At 06:51 PM 1/26/2003 -0500, you wrote:
>Hi,
>
>  I have a very simple question, but I'm asking just to make sure given my bad
>luck with transformers.

Hmm.. let the smoke out, did we?

>I needed to find a regular step-down xformer w/
>centertap for my psu, and I picked up one today from a surplus store, except
>it has 5 wires. It claims that the two black are for primary, and the white
>are for secondary, and the red is for centertap.

Sounds good so far. You can do a rough check with your ohmmeter before
applying mains voltage. The secondary should be much lower resistance than
the primary, and the resistance from either white to the red should be
about half the resistance from white to white.

>Now, the old xformer I used
>had 6 wires... one centertap for the primary, and one for the secondary. This
>made sense to me, since I hooked up all three wires from mains (hot, neutral
>and ground) to the primary coil,

NO! NO! NO! NO! NO!  Never hook the ground to anything connected to the
primary
of a transformer!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
NO!!!!!

If it really is a center tap and you're in North America you'll destroy the
transformer almost instantly, and you could get a FATAL shock if the ground
is not in place!!!

>and the corresponding ones to the secondary.
>But now I only have one centertap? Would it just be the same thing as
>connecting the mains ground to the centertap, and also running another wire to
>the rest of my circuit? Because if so, what is the point of the centertap...
>why not just connect mains ground to circuit ground???

ONLY use the safety ground to ground the floating metal like the housing, and
possibly galvanically ISOLATED parts of your circuit such as GND on the
power supply.

Be careful!!!

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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