All this reverse EMF stuff has me concerned about some KAPOW events seen in
the past. I saw one event that destroyed circuit board traces in the
primary circuit of a power supply located in another part of the shack.
I would like to do some testing for this and would appreciate any input on
the proposed method.
What I have in mind is connecting a single diode and 1uF capacitor to the
transformer primary. At turn on, it will charge up the peak AC line voltage
value, approximately 160vdc. At turn off or load disconnect or other
scenario, any counter EMF voltage spike would increase the reading across the
capacitor.
I am assuming the primary winding would provide a low impedance voltage
source to charge the capacitor to peak value pretty much instantly. A high
impedance (11 megohm) DVM could then be used to measure the voltage before it
had time to discharge very much.
The goal here is to quantify the problem and evaluate suppression and
switching techniques.
Any thoughts?
Thanks/73,
Gerald K5GW
In a message dated 1/17/2011 5:09:55 P.M. Central Standard Time,
dezrat1242@yahoo.com writes:
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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