Newbie w/ Pedal Board Power Supply Questions?

[campj0le]

Want to make a Pedalboard with 10 power outputs. All will be on seperate boards for the isolated thing.

Parts:

Radio Shack:
Transformer Specifications:   
? Primary voltage: 120VAC   
? Secondary voltage: 12.6   
? Maximum Output: 300mA   
? Body in inches: 1?1/4 x 1?7/16 x 1?1/16?

Full-Wave Bridge Rectifiers for Power Supplies
4 amps, 50PIV.

Voltage Regulator (Not Radio Shack)
MC7809
Vo = Min. 8.65, Typ. 9.0, Max. 9.35

Capacitors
220uf
10uf

Here are the questions.
I have 120V to transformer. Out of transformer is @ 12.6V into Full-Wave Bridge. Using the calculation, Vpout = Vs - 1.4 = (N2/N1)Vpin - 1.4 = (1/10)120V - 1.4 = 12 - 1.4 = 10.6. So, 10.6 volts coming out of the Full-Wave Bridge into the LM7809 Voltage Regulator. Then out of the LM7809 should be 9VDC/1A(100mA)

When I looked at the digitech power supply it has an output of 9VDC/300mA, and I cannot find a regulator for the 300mA.

Will this combination be able to drive all the boards equally from say, 10 trans., so on and so on. Also what do the capacitors really do to the before and after the Regulator, I understand that they try to keep the positive peaks to a more level DC, but will I encounter a current drop that I need to be worried about.

I have the case figured out and would like to be able to use many different pedals, Digitech, Boss, etc. Most of the Pedal information sheets that I have read say that the mA do not go above 100mA.

Any hints or help is appreciated. :roll:

[Gearbuilder]

Hi,

 How many pedals would you  wany to connect  and what is  their total amperage ?
The  sum should be equal at 1 amp.For my part i've got a bigger power supply(2 amps ) ,like that ,i've got a big headroom before i smoke my power supply.
  I think it's better to use output caps ,pedals stay quiet.
Bruno

[Peter Snowberg]

Welcome campj0le,

The math for what you get out of a bridge rectifier is more like Vout = Vin * 1.41414. If the transformer is putting out 12.6 volts, that would give you roughly 17.8 volts out. No keep in mind that the transformer is rated to deliver 12.6 volts when you are pulling the rated current from it. With less draw, the voltage will be higher than that. 78XX series regulators need about 3 volts above the output voltage as a minimum to give a stable output. As you raise the input voltage, the regulator will produce more and more heat. Be sure to use a good heat sink.

The Digitech supply you're looking at is unregulated and it relies on things being "close enough for Rock & Roll". Your supply will be much better and cleaner.

If you want isolated outputs, you'll actually need a separate transformer, rectifier, and regulator for *each* effect. This will eliminate hum from ground loops, but obviously it's much more complex.

The number of effects it can drive will depend on the effects. I have designs that take a couple hundred microamps and ones that take a couple hundred milliamps. Most will be a few 10s of milliamps. The only way to really tell is to use a meter to check. I wouldn't trust the labels too much. Reality beats printing and a decent meter costs $10-$20 US.

The caps try to average out the voltage and eliminate the  "gaps" between each positive peak. They're not really for removing the peaks, the regulator does that. They eliminate the valleys if they're large enough. I would suggest using a MUCH larger cap in front of the regulator. 2200uF is a good start. 4700uF is even better. The cap after the regulator is there to keep the regulator stable as load from the circuit changes. You may want to add another cap of 0.1uF across that cap for increased stability.

Most pedals will work together without issue, but PNP fuzzfaces are a notable exception because of their reverse polarity. They will require a separate supply or else they'll get smoked when you plug them in.

Good luck!

Take care,
-Peter

[campj0le]

Gearbuilder:
10 total pedals. One main supply with 10 different boards so that they are isolated and then will have two fans at either end, one blowing in and another blowing out. How did you get the two amps out. I am under the impression that it depends on the Voltage Regulator? Please correct if incorrect!


Peter Snowberg
So you think that everything looks good except for the things that you have added about the caps. I want to make the quietest power supply that I can, without re-engineering the thing.

Thanks for the replies!

[smallbearelec]

If this is your first DIY project, I wouldn't suggest a power supply at all. Wiring anything to line voltage--much less multiple transformers--takes a lot of care and skill, or you will light yourself up like a Fender. If you do want to work on a simple, quiet power supply and learn some construction ins-and-outs in the process, check out the Small Wart on my site. If you want a design for multiple outputs that's known to work, check out the four-output supply at GeneralGuitarGadgets.

Regards
Steve Daniels
www.smallbearelec.com

[R.G.]

You refer to yourself as a newbie, but you obviously are no stranger to some parts of this.

Your scheme is good, if somewhat elaborate. I proposed something very similar in the Spyder power supply (at GEO -http://www.geofex.com). Clearly I like good, if elaborate, too 8-)

Also see the article on power supply design, although you seem to have a pretty good handle on it.

Peter is correct, the raw DC is likely to be 17 to 20Vdc; it's nominally 12.6Vac, but is actually more like 14 or so to allow for loading, and the rectifier picks up the peak value of Vpk = Vrms* SQRT(2).

The current specs are somewhat more confusing. If you have a constant voltage regulator, it will put out any amount of current from 0 up to its max current limit to keep the voltage constant. The pedal can use as much or as little current as it wants within that range and be fine.

Your transformers are rated at 300ma. That's an RMS (heating equivalent) rating. Full wave rectifying an AC voltage outside a transformer makes the transformer see something like 1.8 times as big an RMS current as the DC current going out. That means that the "300ma" transformers can supply maybe 300/1.8 = 167ma before the transformer gets to its rated current handling. The regulators can supply up to 1000ma, so they wil just keep putting out current until the voltage from the transformer falls below their minimum input voltage, at which point they'll saturate and start letting ripple voltage through.

You might want to consider using 78L09's. The "L" version current limits at 100ma and comes in a TO-92 package, which may save you some mounting space.

Lastly - Steve is also right. Be absolutely, 100% sure you can wire the AC power line connections safely. If you don't already know you can do this, you should get help doing it or stop. DIY effects aren't worth dying for.

[campj0le]

Sorry that I did not clarify before. I do have experience with 120, 240, and also 480. In school it was all theory and no practical application, so if I seem alittle off on some things I apologize (That was back in '97 and have had desk jobs since, until now). Sad to say I just made it through and music was supposed to be a full time gig. Never made it to the music part but was ready to audition to some guitar programs, Classical. Actually am working as an Electronics Tech, yeah I know,I know!

Starting to get back into the band thing again after a slow down due to wife and kid. Had rack effect processors before, but never really was to inthused, I liked the stomp boxes better, also cheaper.

R.G.
I saw the Spyder before and I thought that it was a great idea. The board that I want to make will be curved so that I will not have to move my foot as much. Also will have an incline with less upward toe travel. Sounds dumb, but I think that it will be neat.

[Peter Snowberg]

That sounds like a great project. I'm curious which 10 pedals you're going to put in this thing?

If none of the pedals are tube based, you shouldn't need any kind of fan. Fans get brutal when it comes to interference too. Even with tubes in there, a little attention to layout should eliminate any kind of heat issue.

The 220uF you mentioned at the top would work, but you might have some hum. I figured you wanted silence so I suggested the size increase .

Take care,
-Peter

[Gearbuilder]

Hi,
Yes it's depending of the regulator and the heatsink you use with.
My power supply is not a DIY ,i've  just modified the output and i plan to do the mod from Geofex(Thanks R.G. )that allow to simulate an old battery.I'm agree with Steve Daniel ,it's an hazardous project for a newbee but if you take care to disconnect the main plug,discharge the capacitors with a resistor drain and insulated alligators plugs and never touch anything with your finger and Always a hand in your back when you probe ;i think you can DIY,Take care .
Maybe you could find some  good regulated power supply(the models where you can choose 6,9,12 volt)inb your electronics shop.They are ok for that use and affordables.I bought mine 22$ in a E;shop .The cost of a transformer ,regulator and other pieces like the rectifier and the box is not so interesting for doing yourself project  .They are well insulated and have multiple prong,you'll just have to take it in your rack with 3Mvelcro and find a female plug to connect 9V with your pedals.
Regards
Bruno

[campj0le]

Well I made my test board and am only getting 7.5V out.

Parts are as follows:
Input was 112.0V
Transformer
120/12.6, 300mA
Bridge
4A/50PIV
Regulator
LM317T
1.2 - 27VDC
1.5A
Only using 2 caps and nothing to tune the regulator at the moment.

No matter what cap I use I still only get the 7.5V out. Anyone have any suggestions.

[Impaler]

If I remember, the 317T uses a resistor to set the output voltage externaly. I could be wrong tho...

[Gearbuilder]

Yes you could fix the voltage with a preciscion resitor and put a pot to the output for adjust but you could put a resistor to the fixed resistor to adjust the regulated voltage .
Bruno

[campj0le]

Well finally finished the prototype.

Got:
9.3V DC
.163mAAC
.001mADC

Does this sound right?

[Gearbuilder]

Hi,
1 mA seems to be small cause any effects you try are pumping at least 80mA DC

Bruno

[smoguzbenjamin]

1 mA won't do anything. But how did you measure that, I mean you have to have a load for current to flow in the first place, just using your DMM won't help.

[campj0le]

Thats what I was figuring also? If I take the Digitech Multi Chorus, and it says that it uses 4.8 Watts of power.

Voltage out = 9.3V
DigiMC = 4.8W

I = P/V
I = .516A
 = 516mA

Now what I do not under stand is that my transformer is only rated at 300mA max output. I just am not seeing something or I have something hooked wrong or I am right and the board will not run the pedal and I need a larger transformer.

[niftydog]

it says that it uses 4.8 Watts of power.

DC or AC?  You must always make the distinction!

300mA AC does not equate to 300mA DC.  The transformer is current rated at AC, your pedals are likely to specify the DC current draw.

However, 516mA DC is quite an impressive current draw for a pedal!  It seems to be refering to some other spec... lets see if I can find the manual...

<time passes>

The manual says 4 hours battery life... there is NO WAY you can draw 516mA DC from a 9V battery for 4 hours.  I would expect that a 9V battery could deliver a bit less than 100mA for 4 hours before it was TOTALLY shagged.  By the end it would be more like a 4.5V battery!

I beleive the 4.8W refers to the AC INPUT to the digitech PS200R power supply.  This seems a much more reasonable figure.

BTW; can I just say, I ABSOLUTELY HATE both the BOSS and Digitech websites...  what is it with high priced web designers these days??

Here's how to get a better idea of power consumption.  (Sorry if I'm telling you what you know already, but others may benefit from this)

1) Get yourself a good, new 9V battery & a spare 9V battery "clip" thingo with unterminated wires...

2) Connect it to the pedal, plug the pedal in and turn the effect on.  Measure the voltage accross the battery and make a note.

3) Now, disconnect the battery from the pedal.  Somehow wire the positive of the battery to the mA input of you multimeter - then the "common" terminal gets connected to where the positive of the battery would normally be.

4) Wire the negative of the battery to where it normally should be.

5) Switch you multimeter to an appropriate DC current range (should normally be less than 100mA DC)

6) Then, plug in, turn effect on, note the current.

From this you can work out the DC wattage requirements.


Working backward from there to work out the AC input of you power supply is a whole other kettle of fish.  There should be info available on the net for these types of calculations.

[campj0le]

thanks a lot for the info, I must be having a massive brain fart. What you say makes sense and it is very much appreciated. Thanks again.

[niftydog]

I must be having a massive brain fart

I've had days of extreme brain flatulence too... usually in the middle of exams at tech college!