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Subject: Re: Protecting DC circuits from MAINS

Date: 09/05/02 at 8:27 PM
Posted by: Ray
E-mail: rrogers@plaidheron.com
Message Posted:

In Reply to: Protecting DC circuits from MAINS posted by Darren on 09/05/02 at 3:16 PM:

Depends upon the ciruit and how Paranoid you are. I would place a filter after the varistor. The filter would be designed around a 115VAC*1.414 pulse of 25ns duration. Put this pulse through your proposed filter and see if the result exceeds _any_ of the stated maximums for parts exposed to it. One of the cheapest and most efficient ways might be to use a thin trace as an inductor and terminate that trace in a capacitor. For instance 5nH with 1uf would have a rise time of about .7 microseconds. I haven't done an extended calculation so I don't know how much of the 115v pulse would get through.
You then have to consider ringing in the LC with your load. This is all done with elementary circuit theory. Elementary doesn't mean it shouldn't be done.
A larger problem is the varistor hanging in there until the fuse blows. Most fuses are slowwwww so the varistor would be exposed to the line for quite a while.
In other words your problem is serious, unless you shell out for fast Hydralic circuit breaker; probably a waste of money.
More effective might be a FET that you could "instantly" (within 10-20ns) switch off the input with when the circuit input gets to high. These might cost a little.
A start might be:
http://www.fairchildsemi.com/an/AN/AN-7504.pdf
An IBGT but you don't care.

Think carefully, it's embarassing when your cirucit goes bang or up in flames.

Ray


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