"Tailoring" wallwart voltages?

Started by GibsonGM, September 11, 2007, 06:50:41 PM

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GibsonGM

Hi,
Is it possible to 'tailor' the voltage a wallwart produces?

I have a 30vDC 400mA computer power supply.  It's pretty thin, and would fit well under a homebrew pedal board; I am trying to avoid a bulky transformer.  I would like to regulate it to 9V...but the high 30V to start with tells me it would run really hot.   So, if I use a simple (and properly rated) voltage divider, something like 50 ohms and 68 ohms, I should get an output from the PSU of about 17.3V, with 254mA running thru the divider.  This could then feed an LM7809 with filter caps, and run to my pedals in parallel - I should be running much less than 200mA off of them.  Maybe 10W resistors would be in order here, possibly near 8W of power will be present thru there. 
The other thing that comes to mind is a zener circuit off the PSU, but the divider idea seems more attractive and less sensitive, and would allow a smaller heatsink....

Thanks!

~MJP
  • SUPPORTER
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...

R.G.

Reasoning about power supplies is sometimes less than obvious.

In this case, the power supply is going to put out 30Vdc. Your options are only to eat more or less current, up to 400ma.

Probably the simplest thing to do is to put a single resistor in series with a 7809. This resistor should be sized to burn off 30V-11V = 19V at 400ma, or R = 47.5 ohms. 47 ohms would be fine. Power is 7.6W, so use 10W worth of resistors in a single or series/parallel. When the current drain is lower, the regulator eats more power, but is still within its specification because as current goes up, the resistor lowers the voltage the regulator sees. 200ma is the current with the max regulator power, that's 2.3W. You'll need a heat sink, but not an abusive one. If you're SURE you'll use less than 200ma - or any particular current, increase the series resistor to give the regulator 9V+2V at the maximum current. This minimizes the power dropped in the entire thing.

Have you considered using a switching-type wall wart power supply that is half the size of normal linear wall warts, puts out 9V directly, is designed for running high gain distortion pedals, costs under $25.00 new and is tested to not radiate noise into pedals?

:)
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

John Lyons

How about this, I'll trade you your 30v supply for a regulated boss 9v PSU?

Is it a Laptop supply or a heavy (actual transformer) unit. Heavy is good with me...

John

Basic Audio Pedals
www.basicaudio.net/

96ecss

Quote from: R.G. on September 11, 2007, 07:31:24 PM
Have you considered using a switching-type wall wart power supply that is half the size of normal linear wall warts, puts out 9V directly, is designed for running high gain distortion pedals, costs under $25.00 new and is tested to not radiate noise into pedals?

:)

Hey, that sounds like my Visual Sound 1-Spot.  ;)

Dave

GibsonGM

Yeah, maybe you're right, R.G....thanks for the info :o)  I'm always trying to find ways to use up these power supplies i have lying around, and old trafo's that don't seem to be good for anything!   I'll probably buy one of those $45 units that have the ability to run all your pedals, with the multiple barrel plugs.  Was just wondering if there was a quick & dirty alternative.

Thanks for the offer, John, but then I'd just have 1 supply, and my idea was to power 'em all with 1 unit....
  • SUPPORTER
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...