Stupid TI power supply

From: Eric Smith <eric_at_brouhaha.com>
Date: Thu Dec 16 14:03:29 2004

I wrote:
>> 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.

der Mouse wrote:
> If the centre-tap is left open, there will be no loads, balanced or
> not, between it and anywhere;

Incorrect, as explained below.

> it then becomes equivalent to an
> otherwise similar transformer with no centre-tap at all.

The transformer, yes, but the loads will have problems.

Imagine a circuit with a transformer with a 24VAC RMS
center-tapped secondary:


     ______ ________________
           \ || / __|__
           / || \ load1 20 ohm resistive load
           \ || / _____
  120VAC / || \_______________|
           \ || / __|__
           / || \ load2 60 ohm resistive load
           \ || / _____
     ______/ || \_______________|

With everything wired correctly, the loads will each see 12VAC RMS.
Ohm's law tells us that load 1 will draw 600 mA RMS, and load 2 will
draw 200 mA RMS.

Now break the center tap connection. From your statement you apparently
expect that the loads will get no power at all. But that's not what
happens, because the loads are in series and there is still a complete
electrical circuit, albeit not the intended one. Instead, now the
loads form a voltage divider. The current through both loads must
now be the same. Since the combined load has a series resistance of
80 ohms, the current must be 300 mA RMS. Using Ohm's law we can
calculate that load 1 will see 6VAC RMS, and load 2 will see 18VAC RMS.
If load 2 was not designed to tolerate the higher-than-expected voltage
and current, it may be damaged.

And that's for constant purely resistive loads. When the loads vary,
as is typical with most electronic devices, the voltage across each
load will also vary.

As I stated earlier, this is exactly the kind of problem you have with
split-phase or three-phase electrical mains wiring when the neutral
connection in faulty, and that commonly leads to destruction of
equipment. I have a wall-wart that originally had a flat front surface
(the side opposite the plug), but due to a lost neutral on a three-phase
circuit, the front is now hemispherical! I'd have to conclude that the
electrical and/or thermal fuse in the wall-wart did not do its job
correctly; it eventually fused but it should have done so much sooner.

Eric
Received on Thu Dec 16 2004 - 14:03:29 GMT