[truman]

I know you guys get bombarded with powersupply questions and I have read most of the posts on the subject but I really need to know how to find out the polarity on my transformer because its wires are two red on the mains side and two green and a black on the other side I take it the black is centre tap as this is how I wired it the first time, I had everything working then took it apart to mount in the enclosure so I unhooked the drive board to test to make sure the polarity was right and the transformer started to hum then the magic smoke came out this part scares me because I don't want to fry my drive board! Here is a pic of the one I fried.

[Al_The_Man]

It looks to me the green is the centre tap for the blues, check it with a resistance meter, if you don't need the centre tap, then tape it up.
That looks like a bridge rect. so normally you would not use the centre tap.
You would use two rectifiers in place of the bridge IF you were using the centre tap supply as a Full wave circuit.
Check all windings with a ohm meter for continuity to tell them apart.
Al.

[truman]

thank you Al but what I really need to know is which red and which blue are + and which would be negative

[Al_The_Man]

You don't need to be concerned with polarity on AC (it reverses 60xsec).
All you need to do is distinguish which is primary, secondary and identify centre taps.
The only time you need to observe 'polarity' per-se is when you are paralleling windings, then this is referred to as phasing.
( I assume we are ONLY talking AC here)
Al.

[truman]

so what you are saying is that polarity is determined after the rectifier or the cap? I know these may be anoying questions but I need a firm grip on whats going on

thank you

Rob

[Al_The_Man]

so what you are saying is that polarity is determined after the rectifier or the cap?

Determined by Rectifier only, cap or not .
Al.

[truman]

thank you Al that is the answer I was looking for now I can get my machine running finished the mechanical part yesterday.

thank you

Rob