Stupid TI power supply

From: Eric Smith <eric_at_brouhaha.com>
Date: Wed Dec 15 19:01:59 2004

Vintage Computer Festival wrote:
> But across which pins??? The power connector on the terminal has three
> pins. Which ones want voltage and which one wants ground?

Gordon wrote:
> Look inside it. In any case, since it's AC, it doesn't *matter* which
> is voltage and which is ground. There's no "polarity" as such, since
> the secondary of the transformer is going to be floating.

Sure, but the third pin isn't necessarily floating with regard to the
other two. For instance, if the third pin is a center tap on the
secondary, leaving it open may cause problems. Possibly including
damage to the device, since unbalanced loads between the ends and the
center tap will drag it around. For instance, suppose it's 18V RMS CT.
The peak voltage between the ends of the secondary will be about 25.4V,
and the peak between either end and the center tap will be about 12.7V.
But suppose the center tap is open, and there is three times the load
between one end and the center tap as between the other end and the
center tap. The voltage between the ends will still have a peak of
25.4V, but the third terminal will now peak at 19V from one end and
only 6.3V from the other. If there's electronics in the device that
was designed for the nominal 12.7V peak and can't withstand the 19V
peak, damage may occur.

This is just like losing the neutral connection on AC power
distribution.

On the other hand, the third pin of the connector could be a safety
ground that may or may not be electrically connected to the transformer
secondary.

Sellam is quite right that he needs to know what the third pin is
supposed to be.
Received on Wed Dec 15 2004 - 19:01:59 GMT