high voltage fuses

[w4bfs]

a while back the forum hashed over high voltage fusing and how expensive commercial fuses are ...

take a look at 6 x 40 mm ( 1/4 x 1 5/8 inch) chinese microwave fuses ... 750 to 900 mA  with a 5 kV rating ... lotsa available on the cheep from honorable Ebeey ... would you trust them ?

n.b.  I ordered some today to see how they look ... 20 cents each    .... how do they do it?

[Jim, W5JO]

.... how do they do it?

Right now their economy is hurting and manufacturing plants are closing all over China.  I ordered a 12 dollar battery from over there, it took a week to get and shipping was free.  Did you get free shipping? 

[w4bfs]

.... how do they do it?

Did you get free shipping? 

rodney rodney

[KD6VXI]

They undercut everyone globally to close the competition.

Someone forgot to tell them that once the competition was done for,  they had to raise their rates....   Lol

Tongue (kinda)  in cheek.

--Shane
KD6VXI

[K4RT]

 ... 20 cents each    .... how do they do it?

No R&D costs, no unions, low pay, hit or miss QC, and many factories are using machines & tooling pulled out of factories here and shipped there.

[K1JJ]

China cutthroat prices?  We're gonna build a wall. A great big wall.  It'll stretch from San Diego to Anchorage .... :-)

T

[ka1tdq]

Don't build THAT wall!  I still need my cheap Chinese DDS VFO's.  While on that thought, I also need my cheap yard workers.  It's hot here.  Believe me, it's so hot that I'm not even motivated to do antenna work.  So don't build the other wall either.

As for the Chinese HV fuses, I'm very curious myself how good they are.  I'd almost be too scared to try them though and just use the thin piece of wire technique. 

Jon

[WD8BIL]

Remember, fences make for good neighbors! 

[n2bc]

Yes, the Bussman HV fuses have gotten expensive.  15 years ago +/- I bought some Bussman HVB fuses for $4 each, now they are $22.   The other thing I note is that the HVB 2.5KV fuses are over 4" long, the higher the voltage, the longer the fuse.  I suspect this for 2 reasons, 1) you are forced to use the proper KV rated replacement, and 2) the total length is to ensure any plasma is quenched.   Maybe the Chinese jobs are some newer innards to kill the plasma (smile).   If I still had my KWS-1, I'd spring for the $22.
73, Bill  N2BC

[K1JJ]

 The other thing I note is that the HVB 2.5KV fuses are over 4" long, the higher the voltage, the longer the fuse.  I suspect this for 2 reasons, 1) you are forced to use the proper KV rated replacement, and 2) the total length is to ensure any plasma is quenched.   Maybe the Chinese jobs are some newer innards to kill the plasma (smile).   73, Bill  N2BC

Do the commercial HV fuses use a vacuum inside to keep the arc under control?  It would make a huge difference by quenching faster, if so.

I use two ceramic pillars spaced about 3" apart with a fine RG-213 coax shield wire strand, and still get situations where the arc will become sustained and create a lot of soot.  No problem as long as there is lots of room around it. Otherwise the arc can propagate and burn surrounding parts if too close.  4-5KV can be sporting, but this technique has saved my diode stack for 25 years.  The 240VAC primary breaker alone is not enough to protect it, from my experience.

T

[n2bc]

I cut open one of the Bussman HVB types I described, it was filled with a fine powder.

[w4bfs]

I saw a piece on "How its Made" on large fuse construction .... it was also sand filled

[KD6VXI]

Sand / Silica is popular for HV fuse construction.   Reason being,  when the plasma appears during the event,  the heat turns the silica into a couple very small beads of glass.   The process extinguishes the plasma VERY quickly. Easy peasy cleanup,  and doesn't make a mess from the plasma.

We use those on feeders at work.  I've replaced ones on 21 kv,  we have fuses rated to a hundred kv.

--Shane
KD6VXI

[w4bfs]

the microwave fuses came in ... some clips also came in ... I plan to use some perf board to mount the clips on ... I will take one of these .75A fuses and pass 10 Amps or so to blow it then put the blown fuse on the hi-pot tester and run it up to 4.4 kV max and see if it leaks current ....let you know

[Opcom]

Is 10A enough? Seems like a high voltage large C filter cap discharge would be a better test to see of the QRO size arc is really stopped or contained.

[kb2vxa]

I think what you need do here is simulate actual HV fault conditions such as throwing the secondary of a plate transformer at it. Just melting the wire isn't enough, you need to create a plasma arc.

[Opcom]

It should also involve the joule storage of the filter caps and any filter chokes. As we know an inductor can keep an arc going nicely once it's started and a lot of current may not be needed to keep it supplied depending on the choke.

But I agree to the testing proposal to find out how the fuse handles a large but 'normal' overload. In the microwave oven where it is intended to be used the HV transformer is usually somewhat current limited by shunts.

[w4bfs]

I think these are excellent ideas ... I may have to sacrifice more than one to science ....uggabugga !

[w4bfs]

got to chat with Reed, W2CQH on 75m this morning ... ran this hv fusing question by him and got a thought provoking response ... he asked "Are you fusing ac or dc ?" ... well I don't know where the fuse is placed in the chinese built microwave oven ... I have not torn into one for at least 10 years ....

it turns out that where the fuse is placed is very important because in an ac scenario the zero crossings tend to help quench an arc that has formed ... this is not what is happening with dc and Reed reported 1 inch drawn arcs possible with as little as 240 Vdc in inductive circuits.

time to fess up ....years ago (1976) when I built my first 4-1000 amp I fused my 4400 V B+ supply with a plain ole 3AG 1A 250V fuse suspended in a plastic fuse clip (not panel mounted) because it was what I had on hand ...very bad idea ... something arced over and the fuse tried to blow .... the fuse looked like a mirror from the fusing metal vapor deposited inside ... it was removed

[WD8BIL]

I usually put a pair of parallel 1 ohm 1 watt resistors inline at the bottom end of the plate choke. This helps limit current during tube flashes and shorts.

[KD6VXI]

From the,  'Go big or go home'  department:

50 ohms,  200 Watt CCS.    My glitch resistor for VOB.

This is directly in the path of the blower intake.   Should help keep it cool during brick on key transmissions.   :-)

This will allow crow barring the output of the power supply without having too many problems.

30 uF of filter c,  12 kva ccs xformer and 10A rectumfriers.   Didn't want anything blowing.....   CQWW AM here I come lol


--Shane
KD6VXI

[Opcom]

Say that blower may suppress the arc! Or ingest like the scene from ST: Into darkness where Kirk throws a rifle tied to a fire hose into the engine intake of Kahn's aircraft.

What is really appropriate? How can the unit be made bulletproof without the safety costing as much as the supply?

What benefit would a fuse in the position of F1 give? Maybe just protect the transformer from a regular overload.

Some locations are probably not a good idea for one reason or another. One bad thing that can happen is instantly interrupting the current through the choke if one is used. I keep noting that the rectifier can provide a freewheeling diode for the choke, which can be good or bad. What happens to the choke if F2 opens? A spark gap across the choke is probably a good idea.

Is the greatest hazard an arc in the tube? Most people put them where F3 is to protect the tube? If there is an arc in the tube and F3 blows, there will be a transient in the HV supply as the choke current charges the cap.

Trying to protect 'everything' from the mean old capacitor bank as with F4 has its own risks because it can leave the cap charged. An auxiliary bleeder could be useful there. If there is an arc in the tube and F4 blows, the choke current still has somewhere to go and then maybe F1 can be looked to, and blow at 2-3x the rated transformer current or so. Attention to the right profile for soft start is as must for something like that.

[w4bfs]

Reed suggested F2 position for this particular microwave oven fuse, if I understood him correctly .... does someone have a hv fused oven schizmatic diagram ?

[w4bfs]

well finally got around to the microwave fuses .... ratings are 5kV   .75A ... note the spring has fully retracted ... metal vapor depositing is hard to see, if any .... I applied 20Vdc at 10A and the fuse blew quickly with no flash that I could see