets/parts/datasheet/2967/62021-P2S02-pdf.php
Should be great for what I want to do...
not sure how to use the dual secondaries.......I don't remember how it was wired in my neovibe!

[GibsonGM]

The only problem, for me at least, with doing 'chained' regulators that way is that that LM317s cost me a lot more! 

According to your specs, you CAN do what you want but must be mindful that you don't have a ton of ampacity there.   For pedals, you're probably going to be FINE.  I'd limit each 'power supply' to < 100mA.      When you breadboard it up, try loading each regulator, see what happens.    If you can get 100mA out of each at the same time, you're doing pretty well with this idea, I think.   Just remember to USE the caps on input/output of the regulators from the data sheets, and be aware you might need more if the supply is noisy.     Generally a cure-able situation, tho.

Can you look up how you wired the neovibe?  I'm not that familiar with toroidal trafo's; when I encounter them, I just use a DMM to figure out which wires go to which windings.    I'm sure you'll get an answer on that soon, tho.

[momo]

I try to play the least possible with AC stuff.......
I will put check the voltages on the wires, but only after my worktable is cleaned and free of any metal and wires...

[davent]

I try to play the least possible with AC stuff.......
I will put check the voltages on the wires, but only after my worktable is cleaned and free of any metal and wires...

Something you can do when checking out the transformer is power it with a small AC wall wart, i have a 12vAC i'll use for this so voltages are 10% of normal. Once sorted can move up to mains voltage.

The free wires of a transformer under test i mount in the seperate compartments of a terminal block.

Different transformer testing but  to show the terminal block.

[Jdansti]

At 278mA, it seems a little on the low side. As mentioned, be careful not to overload it.

I assume you're powering this from mains 120VAC? There's a whole 'nother discussion on how to wire it up safely. As has been said before on the forum, that information is best not learned on the internet, but maybe you already know how to do this.

[momo]

I had not noticed the low current specs. It is low....
So then I will have to use another transfo for this project and I will put the torrodial directly in my neovibe....
I have tons of transfos, I just need to figure out which one would be good.
So I guess when one goes past the current ratings, the transfo heats up and explodes??

[karbomusic]

When you breadboard it up, try loading each regulator, see what happens.

That's a great point and why I ended up going with a 1.6 amp transformer. I forgot that much of my testing was with a scope/meter and adding load in multiple configurations to make sure I had more than enough reserve AND a steady voltage with no sag. It was a great learning experience and I'm glad I didn't know how much thought it required before I started. 

[Jdansti]

I had not noticed the low current specs. It is low....
So then I will have to use another transfo for this project and I will put the torrodial directly in my neovibe....
I have tons of transfos, I just need to figure out which one would be good.
So I guess when one goes past the current ratings, the transfo heats up and explodes??

I guess an overloaded transformer would get very hot, and if it got hot enough, the insulation on the wire would break down and the windings would short, hopefully tripping a breaker or blowing a fuse and not stating a fire.

One thing you could do for overload protection would be to place a 250mA fuse on the secondary.

If it were me and I didn't have any pedals that were super sensitive to power, I'd use a 1A to 2A transformer and let the caps handle cleaning up the ripple. I'd at least fuse the input to the primary side in the "proper way", ground the chassis per "established conventions", and do all of the other stuff to avoid killing yourself and/or burning your house down. Notice I'm being vague because I personally don't know the correct engineering practices to achieve this. 

[karbomusic]

I guess an overloaded transformer would get very hot, and if it got hot enough, the insulation on the wire would break down and the windings would short, hopefully tripping a breaker or blowing a fuse and not stating a fire.

For wall warts, many times they melt, deform, smell and then stop working. Guess how I know that?   Getting too warm is one surely of the early symptoms of an overloaded transformer.

On a more serious note, not having enough current will make it frustrating because that limit will always be there along with the stress/worry of not overloading it.

[GibsonGM]

yeah, there is nothing more frustrating than putting something together, getting your caps/R's all set up to filter, you have a nice desired output voltage...then you load it with something near your expected current draw, and that awesome 9V drops to like 800mV, LOL.    Then you know that trafo is garbage for what you're doing. 

Using a beefier supply is *always* better.....the 1A being very common with what we do here.  You draw 60mA or something, but everything stays rock-solid.  Ampacity on tap No heating...the things just idle along...     

Fusing is a GREAT (necessary) idea!   You plug something into the wall, and if your secondary is calling for too much more than what can be delivered, you might have a fire.   Or, you burn a fuse; an annoyance and warning for you to look again.

Momo, maybe do some more reading about building power supplies, how to deal with connections to the mains? Start with known good projects, like a basic 5V supply or something, based off a wall wart!   Davent's test board he presented above is THE way to mess with those unknown transformers, and it's how the old ham radio guys would do it.   Isolation is key - makes things much safer. BUT, you have to hook up the isolation transformer, ha ha.    If you feel a little nervous, that is good, and normal, but may mean you're not ready for mains yet....

Old ARRL books (again, ham radio) are great teachers about this, as are many sites out there.

[duck_arse]

^ what everyone says.

if you look the datasheet you linked to, pp17/18/19 has the colour-codes for yr trafo. no need to mess about. there is also plenty in that doc about recifiers how's and don'ts. there will be some info on fuses there too, I'm sure, with some sorta note about slo-blo-ing toroids.

rg and prr always say fit the reverse-discharge diode from output to input on 3 term regs, who am I to say otherwise? and don't forget heatsinks. you can dig good ones outta old 486//pentium pc's, many will have a fan attached.

[GibsonGM]

^ Yes, forgot the heatsinks!!    Dropping 9V across the regulator(s) will make it hot.   I suggest thermal compound with a big chunk o metal.   PCs are extremely good sources for sinks!   Rip one apart.

[greaser_au]

two words:   AC wallwart...    These things are isolated, internally fused, thermally protected, AND UL/CE approved...   16VAC  20VA  is fairly common in the security alarm industry.
You'd have to *really* screw up to toast one of these babies (and if you can do that - to be blunt- you'd be well advised to *stay well away* from mains power).

I've said it before, but using one of these means that there will be one less set of questions to answer if a disaster happens  (my homebrew pedalboard supply uses 16VAC/1A wallwarts).

Put whatever you want on the end: a rectifier/filter/regulator  or a C-W doubler/tripler  - whatever. With clever design and big enough rectifiers/filters you can even easily manage a bipolar supply.

david