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???
01/31/10 22:23
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#172824 - It is very difficult...
Responding to: ???'s previous message
...to safely handle this enormous involved power with a "tiny" protection circuit!


- 640V (450VAC) does not allow the use of Polysfuses!


- A normal melting fuse must be able to handle 640V and to stop huge currents. So, a fast acting, sand filled 1000V fuse should be used, like those used in Fluke DMMs for instance:

http://docs-europe.origin.electrocomponents.com/webdocs/0...279c60.pdf


- A fat wirewound power resistor, like this here

http://www.vitrohm.com/download/series/24/

could be used, which can withstand huge currents for a few milliseconds (when properly chosen!) until the fuse breaks. Eventually more than one must be connected in series to withstand a short to 640V.


- A very very fat PTC could also be used, eventually in combination with wirewound power resistor(s).


- One or several 5KP30A Transzorbs in parallel

http://www.vishay.com/docs/88308/88308.pdf

could be used to clamp the input voltage to a safe level. This 5kW Transzorb can handle a few kW during a few milliseconds!


- A LM2574HV switcher (see the "HV"!) should be used, though.


Make no mistake, I didn't say that it is simple to design something like this!!

An example for a scheme working up to 120VAC can be seen here (figure 11):

http://cds.linear.com/docs/Datasheet/178591fc.pdf

But remember, a Polyfuse will not work in your application, as there doesn't exist any that can withstand 640V, as far as I know.


Kai Klaas

List of 12 messages in thread
TopicAuthorDate
LM2574-5 AC voltage protection            01/01/70 00:00      
   of course not            01/01/70 00:00      
   no AC protection            01/01/70 00:00      
      Thank you both            01/01/70 00:00      
         fire brigade            01/01/70 00:00      
            suggestions?            01/01/70 00:00      
               something like this            01/01/70 00:00      
               already made            01/01/70 00:00      
                  just one more thing            01/01/70 00:00      
                  thank you            01/01/70 00:00      
                     It is very difficult...            01/01/70 00:00      
                     very small components are not recommended            01/01/70 00:00      

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