Microwave Transformer Isolation

[Proschuno]

So I'm using a step down- step up arrangement; could the fuse maybe be placed somewhere in between? My thinking is that the output current is obviously dependent on input current (which our output is dependent on the load), so limiting the input current (which would be at a 40 V max) to the step up transformer should limit its output current.

I removed the magnetic shunts and maybe plan on tearing apart another transformer for its copper so I can put more turns on the primary to increase coupling. I guess this should slightly reduce the output voltage, but rectified and filtered it would already be 1131 volts, maybe putting on a number of turns should hopefully only lose enough to make 1000V?

 

[john monks]

Proschuno, this would be unsafe but could be done in my opinion. I have such a transformer and considered using it myself in a similar situation and I did not because I can't figure out how to mount the transformer keeping the core isolated from my chassis because of the weight. If you figure it out and your still here tell us how you did it.

 

[Proschuno]

Yeah, I just posted that I desoldered one of the secondary leads from the core, which resistance testing proved that there was no connection afterward. I figured to that I would place an MOV or something like that rated for 1000V on the output.

 

[Proschuno]

Question though, since the fuse pretty much acts like a short (and hence will see no significant voltage drop), could I theoretically just use a regular 250V fuse? I assume that's what the voltage rating means, unless someone can correct me, but the voltage limit on fuses almost never gets exceeded due to this fact.

BUT, looking up on mouser AND digikey reveals 7$ fuses with the exact voltage and current rating, however.

 

[(*steve*)]

A 250V fuse is rated for 250V because at that voltage it will successfully blow and open the circuit.

At higher voltages it may sustain an arc, leaving the device powered.

You may have seen similar behaviour in light globes. Sometimes when they blow as you turn them on, the make a buzzing sound and remain (kinda) lit. If you turn the power off then back on again, they will not light up. In this case the filament has broken, but an arc has been generated across the break and this sustains the current.

Whilst this behaviour is harmless (and even slightly amusing or entertaining) in a light bulb, it has the potential to be far less so if it happens in a fuse when a fault exists.

 

[Proschuno]

I see... and so it is a smart idea to put a fuse in series with the HT? I've looked at a couple of other high voltage supplies and I don't see this. Any ideas WHERE I should put fuses? I know to put one between the step down transformer and the mains.

 

[(*steve*)]

No, it's not a smart idea to put a fuse in the HT unless you can get a very high voltage fuse. It's likely to be expensive.

You fuse the low voltage side where the current is higher. The fuses are cheaper for both of these reasons.

 

[Proschuno]

Ok I see, so would it be overkill to put a fuse between the mains and the step down, AND in between the two transformers? I'm using a step down - step up arrangement if hat hasn't been mentioned already