Power supply advice

[2tonewarrior]

I´m planning to build a power supply but I need help as I know only the basics.

The transformer I´m planning to buy is that http://www.ebay.es/itm/50W-R-Core-Transformer-220V-in-out-20V-2-15V-2-12V-Power-AMP-/230732843466 or something similar (any advice on choosing the right transformer is welcome, I have no idea on what It´s best for this project but keep in mind I´m in Europe, I can´t buy it from Weber, for example).

Attached is a picture of what I am working on (with more outputs): IC is lm7809 or similar and diodes are 1n400x (the diode between cap and Q1 is for protection, I don't know if it is needed)


So here are a bunch of questions I hope you can help me with.

1. Do I need other elements like a little filter cap for the regulator?
2. How I calculate the value of the filter cap(s)?
3. Since I use different bridge rectifiers, filter,... for each output. Would it be number 1 isolated with 2 and 3 but 2 and 3 themselves would´t between them?
4. If I get 0,6A from the secondary transformer on each 20/15/12V output. What current will I have on 2 and 3? 0,3 each?

Thank in advance for the help

[patrickbrose]

Can I ask why? If you are creating a daisy-chained PS anyway, it would be far cheaper and simpler to just buy a onespot. That transformer is almost $60 by itself. A Onespot is about $20.
-P

[stecykmi]

1. i would put a large electrolytic capacitor across the output of the regulator, just to help smooth out the output a little further. a 10uF cap should work fine, but you can sorta put any large-ish value there. if you want to be extra cautious, you could put a large mylar cap in parallel with the electro (say, 0.1uf or so). in theory, this may help reduce high frequency noise.

2. it's a difficult calculation because it has to do with the output resistance of the regulator and the input resistance of the circuit being powered. it should suffice to say that any large value cap should do and the larger value the better.

3. yes, 1 is isolated, however 2 and 3 are not. this is less than ideal, i would consider redesigning so you can provide more isolated outputs. if you're going through the trouble of building this device, you may as well build it the best you can.

4. just to clarify, the 0.6A represents the maximum current draw before the transformer stops functioning. it does not mean 0.6A is constantly flowing through it. in other words, you should not have any pedal or pedals connected that draw more than 600mA (which is a fairly huge amount for most analog pedals). in the case of 2 and 3, any combination of pedals should equal less than 600mA.

good luck!

[2tonewarrior]

Can I ask why? If you are creating a daisy-chained PS anyway, it would be far cheaper and simpler to just buy a onespot. That transformer is almost $60 by itself. A Onespot is about $20.
-P

Of course you can. The reason is I enjoy doing it and learning in the process plus then I feel good if I play with the guitar with things built by myself anyway I want to make all the outputs isolated, not daisy chain. The image was just to ask in the forum how does the current works in these situations.

1. i would put a large electrolytic capacitor across the output of the regulator, just to help smooth out the output a little further. a 10uF cap should work fine, but you can sorta put any large-ish value there. if you want to be extra cautious, you could put a large mylar cap in parallel with the electro (say, 0.1uf or so). in theory, this may help reduce high frequency noise.

2. it's a difficult calculation because it has to do with the output resistance of the regulator and the input resistance of the circuit being powered. it should suffice to say that any large value cap should do and the larger value the better.

3. yes, 1 is isolated, however 2 and 3 are not. this is less than ideal, i would consider redesigning so you can provide more isolated outputs. if you're going through the trouble of building this device, you may as well build it the best you can.

4. just to clarify, the 0.6A represents the maximum current draw before the transformer stops functioning. it does not mean 0.6A is constantly flowing through it. in other words, you should not have any pedal or pedals connected that draw more than 600mA (which is a fairly huge amount for most analog pedals). in the case of 2 and 3, any combination of pedals should equal less than 600mA.

good luck!

Wow, thanks for the information! I´m gonna read carefully all the points

[davent]

Required reading from GEOFEX.  http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm

http://www.geofex.com/Article_Folders/Spyder/Spyder.htm

I would go with multiple small transformers as opposed to one transformer with multiple secondaries. Small, dual secondary transformers should be easy to find, the ones i have have a dual 120v primaries  so are good for N.A. and Europe.

[2tonewarrior]

Required reading from GEOFEX.  http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm

http://www.geofex.com/Article_Folders/Spyder/Spyder.htm

I would go with multiple small transformers as opposed to one transformer with multiple secondaries. Small, dual secondary transformers should be easy to find, the ones i have have a dual 120v primaries  so are good for N.A. and Europe.

Curiously this was going to be my next question The benefits of use multiple transformers with one secondary or one with multiple secondaries. I´ll take a look at those articles, thanks!