DIY Power Supply help

guitarboy2828

I have 4 of these transformers lying at home: http://www.mantech.co.za/ProductInfo.aspx?Item=50M0403
and I found 2 of these: http://za.rs-online.com/web/p/pcb-transformers/0504397/

So, that's 12 isolated outputs. It's not going to be small... or light.. BUT, I'll mount it under a slanted DIY pedalboard (much like the one Alan built and then sold).

I'm using a super simple layout. Transformer -> Bridge Rectifier -> Cap -> Regulator -> Cap ->

What I'm trying to figure out is.. What is the best terminal to use at the end of it? I've thought of using RCA connectors (I think Coiks do that?). I've also thought of using the normal 2.1mm sockets. But, just seems like a waste of money to buy sockets and then have to make up 12 patch cables? with male plugs on each end.

So, I wondered about using something like this: http://www.mantech.co.za/ProductInfo.aspx?Item=64M0071

I would use 3 of them. Then make a cable with the Male plug with my leads with 2.1mm jacks.

Would this work? Anyone got cooler, nearter, simpler, cheaper ideas?

Lastly, are those transformers fine? I know they're big, but other than the size and weight, they should work okay?

Ibanezguy

hey Paul E,

From what I can tell those option in transformers are a bit light weight current wise if you want to even run one delay its going to struggle. You need at least a 2amp 12v ac to build a half decent 9v psu.

if I can help you out with any questions you have just give me a shout

guitarboy2828

Thanks for the reply Ibanezguy.

To be honest, the power supply is just to supply some DIY stompboxes. Nothing digital. So, I plan to just get 100mAs out of each secondary. When (oh please be when and not if) I get a digital delay or strymon then I'll invest in a proper power supply.

For now I'm wanting to not spending money, but power the 10 pedals i've built. ?

petercornell

Paul

There are a few more small things to do to improve the PSU. Do this for all channels. I have not shown them all.

1. Add in the protection diodes as shown, 1N4004/4005/4006/4007 will do here.

2. Add in the 100nF cap across the output electrolytic. Also add one across the first electrolytic as well (not shown).

3. Between chassis/ mains earth and the Ground of each channel, add in a 10 ohm 1watt resistor to prevent earth loops.

As always, observe safety measures and out in a fuse for ultimate happiness!

Peter

guitarboy2828

petercornell wrote: Paul

There are a few more small things to do to improve the PSU. Do this for all channels. I have not shown them all.

1. Add in the protection diodes as shown, 1N4004/4005/4006/4007 will do here.

2. Add in the 100nF cap across the output electrolytic. Also add one across the first electrolytic as well (not shown).

3. Between chassis/ mains earth and the Ground of each channel, add in a 10 ohm 1watt resistor to prevent earth loops.

As always, observe safety measures and out in a fuse for ultimate happiness!

Peter

Thanks so much for taking the time to reply Peter!

After doing some more reading, I learnt that I need to add a diode where you did. I didn't know about the 100nf, so thank you. What does that capacitor do? Does it just help smooth out the voltage further?

Great, I wondered how the Earth was connected to the circuit, thought maybe I just left it out. Will definitely add in the 10R!

Thanks again! ?

Gearhead

Paul E wrote: What does that capacitor do? Does it just help smooth out the voltage further?

It's my understanding that the small cap is a lot faster than the electrolytic and that it helps smooth out any high frequency noise.

petercornell

As gearhead said, it helps to get rid of HF noise.

guitarboy2828

I finally got some more time to invest into my power supply.

Would love some more input if you guys have any.
Here is the updated schematic:

I took the transformer off the main schematic because I'm using the free version of Eagle PCB, so, I couldn't fit it all onto the free dimensions and I can't afford to buy the full version. So, I've split it up into two boards. Transformer board will have pads that'll link to the pads in the above schematic.

Here's my board:

Here it is clean:

Any other input or suggestions?

Are these bridge rectifiers fine? http://www.mantech.co.za/ProductInfo.aspx?Item=35M0841

Thanks for all the help so far! Really appreciate it ?

petercornell

Bridge rectifiers are fine.

Any reason you are using the 78L08 (8v) rectifier? The part should be a 7809 for 9v. The 'L' regulator is only rated for 100mA, so if a pedal that draws more than that is plugged in, the rectifier will blow. Rather use the 7809.

Up the output cap to 220uF or 470uF for added smoothing.

I did not check the PCB layout, but the schematic looks good.

Peter

guitarboy2828

Hey Peter, thanks again for your help.

That was supposed to be 78L09, clicked the wrong part. Oops. This is why I'm using those instead of the 7809 (help given by a friend).

"Determining "enough" in your case is problematic. If you use 7809s, the chip won't current limit until it's putting out about 1-1.5A. The one transformer spec I looked at had the winding current at about 225ma, and that's only good for about 100-125ma of DC out of it because of the peculiarities of rectification and filtering. So the regulator will let the transformer overheat if it's shorted. Changing to 78L09 regulators or LM317L regulators would be good, as these limit in the 100-150ma range. The transformers may self limit - some of them do. But the voltage may also sag and let the regulator saturate. Ripple hum will get through if that happens, and you'll hear it."

Is that good advice? I'm assuming I won't need more than 100mA as it's just for some DIY stomboxes I've made. Nothing digital. If I get anything digital, I'll use a separate supply for that or cash out and buy a proper power supply.

Thanks for the advice on the output cap, I'll definitely do that. Thanks!

petercornell

In that case they should work fine. The regs are short circuit protected internally, although I have never experienced that myself.

If you really want to be safe, you could add a fuse (about 200 mA) after each bridge rectifier, that would protect each supply individually.

Peter

Gearhead

Here's my humble opinion: in any power supply one should have current limitation on both sides of transformers. I tend to use automatic fuses which make it WAAAY easier to continue mid-set after a trip, because one can reset them instead of having to rummage through the band's collective gear cases to find the right size fuse. Yes more expensive but if you're not completely sure about the design (and what amateur is?) gives a a lot of peace of mind.

Example

guitarboy2828

Interesting, i've never heard of automatic fuses. Could you possibly link me to one Gearhead?

Would you put automatic fuses both before and after the transformer?

petercornell

Good point Gearhead.

Skylark

At the cutoff current it switches the circuit off and then just needs to be reset/manually switched on again to be back in business again.