DIY Pedalboard power supply

[PRR]

> spec sheet and its input min voltage is 15v

No. (You mean page 10?) That is a Test Condition, not what you can get away with. Running with ample in-out differential improves a few specs somewhat. You can certainly be fine with 3V in/out differential. See Fig 28 page 33. Dropout voltage (gives up) for 1A output is 2V. Pedalboard load is likely to be <500mA, 1.8V overage needed to keep in-regulation. 12V in will give 9.0V out for most suitable situations.

5V headroom allows 2 Amp output, but that's massive for pedals.

[antonis]

antonis - if I was going to balance two regs w/ resistors, what value resistors would they be? I thought I would have thought maybe 0R47 or something, but I don't really think I know.

Now I've seen it duckie, so sorry for the delay..

Resistor's value should be the higher for an affordable voltage drop - the higher the value the better the current sharing..

If you use variable voltage regulators (like LM317-350) it's better to adjust output voltage a little higher to compensate for voltage drop on balance resistors so you can use relatively "high" value resistors (over 1 Ohm or so..)
Same of course stands for fixed voltage regulators by placing a resistive voltage divider between out & adj pins and deriving output voltage by same formula with just replacing the 1.25V (which stands for LM317) with the fixed regulator's nominal output value.
(i.e. if you use R1=2k2 & R2=150R on a 9V fixed regulator you'll get 9.6V on output..)

P.S.
I'm sure that R.G. will slap my hands but sometimes you need solution "on this very moment", so.. 

[Les Turnbull]

Just checked the back of my stomp boxes for there milliamp draw The 2 Marshall digital say 80 ma min , the Danelectro Fab Metal says 300 ma max , Morley classic wah 300ma , The Arion stage tuner - Behringer Ultra Metal and Ibanez Tubescreamer have no details on the back so looks like i will have to rig it all up then run it through my Milliamp meter to see what the total draw is .
  Need a larger pedal box as my old casset holder case is way to small . Lent out a Electro Harmonix Little Muff around 30 years ago , It never came back .

 

[Rob Strand]

Like a lot of stuff with parallel devices it only works when you get devices from the same  batch. 
From a design point of view it leads to a bad design.

If you look at the spec sheet  the output voltage of a 9V 3 terminal reg can be as low as 8.65V and as high as 9.36V.
We put one of each extreme together in a sharing configuration.  Suppose there is a sharing resistor R at the each output. 

The higher output regulator is at 9.36V, when the voltage on the load side of the resistor is at greater or equal to 8.65V, the lower output voltage regulator will not supply any current (as it cannot sink current).   
If the upper regulator is supplying 1A then this situation is:

Vout = 8.65 = 9.36 - i * R
R   = (9.36 - 8.65) / 1  = 0.71 ohms
So if the sharing resistor is less than 0.71 ohms the upper regulator will supply all the current
and the lower regulator does nothing!

So what we have done is put in 0.71 ohms,  got 0.71V drop because of it, and still the lower voltage regulator is not giving us any increase in output current - despite the other regulator is running at full current.

It's not good!

[PRR]

True, sharing is dubious.

One great thing about ultra-cheap regulators is that you can use one-each for every load, or any group of loads that is under 1 Amp. That's how they got so popular. In the old days we would pay $50 for one 3A regulator. Today we can put a $1 reg on each of five loads and come out way ahead.

[Rob Strand]

One great thing about ultra-cheap regulators is that you can use one-each for every load, or any group of loads that is under 1 Amp. That's how they got so popular. In the old days we would pay $50 for one 3A regulator. Today we can put a $1 reg on each of five loads and come out way ahead.

Agreed.

I remember the days of the expensive TO-3 regulators.   Years ago I inherited a  project using a 5A regulator TO-3.  It was insanely expensive, hard to get, and went obsolete relatively quickly.   A few PC printers, maybe 2000 to 2010 era, used TO-218 package 3A regs.

[PRR]

I was thinking before LM105, LM101, even before the '723, when we rolled our own without even an app-note to crib from.

[Rob Strand]

I was thinking before LM105, LM101, even before the '723, when we rolled our own without even an app-note to crib from.

Before my time - I'm LM723 and LM309K era.   The opamp + transistor designs carried through in test equipment and some pro audio stuff upto maybe mid 90's - performance is pretty good.    Plenty of zener + emitter followers in CRT monitors - performance not so great.

[Les Turnbull]

Boxed up the 9v reg today and working good , Got given this massive Diago pedal box off my nephew yesterday so ill have to get cracking making some pedals to fill it .