Pedal Power Supply Question/Idea

[yettirockc]

Are there any schematics/ideas out there that relate to "padding" the output of a typical 9V wall-wart with capacitors in order to negate small, instantaneous power drop outs to the wall wart itself?

The reason I ask... last night while gigging out with my Ampeg VT-40... every time I hit a loud power chord... my pedal board would reset, like the power to the wall wart got interrupted or something.  I know Ampegs draw alot of current, but damn!  The house wiring must have been crappy. 

Either way, I'd like to know a way to make sure this doesn't happen again... and I thought a cap across the 9V output of the wall wart might help.

Ideas?  Thoughts?  Suggestions?  Criticisms?

thanks,
Joe

[R.G.]

It depends on the dropout time and the amount of current you're pulling. It's likely to be impractical

For instance, if your pedalboard uses 250ma, just to pull a number out of my hat, and the dropout is half a second, you can compute the size capacitance you'd need to hold it up to some voltage.

For instance, if you have 9.0V normally, and you want it to not go below 7.5V, which is still a valid - if low - battery voltage, then the voltage change is from 9 to 7.5V, or 1.5V. All capcitors obey the law that the current out of them is equal to the capacitance time the voltage change per time. In this case we have:
I = 0.25A = C * (9-7.5)/0.5S and we can solve that for C.

C = (0.25*0.5)/1.5 = 0.125/1.5 = 0.083 F

That's 83,000 uF. You can indeed get caps almost that big. They're usually in the 2 to 3 inch diameter by 3-4" long category. Your wall wart may have a hard time *starting* a cap that big up, but it's possible.

A possibly better way is to use a radio shack battery holder and put 6 D cells in it to get 9VDC from the batteries, and then diode-isolate the D cells so they feed the filter cap as well. When the wall wart is running, it holds the voltage at 9V, so the D cell's diode is not forward biased and no current flows out of them. When the wall wart drops out, the D cells pick up when the wall wart voltage drops below 8.3V and turns on the D cell's diode.

[Gus]

some one F supercaps.

  Problem is they are rated at around 3.3V so you would need 3 with eq/bleed caps(so the voltage divides correctly) across them to get the voltage rating.

Three 1 F in series would be about .3F.

Other problem can the wall wart supply the start up current without popping the internal fuse

[yettirockc]

Great.  Thanks for the advice.  I think I'm going to try out the battery/diode idea.  Its a terrible thing for your rig to cut out mid-song.  Quite embarassing, actually.

thanks,
Joe

[Processaurus]

Thats a weird problem you had, are you sure your amp had anything to do with it?  you might measure how much current your pedalboard is drawing (when you hit a big power chord) and if its near the limit of the wall wart, that may have made it shut down momentarily (if its a regulated type).  The boss and danelectro types are rated to 200mA, you can get there quick if you are daisy chaining pedals together, especially digital pedals.

I do like the idea of a 9v pedal UPS, but you may want to try a beefier power supply, maybe the switching kind (like the 1 spot, which can deliver 1700mA) if you find you are near the current limit of your current power supply, or have a seperate power supply for some of the pedals that take more juice.

[yettirockc]

Thats very true... pedal power supply capacity was something that totally slipped my mind!  I guess that should have been the first thing to check.  I'm running several pedals... Boss PH 3, Boss CH 5, Fulltone Fulldrive, Fulltone Dejavibe, Boss EQ...  do those seem like they would add up close or more to 200 mA?  I'll measure tonight...

thanks,
-Joe

[bancika]

They hardly have 100mA between them 

[petemoore]

radio shack battery holder and put 6 D cells in it to get 9VDC from the batteries, and then diode-isolate the D cells so they feed the filter cap as well.
   I can't seem to form a visualization of this schematic or setup.
   I can't even tell you which of the polarized PS leads to connect to which pole of a battery to charge it...I suppose + goes to +.
  I think I need a schematic of this one. If that's possible?

[Processaurus]

http://www.stinkfoot.se/andreas/diy/power/list.htm

Heres a good link that has current consumption for a lot of commercial pedals.

Recently I made a contraption for testing current and voltage for pedals with 2.1mm DC jacks, because I had a couple projects and repairs that I needed to know the current a pedal took, and what the voltage of different was with their intended load was, without wanting to open the pedal up.  Basically it is a 2.1mm jack, that goes to a 2.1mm plug, but there is a break in the + wire, with alligator clips on each end.  That allows you to put your meter into the circuit to check current, or to test voltage by clipping the clips onto the + meter lead, and sticking the - meter lead on the chassis somewhere. 

Not something a regular musician needs, but its great for builders.  You can quickly figure out battery life on any pedal with a DC jack too.

[Paul Perry (Frostwave)]

That's a coincidence Processaurus! I built a box for running power cables in and out, so you can plug in and measure current and voltage while a power supply is under load (I have phono jacks connected to multimeter leads that I use to check modular analog synths.) Well the first thing I found was... all my meters need new current fuses    don't know how that happened, it's either me or the cat..