[Amps] HV fuses

[John G3UUT]

In a microwave, I believe that these fuses are before the rectifier in 
the AC part of the circuit.  I'm sure everybody knows this but I'll 
mention it again.  As AC drops to zero volts during a cycle, the chance 
of getting long lasting destructive arcs (plasma arcs) is much lower, 
therefore using them on DC in an amplifier is potentially problematic.

Having said this I've used a microwave fuse in an amp, putting it before 
the glitch resistor on about 2.4 kV and it seemed to blow ok.  They are 
very inexpensive and certainly attractive for use in amps and so a bit 
of research on their usability would be very useful.   The fuse I used 
was in a white plastic insulating enclosure so that if it blew up 
removing the bits of glass wouldn't be difficult and I had a glitch 
resistor to limit the potential arc current.   I suspect the voltage and 
the value of the glitch resistor are going to be the biggest factors 
determining whether there will be an arc.  Take care doing the experiments.

73 John G3UUT

On 25/10/2013 07:49, Fuqua, Bill L wrote:
> I would wait until New Years or July 4th to give one a try.
> Hi Hi
> 73
> Bill wa4lav
>
> ________________________________________
> From: MU 4CX250B [4cx250b at miamioh.edu]
> Sent: Friday, October 25, 2013 1:48 AM
> To: Fuqua, Bill L
> Cc: Amps at contesting.com
> Subject: Re: [Amps] HV fuses
>
> It didn't sound right to me either, Bill. The seller provided no data
> sheets other then claiming a 900 mA/5kV rating for the fuses. I think
> they were intended for microwave ovens, but am not sure. As I said, I
> bought them out of curiosity because they were so cheap.
>
> Sent from my iPad
>
>> On Oct 24, 2013, at 23:38, "Fuqua, Bill L" <wlfuqu00 at uky.edu> wrote:
>>
>>    This does not sound right. I assume you are saying that they are glass fuses.
>> I suspect if you blow one in a HV powe rsupply it will shatter or explode into pieces if
>> you have a reasonable value of filter capacitor.  Do you have a data sheet on these
>> fuses?
>> 73
>> Bill wa4lav
>>
>> ________________________________________
>> From: Amps [amps-bounces at contesting.com] on behalf of Jim Garland [4cx250b at miamioh.edu]
>> Sent: Thursday, October 24, 2013 7:58 PM
>> To: Amps at contesting.com
>> Subject: [Amps] HV fuses
>>
>> Out of curiousity, I bought several 900 mA/5 KV fuses from an eBay Chinese
>> supplier. They arrived today. They fit fine into 3AG fuse clips, but are
>> about 50% longer. Internally they have a spring connected to the fuse link
>> which pulls the connection apart when the fuse opens. If they work, they're
>> pretty cool. and VERY inexpenisve.  I'm wondering why one couldn't parallel
>> two or three of them to multiply the current rating?  Assuming they're
>> reasonably well matched, so that the current divides equally among the
>> fuses, it would be a very compact and inexpensive HV protection circuit. I
>> would mount fuse clips on a piece of G10 circuit board and mount the
>> assembly on ceramic standoffs. When one fuse opened up, all would open up,
>> but replacing three fuses would cost  only a dollar or so. I'll test some of
>> the fuses ( I bought twenty) to see how accurate their current rating is.  I
>> have no convenient way to test their HV rating, but I'd use them only at
>> 2500V in my dhomebrew amplifier.
>>
>> 73,
>>
>> Jim W8ZR
>>
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