due to popular request how to use this transformer 14VA 15V :
http://uk.farnell.com/myrra/45046/trans ... dp/1777953

or
http://www.selectronic.fr/transformateu ... res-3.html

check those pics : fuse could be before the transformer (ideally) but it is expensive for 80/160 mA (yes, i'm greedy ) and finally i placed the IEC connector on left and switch on right for a centered Lexan

yes, please any help with instructions is greatly appreciated...! You wouldn't want us to electrocute ourselves, now, would you?

@julienvoirin : thanks for these information, I will maybe choose for this internal power supply

@ElSchtroumpf : is there enough room in the box for this power supply ?
If yes i will change for a C14 connector

@misa : seems not to be too complicated, look at the the datasheet for the transformer connection and at the drawing of julienvoirin for the rest

My enclosure will be higher than julienvoirin's, if you use a low profile transformer you should be fine!

I have two questions re: Myrra: it says it's a 115-230 V transformer -- but the wiring for 115 seems to be very different from the 230 wiring.

First off: why? (can someone explain to me in layman's terms why the wiring needs to be different for the different voltages, please?)

second: this means that I *can't" just plug in a 110 V cable when I tour the USA, but need a step-down converter, right?

bonus question: is there a solution that would allow me to use 110V *or* 230V? I'd like to travel with Yocto and play out live, that would make things easier (and presumably more reliable...). Otherwise it might be a better solution to use a wallwart PSU?

Thanks!

some physics :
P = U x I = constante = 14VA
P : Power

on primary windings :
Utotal = U1 + U2 (voltages)
230V = 115V + 115V
--> a single winding has a maximum intensity output.

secondary windings are in parallel to add intensity :
i total = i1 + i2
and deliver a single 15V AC of 0,4 + 0,4 A.


you can have a DPDT switch for EU/USA conversion as used on old computer that will route the electric "path" around the transformer (remember this switch 115/220V on the rear of apparels).

nota : windings of primary and secondary are phase opposed, explaining this crossing serial connection

HI Julien,

thanks a loot for the answers! I will think about the switch solution. ++

A few important things to bear in mind here:
- metal cases MUST be earthed (not doing so means you potentially could have a fault that makes the case live). Julien's diagram has this noted, but I wanted to make this very clear.
- use an internal fuse on the mains-side, not just the low-voltage side, on the live wire (not the neutral). 1A fuse would do.

If you are unsure about the wiring for the transformer, switch, fuse etc., i suggest you simply do not attempt it. Just use the wallwart as the designer planned. Any mistake on the mains wiring could electrocute you and/or cause a fire. You also may have live wiring exposed when you have the case open while testing it etc.

If you do attempt it, you can use an insulating "boot" over the mains socket and fuse or use heatshrink sleeving to cover it. Not important for the low voltage side.

Here is a voltage selector switch of the type that could be used to switch the primary from series (240) to parallel (120)
http://uk.farnell.com/schurter/0033-450 ... dp/1211994

Another thing to bear in mind is that putting the mains transformer close to the audio circuits might cause more mains hum pickup - hard to get rid of.

I do have a question about this... if using a metal case but powering using the AC wallwart, which ground point in the circuit should the case be grounded to in order to best reduce the hum?

Stay alive!

Thank you for all those information. I finally understood almost everything. The last question is something I often asked me, someone would be able to answer?

But mainly I have two questions about the technical data sheet of the voltage selector switch linked above by 8Bit:
- What means the diagram A) ?
- On the diagram B), what means the line linking the 1 to nothing ?