AC in for dual regulator circuit

[harps]

hello there

I am wondering regards the schematic (jpg attached) I am hoping to tune this so that it gives out DC 18v + & - with a ground)

On the schematic, CON3  (AC in 3 inputs). Do i need a special transformer that gives out  2 x 24v and a ground wire  ( 3 wires)?

I have an AC 33v transformer, just two wires + -  , is it possible to use that? 

thanks for help.
 : )

[Jwillis]

I believe you will need a 24-0-24 VAC transformer where the center tap ( 0V )is your common  ground on CON pin 2 .the two 24 VAC would connect to CON  pin 1 and CON  pin 3.

[harps]

thank you Jwills

cheers for the info. :  )

[Audioguru]

A 24V transformer produces a peak of 34V which is much too high for your 18VDC regulated output. Use an 18V transformer that has a peak of 25.5V, the rectifier drops it to 24.5V and the ripple in the filter capacitor reduces the voltage to the regulators to a reasonable 22V.
So get a 18V 0-18V transformer called "36V center-tapped". 

[harps]

hi thanks

Would a 2 x 18v toroidal be o.k? ( it will be for an audio preamp)  like this:

https://www.maplin.co.uk/p/2x18v-15va-toroidal-transformer-a76qg

[harps]

hello

The regulator can handle 40v.

I wonder, how does an 18v transformer provide a constant voltage of 22v for the regulators?

i understand that the regulator requires a higher voltage of at least 3v than the desired output, as it says this in the pdf. that makes sense.

That is why i thought a 24v transformer would be correct. (and that an 18v transformer would be too low)

"
the LM317 Operation With Low Input Voltage.

The device requires up to 3-V headroom (VI – VO) to operate in regulation. The device may drop out and OUTPUT voltage will be INPUT voltage minus drop out voltage with less headroom
and  3-V headroom required to support maximum current and lowest temperature."

[rdl]

How much current does this power supply need to provide?

[harps]

Hi

I have the need to power:

2 x mono mic preamps 17v dual  ( not 18v , as i thought)
1 x phantom 48v ( I will likely use a step up converter circuit which requires 10V to 38V input (24V nominal) maybe using another transformer for that?)

Then i need something similar for the ADC analogue to digital converter.

The ADC requires:
( 15v dual) for its analogue power
5v for  its digital power

So I thought 2 transformers for this project.

1.1x toroidal with 2 x 24v = for the pre amps(17 dual) and ADC (15v dual analogue power)

2.1x toroidal with 24v + 9v = for the phantom (48v) and the ADC digital power(5v)

These transformers then give adequate power to the regulator circuits.

I am not sure of the current required though.

Thanks for help.

[rdl]

I don't think you'll need anywhere near the maximum current that the regulators could provide. Considering the amount of input filter ("smoothing") capacitance shown in your schematic, an 18v x 2 transformer should be plenty. As a matter of fact, you could probably use half as much capacitance and still be good to go.

 
https://en.wikipedia.org/wiki/Ripple_(electrical)

[Kleinstein]

It does not sound like you need a lot of current. So way smaller caps should be enough, especially if there is plenty of voltage.
The required current should be known.

For the 48 V one could use a kind of votlage doubling or similar rectifier, as the current is likely low. It is a little like a 50/60 Hz driven charge pump. So no need for an extra transformer or switched mode converter

[harps]

thanks everyone

i had already made the circuit up (as shown in the schematic ) so the capacitors are larger than needed. but hopefully this wont be an issue.

I purchased a 2 x 0-18  toroidal.  15va  was the lowest rating in my price range. but should be o.k

 i shall see if i can power some of this up.

I am not 100% sure on the phantom power, the mics i will use may be rather expensive so i would want it regulated and safe. I might  buy a separate toroidal (2x25 in series to make 50v :?) and use a  regulator tuned to 48v.
, shall have to research a bit more on that

thanks for your help.  : )