Centre tapped transformers.

caps · Nov 28, 2005

As I understand it ( I THINK), if you have a say, 120V AC input to primary transformer with a centre tap...

You will have (1:1 in this case) 60V AC on each leg of the secondary and the centre tap seems to go to ground in alot of schematics I have seen (power supplies). I understand this puts the secondary 60V above ground potential. Is this to limit noise and hash? Am I on the totally wrong path?

Seeing bridge rectifiers are the go mostly these days, what are some other uses of Centre tapped transformers? thanks.

Mark Burnley · Nov 28, 2005

Hi Caps,

I think there are two issues you're confusing here.

It's common practice (in the UK at least) to use a mains Isolation Transformer on construction sites. This has a 2:1 ratio in the UK so that we use 115V tools from a 240V supply. This has a centre tapped secondary connected to mains Earth so that the maximum voltage you see in a fault condition is about 60V. This is basically for safety. This method is also used sometimes for "balanced" mains power.

Within equipment, a centre tapped secondary is used to make rectification easier within a DC power supply. For example, to get a +/- DC supply (common in audio equipment for op-amp circuits) you can use a split secondary. The outer two terminals of the winding feed the bridge rectifier, and the centre tap becomes "0V". A centre tapped winding is identical electrically to two separate secondary windings connected in series in phase with their junction becoming the centre tap. Check this for examples of transformer wiring:

http://www.diyfactory.com/data/transformer_connections.gif

Going back to the reasons for using a centre tap, even with a bridge rectifier, the only way to make a split (+/-) supply from a single winding is to take one side of the secondary as 0V and then feed two diodes reverse-connected to supply the + and - rails. This is only half-wave rectification, and so the ripple frequency will be 60Hz, and the smoothing capacitors will have to be twice the size to achieve the same smoothing as when using a bridge with a split secondary. A bridge with a split secondary has full-wave rectification. This is especially important with a higher current load.

Most modern rack/fx gear fed by a wall wart which has an AC output uses this half-wave rectification to achieve the split rails. Nothing wrong with it, it's just not recommended for higher current demand loads.

Hope this is some help?

Mark

caps · Nov 29, 2005

Yeah thats great, thanks alot Mark. :thumb:

PRR · Nov 29, 2005

This is the reason I wish more members would put a clue in the "Location" field of their Profile. I don't want to know that you live at 414 Oak St Vealtown Utah; I often "need" to know if we are talking 120V or 230V wall-power, if you shop in Euros or Dollars or Pesos: general power and market location.

> It's common practice (in the UK at least) to use a mains Isolation Transformer on construction sites. This has a 2:1 ratio in the UK so that we use 115V tools from a 240V supply. This has a centre tapped secondary connected to mains Earth so that the maximum voltage you see in a fault condition is about 60V.

That's certainly never done in the USA. We bring 240V in from the pole as center-tapped, and use 120V drills, but one side is ground/earthed and we hold the side that is 120V hot-to-ground while standing in mud. Well, most of those guys and their tools are dead (I have a dead-guy's grinder and I can't use it outside), most US power hand tools (yours too I bet) are all plastic "Double Insulated". There are other metal tools that really need to be groundED.

Plastic, 120V CT, and don't you use an RCB too? To my mind, 60V to ground, plastic tools, and a GFI is belt AND braces AND a hand holding up your pants.