e-licktronic Forum

I have made an update to my internal power supply and the PSU of the Yocto in general.
Due to the nature of the Yocto PSU design, when using a toroidal transformer with double secondary windings, you either choose to use a 2x 15V 15VA and wire it up in parallel, or you use a 2 x 8V in series. Both ways work, however I wanted to make use of the toroidal center tap and use it as 0V ground.

Currently the Yocto design has a virtual ground, which is fine if you use it with an external adapter. However, having an internal transformer, using the center tap in a correct way and connect it to PSU ground, can reduce hum.

So, two ways to go, either remove D1 and D2 and replace it my a bridge rectifier, or go less intrusive and just add 2 more 1n4004 diodes to D1 and D2.
This way you change the PSU into a full wave bridge.

So I choose the latter option.

[resize=800]http://i.imgur.com/ft4EqOD.jpg[/resize]

I have also connected the chassis ground to the PSU using a loop breaker intended to prevent earth/ ground loops to prevent hum when systems are interconnected.

[resize=800]http://i.imgur.com/Eq8HsfU.jpg[/resize]

More pictures: http://imgur.com/a/IeBch#3

Thanks a lot. I actually did the same: added diodes for the second transformer output on the three voltages (+5V, +15V, -15V) and added a loop breaker (I chose a simple RC, no diode).

I'm happy to see it works for you too!

I'm getting ready to build my yocto and would eventually like to use an internal power supply as well (in the metal elstroumph).
I need some more info about how to install a (ring) transformer.
I've ordered the switch with fuse with the case.

-How do i properly earth the transformer to the case? (the above example seems to be connected from the 0-leads of the secondary 15v output of the transformer)
-What values do the loop breaker components need?

Hope to learn how to do this, i'll not try if i don't fully understand how to implement it.

Hi deejayraoul,

Have a look at the pictures on the imgur site. Most of them have comments and explain things a bit more detailed.

http://imgur.com/a/IeBch#4

Hey Jeroen,
Thx a lot, it clears up a lot.
I'll study the schematics some more and i think i´ll succeed.
Just one question, you have a ring transformer 2X15v in parallel right?
Would the connection change in a 2X8 series setup? (The extra diodes on the power board?)

Thx again,
Raoul

Hi Raoul,

With transformers that have double (two the same) secondary windings, following happens. If you wire it in parallel, the voltage will stay as specified but the specified max current per winding you can draw doubles. So if you have a 2 x 15V 15VA (500mA per winding) transformer, you will get a 15V 1A output.
If you wire them in series, the voltage doubles and the current you can draw halfs so you will have an output of 30V but with only 500mA.

So to answer your question, you have 3 options:

No center tap and connect to the board without mod;
- Take a 2x 8V 16VA, wire them in series to get 16V 1A out.
- Take a 2x 15V 15VA, wire it parallel and you get 15V and 1A output.

If you go for the center tap mod;
- Take a 2x 15V 15VA, wire it in series. You will then have the center tap as 0V and have 2x 15V 2x 500mA to feed the power supply circuit, as described in my first post.
I personally believe you can probably get away with a 2x 15V 15VA as the Yocto probably never needs more than 1A in total. If you want to go for the overkill (I did as I had one lying around, you can always take a 15V 30VA, however the physical size will grow aswell and I think in time I will get a 15VA one).

I don't get the center tap idea. If you have 2X15V in series wouldn't it result in 30V AC?
Excuse my electronica noobness. I hope to learn.

No worries, we all had to learn someway or another.



The theory behind a center tap transformer is that you create a 3 wire, 2 phase output. The center tap serves as the 0V neutral as it is exactly in the middle of the secondary coil. Between the center tap and each wire L1 and L2 you will have 15V (Va and Vb) in our case. However, if you measure the voltage between the two outer wires L1 and L2, you will see the voltage doubles.

The advantage of using the center tap is that you can supply the two circuits in your power supply (The positive with the 7815 and the negative with the 7915) independently.

These are the advantage of center-tap full wave rectifier:
(http://electronicsproject.org/advantage ... rectifier/)

-The rectification efficiency of full-wave rectifier is double of that of a half-wave rectifier.
-The ripple voltage is low and of higher frequency in case of full-wave rectifier so simple filtering circuit is required.
-Higher output voltage, higher output power and higher Transformer Utilization Factor (TUF) in case of a full-wave rectifier.
-In a full-wave rectifier, there is no problem due to dc saturation of the core because the dc current in the two halves of the two halves of the transformer secondary flow in opposite directions.

Have a read on this tutorial: http://www.electronics-tutorials.ws/tra ... rmers.html

Thx Jeroen, your explanation and the link cleared up a lot.