[Amps] Fusing the Plate winding

[Will Matney]

Jim,

You could make a crowbar clamp circuit for it. The only problem is a  
straight crowbar will blow a fuse. Lets suppose you pick up a small  
voltage across a resistor and place it across a 5-6 volt zener. When the  
zener fires, it will put a voltage on an opto-isolator. The opto in turn  
will hook to the base of a large transistor. The transistors collector  
would go to B+ and the emitter to ground in a clamp to blow a fuse.  
However, if it fired a relays coil with the collector going to say a 12-24  
Vdc coil and the emitter to ground, I think would work. The wire in the  
transformer, I dont think will fuse that quick. The reason being is it  
should be wound at least for 750 cir. mils. per ampere for ICAS. For a CCS  
transformer, 1000-1200 cir. mils per ampere. A fuse wire is way smaller  
than this and way under the conduit rating of a wire. At 500 cir. mils per  
ampere, that's about X2 the current rating of a conduit rating. It dont  
hurt them for a temporary overload like this. Hardly ever would a shorted  
rectifier, capacitor, or even a tube, burn out a winding because the line  
fuse opens before any damage. The rectifiers backwards action your talking  
about is the same thing, acting like a short.

Will



On Mon, 7 Feb 2005 08:21:57 -0600, Jim Isbell, W5JAI  
<jim.isbell at gmail.com> wrote:

> Well, my concern is that by the time a relay clicks the secondary is
> already gone.  Same with a fuse.
>
>
> On Mon, 07 Feb 2005 08:16:27 -0500, Will Matney <craxd1 at ezwv.com> wrote:
>> On Sun, 6 Feb 2005 16:56:02 -0600, Jim Isbell, W5JAI
>> <jim.isbell at gmail.com> wrote:
>>
>> > I am in the final steps of designing and building a linear to
>> > supplement my station om 160 thru 10 Meters.  The chassis is being
>> > built at this time and I will soon be mounting parts.
>> >
>> > It is a single 4-1000A running GG with a plate supply that is
>> > switchable in steps from 3KV to 6KV.  The transformer is a 12KVA
>> > transformer which will obviously be able to handle 2 amps on the
>> > secondary winding.  I will be using a pair of VT-42A Mercury vapor
>> > rectifiers.
>> >
>> > Yes, I know there are "better" ways of doing it, but none would be
>> > prettier.  This is to be an amp capable of full amateur legal limit
>> > that will be a JOY TO WATCH. not just an appliance.
>> >
>> > OK, the questions.
>> >
>> > I have heard from very good authority that the mercury vapor
>> > rectifiers might short on occasion and that the secondary of my plate
>> > transformer would probably "fuse" just as fast (or even faster) as a
>> > FB 1 amp fuse in the plate lead.
>> >
>>
>> Wouldn't a current limiting resistor between the transformer and  
>> rectifier
>> be prudent?
>>
>> > The first question.  If I don't need a fuse any bigger than 1/2 amp
>> > and the secondary is rated for 2 amps.  Will I be safe from blowing
>> > the transformer?
>> >
>>
>> The fuse has the be big enough to carry the max. plate current, what  
>> will
>> it be?
>>
>> > The second question.  If the above answer is NO, is there any circuit
>> > I can build that will protect the transformer?
>> >
>>
>> Some type of current limiter. A series resistor could be all that's  
>> needed
>> or sense the voltage across one and when it's high enough, the circuit
>> would close a relay opening the primary leads.
>>
>> > I would sure hate to leave off the glow of a pair of VT-42As to light
>> > the inside of the chassis around my 4-1000A which will be behind a
>> > window.  This is art now, not engineering, and I sure like that purple
>> > glow.
>> >
>>
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>



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