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Registered Member #1530
Joined: Tue Jun 10 2008, 03:34PM
Location:
Posts: 32
So I'ma fusing my little new variac (used) for use with a halfbridge SSTC fRes at about 350kHz without topload.
I got nice panel meters so I know what is going into it, but I'd also like to fuse the circuit so I don't burn out the variac. It is rated at 5 amps, and I have 8 amp and 20 amp slow blow fuses. I intend on overloading the variac, since the sstc will not be on very long. I'll start out with the 8 amp fuse but...
Where to put it? After the variac, before the rectifiers? Before the variac, after the 15 amp circuit breaker for the house? I also wonder how fast those circuit breakers are.
How much current have you guys been drawing from the wall, and been pulsing through the bridge? I've heard things like hundreds of amps in DRSSTCs, but this should be different.
Of course I'll start out at under 50 volts to make sure the whole circuit works OK. Just wonder where you all have been putting your fuses. I wish I got one of those small circuit breakers. Saves using up bunches of fuses when you push just a bit too hard.
Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
Ideally you should have a fuse both on the input and output side of the variac.
Since a variac is a transformer, if you set it to a low output voltage and short circuit it, you can have hundreds of amps of short circuit current burning the carbon brush and windings, without pulling enough current at the input side to blow the fuse!
Registered Member #1530
Joined: Tue Jun 10 2008, 03:34PM
Location:
Posts: 32
well hopefully the avg current will be fairly proportional to input voltage. I'll fuse the output. Yeah, I noticed that it was a little chunk of carbon. I wouldn't think that it would be a great conductor (pencil graphite) But I guess this is a different material hmmmm..
Registered Member #1484
Joined: Wed May 14 2008, 03:24PM
Location: Cary, NC, USA
Posts: 27
IET Labs (believe they own the Variac trademark currently) recommends fusing the output and most of their larger units include internal fusing on the output, see this PDF for more information.
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Variac fusing for TC
