Converting 9VAC to 9VDC

[Cortexturizer]

Okay, so I bought a SKB pedalboard, the Stage Five one. It is one of the best purchases I have made in my entire life. Absolutely fabulous, and I am having so much fun with it.
The tricky part is - the only power outlet that I have left unused is a 1.3A 9V AC. I use another one of those to power the Line 6 DL-4 pedal. All of the other outlets are being used by other pedals. The only one I have left is the 1.3A AC one.
Now, I bought the EHX Freeze (it's on it's way to me) and it sucks around 170 mA of power, so I was thinking I could convert the unused AC outlet to DC, regulate it to 9V, and feed the Freeze from there.

The whole purpose of buying the SKB pedalboard was to abandon power adapters, cables, and everything, I would hate to have to use the Freeze power adapter or to daisy chain anything (currently nothing on the board is daisy chained), so the last remaining power outlet (the 9V AC one right) is my only option. If I don't convert that successfully than I will have to daisy chain, but I would rather not do that.

So, what do you guys suggest? I am totally clueless about current rectification and all. Any existing schematics, veros, pcbs, that could get me started right away?

Thanks!

[Vallhagen]

Sure there are solutions for your task. I browsed a bit and found this for a start:
http://www.tonepad.com/getFile.asp?id=111

Musicfromouterspace also have a project:
http://www.musicfromouterspace.com (browse site for power supply)

... or you can make your own i guess, just borrow parts from the schematics above. Its basically just to rectify, regulate and filter.

Cheers

[Cortexturizer]

Hm, I should have thought about the tonepad thing. Thanks!
Any components here that are maybe overkill and could be chosen to be smaller in value (therefore in size as well), I want this to be as small as possible, because you know...that pedalboard realestate is always a PITA

[RobA]

You can get smaller parts, especially if you go SMD, but the problem is going to be getting 9VDC from the 9VAC supply. The losses in the rectification are going to take you below 9V. You might be able to do one of the voltage doubling rectifiers, but they'll have more ripple, so you'll need more capacitance to smooth it. It would also have more current loss because you'd need to waste more voltage in the regulator.

You might be able to do it with a bit of step up transformer to get the 9VAC to 12VAC and then rectify. I've been looking for something similar for awhile and I can't find anything that will work.

There are some more complicated ways you could do it that would still be small, but I think they are all going to increase the 140mA draw of the pedal up to about 200mA or higher.

[Cortexturizer]

Hm. And I could swear that somewhere I've read that 9V AC would give me 12.6 or something of volts of DC. Somewhere on diystompboxes.
Man I would hate to have to daisy chain, ughh...why do I even use digital pedals 

[Vallhagen]

... but the problem is going to be getting 9VDC from the 9VAC supply. The losses in the rectification are going to take you below 9V. ...

Really? I am converting 12VAC to +/-12VDC right here myself... with 7812 / 7912 regulators. I should look for the exact schematic i used, for some reason i dont know where to find it atm.

Cheers

[Vallhagen]

... just found that from a google search. It looks pretty much just as i done it.

[Cortexturizer]

This...looks very easily doable. Any thoughts on why this should be a bad idea RobA? Given that you wrote just before that one can't get 9V out of 9V AC...I must say that I am confused, and I do know very little about these things...

[RobA]

It's not a bad idea. You just have to be a bit careful about how you set it up and what the voltage reading of your power supply means. If it's 9VAC RMS, then you should be able to do it. I would just use an LDO regulator to do it with. The problems can come from the drop out voltage of the regulator and the losses due to the diodes.

The peak voltage from your 9VAC (if it's RMS voltage) is going to be sqrt(2.0) * 9.0V.  If you drop 1.1V in the diodes in the bridge rectifier, you are down to about 11.4V or so. That's fine with room to spare for an LDO regulator, but less good for a normal type. So, if you go for an LDO regulator it'll work. Just make sure you get one that can deliver more than 100mV, since that's what most can do.

If you get something like this one http://www.st.com/web/en/resource/technical/document/datasheet/CD00044919.pdf in the SOT223 and get an SMD rectifier and use some 16V SMD ceramic caps, you could make the whole thing tiny.

[Vallhagen]

... now i remember i used the General Guitar Gadgets schematic to build mine (sorry for ranting about with more than one reference here, but the GGG one it was, and it is very lookalike the posted picture above. Also, the link above has a few funny voltages, as 9VAC in --> +/- 15VDC out).

RobA has a point, i just had to refresh my mind: on the first (pair of) big capacitor(s), you get ripply DC, more than 12 VDC if you feed with 9VAC. This voltage is then feeding the regulator(s), and will decrease when loaded. And when heavy loaded, it will decrease so much that the regulator will stop working correctly.

In your case though, your AC supply can deliver 1.3A and your load is 170mA (right?), which i guess (notice: guess) will keep you on the safe side.

Cheers

[RobA]

Yep that's where the issues are. Also, since the peak voltage out from the rectifier is multiplicative and the drop out from the regulator is always the same, you've got more room to spare with higher voltages. 12VAC to 12VDC is easy, 15VAC to 15VDC is dead simple, but 9VAC to 9VDC is on the edge of being problematic.

I've adapted the GGG circuit too for my bench supply. It works well. I've got 12VAC in from a good supply and can actually get up to 15V out from the LM317 regulators I use for the variable taps.

One other reason to think about going with the more modern LDO type regulators is that they are stable with really low ESR caps. They can be used with like 10µF or 20µF SMD ceramic caps and be perfectly stable. Doing something like that could really get the size down. If you could find the right little enclosure, you could pretty much do it inline with the plug.

[Cortexturizer]

Hey guys thanks for the discussion so far. I was just about to order some parts for the SpaceHarp phaser, and I ran LDO through banzai's catalog and this is what they have:

http://www.banzaimusic.com/L4940V5.html
http://www.banzaimusic.com/L4941BV.html
http://www.banzaimusic.com/LM2940CT-12.html
http://www.banzaimusic.com/LM2940CT-15.html
http://www.banzaimusic.com/LM2940CT-5.html

Would any of these serve the purpose I need it for?
Regardless of whether it would or not, I am not sure how to use it, just use it instead of lm317 in a typical tonepad power supply schematic/layout?

P.S. Vallhagen my man did you ever receive that pcb and transistor I've sent you during the PIF rounds?

[Thomas_H]

Hey guys thanks for the discussion so far. I was just about to order some parts for the SpaceHarp phaser, and I ran LDO through banzai's catalog and this is what they have:

http://www.banzaimusic.com/L4940V5.html
http://www.banzaimusic.com/L4941BV.html
http://www.banzaimusic.com/LM2940CT-12.html
http://www.banzaimusic.com/LM2940CT-15.html
http://www.banzaimusic.com/LM2940CT-5.html

Would any of these serve the purpose I need it for?
Regardless of whether it would or not, I am not sure how to use it, just use it instead of lm317 in a typical tonepad power supply schematic/layout?

P.S. Vallhagen my man did you ever receive that pcb and transistor I've sent you during the PIF rounds?

Nope, you will need a 2940-9. 2940 has 5/8/9/10/12/15V types.

http://www.mouser.de/ProductDetail/Texas-Instruments/LM2940CT-90-NOPB/?qs=%2fha2pyFadujrFlYti9Pu2NokJrQhkrziYISQ0GXQcq4%3d

[Cortexturizer]

So wait, just to make it blatantly clear for me, looking at this schematic from tonepad -


I just stick that one in instead of the lm317 and I'm (hopefully) golden?

[Thomas_H]

So wait, just to make it blatantly clear for me, looking at this schematic from tonepad -
I just stick that one in instead of the lm317 and I'm (hopefully) golden?

Aähh, NO.

The 317 is an adjustable voltage regulator and the 2940 is a fixed one.
get rid of all the resistors (2x1k, 240R), D6 and the 10uF cap and it will work.