Building my own multi power supply...

[isildur100]

Hi,

I need you guys expert advice

I just got myself a 12V 3.3A regulated power supply at a local electronic surplus store. I was thinking I could use it to power several pedals since it provides plenty of current. What would be the best way to proceed in order supply half a dozen pedals, for example 4 x 9V and 2 x 12V ? Can I use a voltage regulator such as the 78L09 to obtain 9 volts? I was also concerned with the component ratings such as resistor wattage etc. I tried the adapter directly on my valvecaster and some resistors started to get very hot!  The adapter is rated at 40W ! Will the 78L09 support that 12V 3.3A current?

[Blue_Toad]

Hi,
A simple approach to doing what you're looking for is to parallel a few linear regulators, I'm not sure that the 78L09 is the best choice though as it's maximum output current is 100mA (less in the real world) If I were you I'd use a National LM2940T-9.0. It's basically the same as the 78L09 only it can output up to 1A (without a heatsink you'll be pushing it at 550mA). You could basically just use the little schematic that is on the datasheet and ties all of the input capacitors together, the outputs would all be 9V and capable of driving around 550mA (asuming it never gets hotter than 50 degrees celcius in the box) You could then just pass the regulator to the outside world to get the 12V.

As for the resistor getting hot, which resistors are you talking about? Resistors in the power supply or in the valvecaster?

Pedals will only draw as much power as they want, for the most part your fuzz, wah, transistor / opamp based pedals drawing between 10mA and 30mA. Some of the Line 6 pedals are pigs though. You just have to make sure that what the pedals want doesn't exceed the capabilities of the regulators. Just for piece of mind though the LM2940's will shutdown if they get too hot instead of bursting into flames 

Hope this helps,
If I can help any more, let me know

BlueToad

[isildur100]

Hello Blue Toad! Thanks for the reply! 

I will definitely try the LM2940T-9, but I have a few questions.

1) Would the 7809T which can also output 1A work the same? The component is less expensive...

2) Let's say I need to power four 9v pedals that do not draw lots of current (the total mA would be under 1A), could I use only one regulator to supply the 4 pedals? Or would that cause problems?

3) Could I directly power 2 additional 12v pedals without using regulators? Assuming that the total current draw of all six pedals is under 3A and the adapter is 12v 3.3A regulated?

4) Any issues I might not have thought of? 

cheers

[Blue_Toad]

Hi isildu100,

Glad I could help and I'll try to answer these questions as best I can.

1) Would the 7809T which can also output 1A work the same? The component is less expensive...

I took a look at the 7809T, it looks like it is basically an equivalent part, just remember that the datasheets "lie" when they say that the part will put out 1A. They assume that you have infinite cooling, in practice the part will only do about 550mA before it burns up. This is because of the power dissipation, when you use a linear regulator you have to take the voltage in, subtract the voltage out and multiply by the current to get the power dissipation, for example. Pd(watts) = (Vin-Vout)xI(amps) so at 550mA and 12V in you disspate (12V-9V)x(0.550)= 1.65Watts in the part alone. Then you have to take into account temperature of the junction (150C) and the ambient temperature that you're working at (I'll explain if you need more info, just ask). You also want to make sure that you don't actually push it that hard since these are the max limits of the part. Usually you want to make sure that you have a 25 -30% margin to make sure that it will last a long time. This puts you around 400mA output @9V per regulator. It's a little more disappointing than 1A, but the heat comes into effect. In short, it will work about the same just don't push it too hard.

2) Let's say I need to power four 9v pedals that do not draw lots of current (the total mA would be under 1A), could I use only one regulator to supply the 4 pedals? Or would that cause problems?

Basically as long as the total current is below the max for 1 regulator (about 400mA), you can get away with using only one regulator, you're probably limited to about 400mA as I mentioned above, I've been doing this with a cheap cable and wall wart (been meaning to build a power supply) for years and have not had any problems.

3) Could I directly power 2 additional 12v pedals without using regulators? Assuming that the total current draw of all six pedals is under 3A and the adapter is 12v 3.3A regulated?

Sort of.... the basic idea is right, just remember that a chain is only as strong as the weakest link, this means that the total on the 9V should never be more than the regulator can handle (400mA or so in this application) the 12V end of things will be 3.3A - 0.4A so at 12V you can use up 2.9A but remember to add some safety margin. so you'd probably want to stay around 2.5A or so. If you need more 9V current, you can always add more regulators in parallel, but remember that this takes away from the total that the 12V can handle.

Any issues I might not have thought of?

There shouldn't really be anything else after this, just remember that the way these linear regulators (7809T) work is by turning the voltage from 12V to 9V into heat. Electronics don't like heat too much, and that's why you shouldn't take more than about 400mA per regulator because the part will overheat. (If you let the smoke out, it stops working. Remember all electronics run on smoke  )

You can get away with a few 7809T regulators and some caps and it should work great.

If you have any other questions, or get it working, let me know how it goes,

BlueToad

[isildur100]

Thanks for all this information!

I will probably go with multiple regulators. I will tell you when I finish this, I still have to buy those regulators.

cheers

[Jarno]

Does anyone know an existing PCB layout with multiple regulators (preferrably also a couple negative ones for fuzzfaces)?
It's not hard to build this on vero or perf, but a nice and compact PCB would be even better!

[isildur100]

On the general guitar gadgets web site there are a few power supply projects. You might want to look at what they have. But they all require a transformer.

cheers

[Jarno]

That's a good tip, the one with the four isolated outputs looks good. I don't have such a transformer, but plenty of other ones, so I'll just remove that piece of the layout. Nice 'n compact indeed, fits easily on one of those scraps of PCB material! Might even make another one for neg supplies (although I don't have THAT many fuzzes    ).