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Re: [Amps] Power Supply Turn Off Under Load

To: dezrat1242@yahoo.com, amps@contesting.com
Subject: Re: [Amps] Power Supply Turn Off Under Load
From: TexasRF@aol.com
Date: Wed, 26 Jan 2011 08:14:40 EST
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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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