8 output 9V psu (yes another one)

Zokk · Post by **Zokk** » 14 May 2011, 20:15

Hi

I've been working on a 8 output 9V psu for a friend recently. It is powered from a single 2x18V 40VA choke and using LM317T regulators. The schematic is inspired from Ken Stone's CGS66 bipolar psu with slight modifications (and I've swapped the negative rail of course).
It uses fixed resistors to achieve theoricaly 8,97V on each output, reducing costs and risks of failure from the trimmers which were in the original design.

As it should be quite reliable I will build another one for me soon... and I think it could be interesting to share the pcb artwork here too.

Use 1/2w 1% metal resistors and caps rated over 35V (50V or 63V prefered). Diodes are 1N4007UF.
( please don't mind about component numbers in the layout: it is 8 times the same "module").

cheers

edit: here is a 3D view ( NOTE: the 2 big caps are not correctly oriented in this view, use the layout as reference).

Zokk · Post by **Zokk** » 14 May 2011, 23:59

Wow I've seen I've told something wrong:
you should read "18VAC" and not "2x18VAC".
Also it will work best with a 12VAC input as it will minimize heat of the LM317Ts...

I'm sorry if it's not "plug & play", that's my first pcb post attempt here.
Also I will probably change the LED resistors to drop consumption to 10mA per LED...

rocklander · Post by **rocklander** » 15 May 2011, 00:38

what is the advantage (if any) of using 317T instead of 7809?

Zokk · Post by **Zokk** » 15 May 2011, 12:25

Usually LM317T will be cleaner than the 7809. That means voltage regulation will be also more precise and noise reduced, its overall performances are slighty better than the 7809.
The LM is also a bit cheaper even if it implies the use of a trimmer or two resistors to set up the V out. That's why I think it's a better choice for people who need a real clean psu.

>>> concerning the LED resistors in my previous message; you could use 680 ohms or even 750 ohms to reduce consumption around 10mA per LED.

MicMicMan · Post by **MicMicMan** » 18 May 2011, 11:51

I'm very sorry, but I don't really get the point with having 8 DC output sharing the same ground. A powerful, very well filtered 9V adapter with a daisy chain would do about exactly the same job in my opinion : it doesn't prevent you from ground loop issues. Maybe the advantage here is that it's easier filtering independently each output that a huge one.

The LM317T is a far better device for filtering and adjusting a DC output than a 7809. I've never had any 50 Hz thing with a 317T. It's also very useful for sagging your favorite fuzz, or putting a slightly higher voltage to your favorite overdrive so it gets more headroom. That said, maybe the new K7809 devices are better than 317Ts.

Zokk · Post by **Zokk** » 18 May 2011, 12:27

You're right about the ground loops... but for the moment I've not experienced this problem with my friend's rig (3 tube amps, tons of pedals and tube tape recorder played at the same time). However I think filtering is enough to have a clean DC signal with no 50Hz remaining in any configuration. You could also add a ground switch to the mains to isolate completely the psu from earth ground, shouldn't that work like that?

The 8 outputs are mainly to provide strong DC current to feed several digital pedals which need lots of mA. And the daisy chain is still useful, as it's going to be one or two digital stuff + 3 or 4 analog low consumption effects per output.
So I think you could draw slightly more than 250mA per output when powered with a 12V 40VA transformer (but I'm not good at maths).

I have not heard about the K7809, thanks for pointing it.

merlinb · Post by **merlinb** » 18 May 2011, 20:12

Zokk wrote: So I think you could draw slightly more than 250mA per output when powered with a 12V 40VA transformer (but I'm not good at maths).

With an 18Vac 40VA supply you're likely to get about 22Vdc at about 1.5A after rectification and under a hungry load. Each reg IC can dissipate about 1W without a heatsink, so with 13V across them you can safely supply about 77mA from each. If you want to take advantage of the full 40VA then small heatsinks will be required (the clip-on type would probably be fine).

Fender3D · Post by **Fender3D** » 07 Jun 2011, 17:34

I opted for LM317LZ since none of my pedals draw more than 100mA
With an addictional resistor and a capswitch you may have 2 selectable voltages per output...

And it fits in a 1590B...

coldcraft · Post by **coldcraft** » 07 Jun 2011, 20:56

would you guys share your eagle files (if thats what you used)? thanks

Zokk · Post by **Zokk** » 07 Jun 2011, 21:19

coldcraft wrote:would you guys share your eagle files (if thats what you used)? thanks

No problem for sharing the files, but they're Kicad files, not Eagle.
Interested?

Fender3D · Post by **Fender3D** » 07 Jun 2011, 22:49

Here's my 3D modules for KiCad http://www.wizardinside.it/foto/schemi/modules3d.zip.
Still missing some capacitors and PCB jack sockets... I'll add them whenever I'll find the time to draw 'em..

Zokk · Post by **Zokk** » 07 Jun 2011, 23:08

Thanks Fender3D, the 3D modules are always hard to get, and I've never managed to use Wings3D to create my own drawings.
So I used only the free modules available on the web

Fender3D · Post by **Fender3D** » 07 Jun 2011, 23:24

You welcome

coldcraft · Post by **coldcraft** » 08 Jun 2011, 02:55

I'm not familiar with kicad so I'll have to pass.

I was interested in how Fender3D did the switched output voltages though.

Fender3D · Post by **Fender3D** » 08 Jun 2011, 13:38

The capswitch simply bypass 1 resistor in series with the adj. resistor (R2) to ground.
You may calculate values here http://www.electronics-lab.com/articles/LM317/.
Simply get the value for the lower voltage desired, then find the value needed for the higher.

Rhigher - Rlower = Rtobebypassed

In my case:
R1=240
Rl=1500
Rh=560

Shorting Rh with capswitch gives you 9V, without capswitch you obtain 12V

MicMicMan · Post by **MicMicMan** » 16 Jun 2011, 11:16

This is pretty nice having so many power outputs from a 1590B thing. Great design there!
Maybe one addition I would make is adding a 800mA temporary fuse on the primary AC input to prevent the whole thing from destroying in case of a shortcut. You might have some room left for adding a pc-mount socket for one of those?

Fender3D · Post by **Fender3D** » 16 Jun 2011, 14:16

MicMicMan wrote:This is pretty nice having so many power outputs from a 1590B thing. Great design there!
Maybe one addition I would make is adding a 800mA temporary fuse on the primary AC input to prevent the whole thing from destroying in case of a shortcut. You might have some room left for adding a pc-mount socket for one of those?

Yeah..

hmm one of this might work:
http://it.mouser.com/ProductDetail/Litt ... wYGkXEA%3d
It should be socketed though, otherwise you'll have to remove the board to swap a burned one

MicMicMan · Post by **MicMicMan** » 17 Jun 2011, 11:48

yep !
the same in a slow acting / medium acting / temporary version would maybe fit better, but that's the idea. Also, a polyswitch... There are many options

Zokk · Post by **Zokk** » 17 Jun 2011, 13:16

Why a 800mA fuse?
it seems oversized for this application. I would put a 500mA on a 100VA transformer for a +/-15V psu which sends more than 600mA per rail... but for my design I've used a 250mA with no problem. I don't think Fender3D uses a huge transformer like that.
Could you explain why that fuse needs to be rarted so high?

MicMicMan · Post by **MicMicMan** » 17 Jun 2011, 15:03

Fender3D wrote:I opted for LM317LZ since none of my pedals draw more than 100mA

I just was referring to that.
But sure 800 mA seems a bit overrated. You can adapt to whatever maximum rate your power transformer is up to, since the main goal of such a fuse is to protect this component (and ideally the lm317s before they burn). The ideal solution being a fuse for each output, and an additionnal one in front of the power transformer, but this takes quite a lot of room.