Power Supply Theory

[Natman]

Hey all,
I am about 90% done building my own power supply with a few isolated outs (I won't go into all the details). 

One of the secondaries on my xformer is center-tapped and I intend to wire it up in the bipolar arrangement as found here (bottom diagram):


The only difference is that I will be using 12V regulators instead.

Here's my question:
What happens to the current when I have effects running on both sides?

It appears to me that the supplies are in parallel as seen by the secondary and the overall load would be reduced and current would increase. Is that correct? (and is it a simple question of summing the two current draws?)

My concern is that one pedal draws a lot of current and the rating for the secondary might not handle it.       

[Transmogrifox]

With FX running on both sides the current in the center tap is the sum of the current in both sides.

Say you're drawing 50 mA out of the +9V rail and 100 mA out of the -12V rail, then the center tap has 150 mA.

However, they return to their respective windings when they get to the center tap.  The 50 mA goes to the first side and the 100 mA to the opposite side, so the transformer winding is never conducting more than either single side.

In reality the DC (average) currents are conducting from the capacitors.  The transformer windings are conducting much higher current spikes to keep the output capacitors charged.  As for the transformer rating it's probably rated with RMS current.  The thing to be careful about is the Average to RMS ratio gets smaller and smaller as the AC spikes get more tall and narrow (power loss is I^2*R).  So the peak power loss on each cycle increases by a ratio of I^2.  This means for the same average output current you burn off more heat in the transformer when you charge with large current spikes on short intervals than if you were driving a resistive load directly with the AC output. 

Keeping that in mind you should still aim for DC loads with less than 1/2 the transformer RMS rating as a best practice.  It will get pretty hot if it's running at its rated current.

If you're using a good quality UL listed transformer then the primary is triple insulated and the chance that overload or short circuit will result in you getting shocked is much lower.  If using some cheap foreign-made transformer with dubious compliance marking there's a chance that if it gets too hot and smokes it could short AC line voltage onto your audio ground and give you a good zap.

That said, don't take this stuff lightly just because the output is nominally 12V or so.  Heat and smoke can result in electric shock if the insulation melts and primary/secondary windings make connection through the burning goo.

[antonis]

Well said..!!

I'll just add the need for double primary fussing (both HOT and NEUTRAL) and the use of DPST switch for mains..

I don't know what happens worldwide (safety requirements) but in many countries in Europe there is an interchangability between cable plugs, so you can't be sure which wire is HOT and if it's "active" or not by switching off...   

[Hatredman]

In some cities with 220/240V mains, both wires are hot. So it is mandatory to use a DPST for the power switch.

[Phoenix]

Just might add that seeing as DPDT switches are often more easily available and/or cheaper than DPST switches, the same effect can be achieved by wiring the active/live/line/hot and neutral/return to the switched lugs, not the common, and take the connection to the circuit from the common terminals. This way, when the switch is turned to the "off" position, the spare contacts are shorted to the circuit, not the mains wiring, as would be the case if the active and neutral were wired to the common lugs. Heatshrink should cover all terminals for an added safety margin too.
Also make sure to place the fuses before the switch, so that if the switch fails in any catastrophic manner, the fuses will still be effective.

Be careful if you do not have experience wiring mains connected equipment. Preferably make sure you have supervision from someone who does, and if for any reason you don't have such a person available, make sure that there is someone else around who knows not to simply run to your side if the worst should happen, but rather to run to the mains breaker first and shut off the power, to make sure the area is safe before they assist you.

[Natman]

With FX running on both sides the current in the center tap is the sum of the current in both sides.

Say you're drawing 50 mA out of the +9V rail and 100 mA out of the -12V rail, then the center tap has 150 mA.

However, they return to their respective windings when they get to the center tap.  The 50 mA goes to the first side and the 100 mA to the opposite side, so the transformer winding is never conducting more than either single side.

Thank you so much Transmogrifox!

That's exactly how I imagined it (which is reassuring).
My CT secondary is rated 0.5 A each "side" FWIW so I can assume that I will obtain full current rating for each side then?     

[antonis]

Sometimes transformer's rating is quite confusing..

Ignoring losses and efficiency, the transformer must give to the secondary the same power with which is fed the primary..
So, if we have a primary of 240VAC and a secondary of 24VAC we get 10 times current capability (P= V x I)

Secondary's current rating concerns the TOTAL voltage drop on it's inductor, which in our case is 24V..
If our transformer is center-tapped, we can get 2 x 12V with the same current rating like the total one..
(24VA = 2 x 12VA)


P.S.1
In case of your inrush current is low enough, fuses should be fast-blow type (if your switch doesn't appear spikes..)

P.S.2
"full current rating" must be avoided..