48 Volt Phantom Power Supply with Photo-Schematic. Transformer Question.

[alanbcooper]

Hey guys,

This is my first post and I am happy to have found this website. I am currently working on a project using a PCB and jumper wires to make a 48v Phantom Power supply to be used with multiple condenser microphones. To make a long story short, I found a "mostly" detailed post on another site of someone creating the power supply using a 120v to 25v step down transformer. I am confused as to what is going on in the middle of the picture of the photo-schematic. My transformer only has two contact points and a ground (on the step-down side). It appears as though there is a 3rd wire coming from the transformer traveling through the middle of the picture. Also just to be clear, whoever made the original supply used a turret-style board while I am using a PCB with jumpers like I mentioned earlier. Also I understand I could just buy a wall-wart power supply but I am making this so I can learn more about circuits and PCB's. Hopefully someone will be able to help.

Link for details: https://www.tangible-technology.com/power/Phantom_frying.html

Picture of "photo-schematic"

Thanks in advance,

Alan

 

[JimB]

Welcome to ETO.

The transformer used in the link is a centre tapped transformer.
There are three connections to the secondary, there are two ways of describing that transformer, either "25 - 0- 25 volts", or "50 volts CT" (Center Tapped).

Your transformer has just a simple winding.
If it is described as just "25 volts" then the voltage is too low for what you are trying to do with that circuit.

There may be a work around, using a voltage doubler circuit. It all depends on how much current the supply has to provide.

Woah, hold on there!
I just had another look at that circuit and what I have just written is mostly nonsense.
I will be back in a bit when I have made a drawing.

JimB

 

[JimB]

OK, let me try again!

The circuit of the PSU when drawn out fully, looks like this:

The two diodes and the two capacitors form a voltage doubler, so the 25 volts from the transformer (actually 35 volts if we measure the peak voltage) becomes 70volts at the collector of the transistor.

The transistor and the two diodes form a simple (some may say crude) voltage regulator to give the 48 volts output.

Does this help your understanding?

JimB

 

[alanbcooper]

Jim,

This helps me out a lot! Thank you for taking the time to draw that. Do you think I can still use the transformer I currently have even though it is not center tapped? From your drawing and explanation it appears as though I can still use it. I have already soldered my diodes to the capacitors making sure to reverse them in order order to "double" the voltage.

Thanks again,

Alan

 

[JimB]

Yes your transformer is fine.

I got the centre tap thing into my head when looking at the "photo-schematic" and completely mis-understood what it was trying to tell me.

JimB