Glitch fuse for a pair of 3-500's

[ka1tdq]

I didn't incorporate a glitch fuse into my homebrew linear using a pair of 3-500's.  The rest of the wiring is overkill (including the plate choke), so if something goes wrong, I'm not sure exactly where the failure would be to turn things off. 

In another thread, someone mentioned a commercially available fuse rated at 10kv for around $50.  I'd rather go that route.  What current should it be rated for, and maybe someone could post a link to one? 

Jon
KA1TDQ

[WD8BIL]

A pair of parallel 1ohm 1watt resistors works well also.

[ka1tdq]

I suppose.  But I'm willing to spend the $50 for a nice device to blow, and not arc like a piece of wire.

Jon

[ka1bwo]

I didn't incorporate a glitch fuse into my homebrew linear using a pair of 3-500's.  The rest of the wiring is overkill (including the plate choke), so if something goes wrong, I'm not sure exactly where the failure would be to turn things off. 

In another thread, someone mentioned a commercially available fuse rated at 10kv for around $50.  I'd rather go that route.  What current should it be rated for, and maybe someone could post a link to one? 

Jon
KA1TDQ

Jon,
Just use a 1 amp fast blow fuse on the low side of the HV power supply. 
Joe

[ka1tdq]

Oh, right... that would work.  Thanks!

Jon

[n2bc]

Buss makes HV fuses:

http://www.cooperindustries.com/content/public/en/bussmann/electrical/products/medium_voltage/hva_hvb_hvj_hvl_hvrhvthvuhvwhvx.html

I spotted some for $13, $15 on Ebay.

[ka1tdq]

Using the low side of the HV supply seems to make more sense.  I don't see a benefit to fusing the B+ hot side.

Jon

[K1JJ]

Hmmm....  but when the low side fuse path is BROKEN, won't there be high voltage when measured ACROSS this open fuse just as when the fuse is in the high side?  (IE, when the fuses blows, there is collapsing current still seeking a path and causing an arc?)  



T

[W2PFY]

Look up W2DTC on Google for his web page. He has a pair of 3CX3000A7's in parallel and has a homemade Glitch fuse in there. While his fuse may be too high in it's current rating, you can get an idea from there. What is the rating of your plate RFC choke? The fuse wire should be smaller of that of the RFC otherwise the RFC would become the fuse. My guess is that a four inch length of # 24 wire would do the job. Find the wire tables for the current rating of small wire. You want to have at least four inches for the fuse so that a sustained arc will not occur.

[K1JJ]

Another hesitation about using a fuse in the negative lead...  Just where would we put the fuse? If it is in the negative diode lead of a bridge rectifier or in a negative lead filter choke, (or transformer CT)  then the filter capacitors will still hold residual power for a second or so to dump into the rig after the fuse has blown.

Or, if we disconnect the negative lead going to the rig using the fuse, then  this becomes an unsafe situation.

I think the advantage of the fuse in the positive lead (located AFTER the filter caps to the rig) is that it disconnects the filter caps and all current flow is stopped instantly once the fuse is blown. (and arc quenched, if any)  And the ground lead remains connected to the rig for safety.

T

[KD6VXI]

Proper fusing would be in the primary,  secondary between xformer and rectumfrier,  then another one on the output of the supply.

--Shane
KD6VXI

[W3GMS]

Good subject. 

First you need to identify what your trying to protect.  Then you know where to place the fusing.  We all know that primary AC line protection is good circuit design, but will protect for longer duration faults. That protection is rather slow to respond to a load connected to the power supply, ie, RF Deck.  AC line protection has definite merits for other type of faults.  I have used fusing going to the solid state rectifiers back in the day where solid state diodes were not of the best quality with good success.  Then you have load faults, those things that are connected to the power supply output.  Extreme level of fault energy can occur at that point due to the low ESR/ESL of the output filter caps.  You can have a  HV fuse at that point and when sized properly which will limit RF deck components from blowing up when certain RF deck components fail. 

Its not a one for all type of solution, so determine what you want to protect and that should dictate where you place the fusing. 

Joe-GMS   

[K1JJ]

Its not a one for all type of solution, so determine what you want to protect and that should dictate where you place the fusing.  

Joe-GMS    

Hi Joe -

This is true.  In Frank's case, it appears he is trying to protect the HV path going from the filter caps to the RF deck. This fuse can in effect protect parts of both the power supply and the RF deck, depending upon what happens.

In the case of my 4-1000A plated modulated rig, I have breakers in the 240 line on each side of the mains. In addition, I use a HV fuse from the bridge diode stack  to the filter caps. After many bad shorts in the RF deck, I have never lost a diode stack in 30 years. This HV wire fuse in the diode cathode lead has blown many, many times. It is well worth the effort. BTW, this wire fuse will fatigue over time and pop simply from the current surge when the diodes first charge the filter caps. I use a step start in the AC primary on every key up, so this solves that problem.

And lastly, I have a 25 ohm, 50 watt resistor in series with the HV lead from the filter cap to the RF deck. In this same lead I have yet another HV wire fuse that occasionally gets taken out during an RF deck short.

Over 30 years I can't begin to tell you how many new homebrew rigs have been tested and saved by this simple system.

If that's not enough, the grid, screen, and plate circuits of both the modulator and RF deck of the 4-1000A rig have Hall effect devices that shut down the rig whenever the current of any of these preset tube circuits is exceeded. I am a big proponent of effective shutdown schemes because you never know when a dumb mistake will be made....


T

[W3GMS]

Its not a one for all type of solution, so determine what you want to protect and that should dictate where you place the fusing.  

Joe-GMS    

Hi Joe -
This is true.  In Frank's case, it appears he is trying to protect the HV path going from the filter caps to the RF deck. This fuse can in effect protect both the power supply and the RF deck, depending upon what happens.

In the case of my 4-1000A plated modulated rig, I have breakers in the 240 line on each side of the mains. In addition, I use a HV fuse from the bridge diode stack  to the filter caps. After many bad shorts in the RF deck, I have never lost a diode stack in 30 years. This HV wire fuse in the diode anode lead has blown many, many times. It is well worth the effort. BTW, this wire fuse will fatigue over time and pop simply from the current surge when the diodes first charge the filter caps. I use a step start in the AC primary on every key up, so this solves that problem.
And lastly, I have a 25 ohm, 50 watt resistor in series with the HV lead from the filter cap to the RF deck. In this same lead I have yet another HV wire fuse that occasionally gets taken out during an RF deck short.
Over 30 years I can't begin to tell you how many new homebrew rigs have been tested and saved by this simple system.
If that's not enough, the grid, screen, and plate circuits of both the modulator and RF deck of the 4-1000A rig have Hall effect devices that shut down the rig whenever the current of any of these preset tube circuits is exceeded. I am a big proponent of effective shutdown schemes because you never know when a dumb mistake will be made....
T

Sounds like you have things well under control along with the years of experience to show it works.  Fusing can be tricky and sometimes it becomes impractical to protect against every type of fault.   

Its important for folks to realize that a fuse from last output cap within the power supply to the load will not protect the power supply if a power supply component fails.  It will protect the power supply from being overloaded,  but things like a shorted cap, filter reactor, transformer, etc etc still need protection.  Protection from the xmfr to diode assembly will protect most of the faults within the supply.  A transformer short will be protected by the AC primary protection. 

Yes, flexing occurs mainly due to inrush current at the time of initial load current and the fuse actually flexes and gives up the ghost from time to time!

Joe-GMS     

[ka1tdq]

In my case, the best place to fuse would be on the B+ line right after the caps (like Tom said).  I can't lift the center tap on my HV transformer and fuse that because, if the fuse blows, the transformer would no longer have a center tap.  That's bad for the transformer.  It's from a BC-610. 

Plus, due to how I constructed the amp, there's a convenient spot to put two ceramic standoffs on a teflon board and run a small gauge wire between them.  My plate choke is home-brew and uses 20 gauge wire (I think), so #24 at 4" seems like the ticket. 

My transformer primaries are also fused.  One leg of the 240 goes to the filament transformer, and the other leg goes to the HV supply.  The HV uses a soft-start switch.

Jon

[ka1tdq]

I thought I'd use this PVC electrical conduit box as my 'fuse holder.'  The screw holes are 5" apart, and I can just screw it to my chassis.

Jon

[w1vtp]

I thought I'd use this PVC electrical conduit box as my 'fuse holder.'  The screw holes are 5" apart, and I can just screw it to my chassis.

Jon

Would putting the glitch fuse in a box like that sustain the arc that will result in an overload?  Just asking.  Ken DTC has had first hand experience with a glitch fuse going and he likes open air with some distance between the support insulators.  He uses #30 enameled wire.

http://w2dtc.com/3cx3000f7.htm

http://w2dtc.com/w2dtc-hv-power-supply-page.htm

Al

[K1JJ]

If Jon pulls the wire up into a high arch, like a loop hovering over the PVC holder, it should be OK. The arc usually flows upwards with the hot air it produces.  There will be SOME arc since it is in free air, but as the wire burns and gets farther apart it will extinguish.   It will burn with an upwards wandering flow back and forth, so make sure there is nothing close to either side to touch. An arc can wander a long distance from its source when feeding on the air and wire.  Look at the soot marks and you will get an idea what it does.    In some cases I have seen an open air  wire HV fuse sustain an arc for up to one second or so. This may not protect things as well as we like.

Again, a real HV fuse in a vacuum (or inside a non-oxygen, or other non-burning gas) would solve the arcing problem completely. It would respond much quicker to the short circuit. You could always add one later if you like.


Jon, I would use a smaller diameter wire than what you have in there now. As Al suggested, #30 (or probably even finer) might do what you need.  If you have some old coax, just take a single strand from the outer shield and try it. If it opens during normal use, then add a second strand in parallel.  That's how I found the optimum wire diameter for my various HV fuses.    The worst thing you want is a wire that will handle 3A or more and do nothing to protect.

T

[KB2WIG]

A bit over stated, but it does illustrated Toms' point.


https://www.youtube.com/watch?v=UrdblID5as4

This one has been a favorite of mine.

If you have a few minutes,

https://www.youtube.com/watch?v=yX5TIDLvMyw

klc

[ka1tdq]

I went with a smaller diameter wire, but I didn't want to get too small. My main concern was that I wanted a known weak spot so that the whole system doesn't melt down. I used 26 gauge and it tested fine. My plate choke is 20 gauge. Everything else in the system is either 12 or 14 gauge. Plus, I can see the fuse in the viewing window.

Jon

[K1JJ]

I went with a smaller diameter wire, but I didn't want to get too small. My main concern was that I wanted a known weak spot so that the whole system doesn't melt down. I used 26 gauge and it tested fine. My plate choke is 20 gauge. Everything else in the system is either 12 or 14 gauge. Plus, I can see the fuse in the viewing window.
Jon

Hi Jon,

Well, the problem is that # 26 gauge is designed to safely carry 2.2A without burning up. It might pop at maybe 4A or so, probably much higher?  My guess is your power supply is rated closer to  .5 to 1A, no?   Anyway, the HV fuse would probably never blow - it would always be the AC mains breaker.


For a pair of 3-500Zs  I would opt to start with  #32 wire that will safely carry .53A .  It might blow at 1 - 2A or so, probably much higher. This is something you have to experiment with and see.  That's why using the single strand of coax shield or two strands, etc., is a good way to go.

With my 4-1000A rig, during a serious fault, sometimes the AC mains breaker pops OR sometimes the HV fuse pops - OR sometimes they both pop together. I like the redundancy.  This requires a little time to find just the right diameter wire to use for the HV fuse.

Here's a wire gauge vs: current rating sheet.

http://www.powerstream.com/Wire_Size.htm

T

[ka1tdq]

I find it amazing that that little wire can carry so much.  I'll put in a 30 gauge strand and try it.  For the current one, I went carrier down while staring at the wire and holding onto the transmit switch. It seemed fine though. So... Live life on the edge and try 30.

Jon

[W3GMS]

Wire is typically not rated for fusing current, but fuses are!  So making a fuse out of wire is OK, but the only way your going to know when it opens is to test it.  Saying that, depending on the percent of the overload, the time to open the wire will vary.   This is also true with real fuses.  A overload of say 5 times the fuse current will be slower opening time than the time required to open a fuse that is handling a 10 times overload.  So using a wire is very uncharted territory as to the specific time vs. current causing the wire to open.  Hey, this is Ham Radio and we do this kind of stuff all the time so I am not discouraging using a wire.  Also since the wire is placed between the output filter capacitor and the load, there will be extremely large amount of short circuit current to vaporize the wire upon a shorted overload.  The worse case situation is when you have a slowly increase in current and under that failure mode it will take quite some time for things to open up.

Joe-GMS   

[K1JJ]

Yep, this is really catastrophic short circuit type protection. I think a finer wire than we expect will usually be the right choice.

I have definitely saved my huge stack of solid state diodes as well as reduced the effects of tube flashovers and cabinet arcs using the HV fusing techniques.

A pro would simply buy a ready-made HV fuse. But being hams, might as well see what works for cheap...  

I was wondering if very fine nichrome wire might make a cool fusing experiment. They sell rolls of #30, #38 and #40 nichrome. The little added resistance in the HV lead would be good for surges too.  It would be fun to watch it turn a dull red during a transmission when in its zone...

https://www.google.com/?gws_rd=ssl#q=nichrome+wire

T

[KA2DZT]

Not sure if nichrome wire would fuse that easily since it used to take a lot of heat.