ORIGINAL MESSAGE:
On Mon, 17 Jan 2011 17:36:11 -0500, "Roger (Sub1)"
<sub1@rogerhalstead.com> wrote:
>With an inductive load such as a transformer, IF the circuit is broken
>at the peak of the AC cycle (worst case) the Reverse EMF created by the
>collapsing current will induce a tremendous back, or reverse voltage.
REPLY:
Well.... it depends.
If I am reading the original post correctly, he is asking about
interrupting the AC input while the load is still connected. This is
very different from removing the load abruptly.
In the first case, the load will usually do a good job of damping or
absorbing the inductive transient generated. Think about all the
millions of TVs and other appliances that are turned off by simply
removing the AC power. No harm done.
The second case is much more problematical. Removing a load abruptly
does indeed generate an inductive spike, but whether that does damage or
not depends greatly on the design of the power supply. If the power
supply has a large filter capacitor and a small (or no) filter choke,
that alone may absorb enough of the spike to prevent damage.
The worst case scenario would be a relatively large choke and a
relatively small filter cap. If the load is removed abruptly from that
circuit, voltages way in excess of the normal DC can be generated. I
have tested such circuits and voltage spikes three or four times the
operating DC voltage were developed. This is one reason why choke type
filters are almost never used anymore.
There is more to it than this, but what I've stated here covers the
basics.
73, Bill W6WRT
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