The AM Forum

Guys,

I am thinking about fusing the 2600 VDC supply in my transmitter. Both the 115 and 220 VAC primary circuits in the rig are currently protected by fast-acting circuit breakers.

Bussman offers a line of fast-acting high voltage fuses (their HV series) designed for operation over the range of 1000 to 10,000 volts, with current ratings of 1/16th to 2 amperes. These fuses are relatively expensive; a 0.75 amp, 5000 volt fuse sells for aobut $16.00 each net.

My question is this; why can't any fuse of the appropriate current rating be used, so long as the fuse is kept sufficiently above ground so as not to arc over to ground? The way I look at it, amps are amps, regardless of the fuse voltage rating or the applied voltage. What makes this fuse a "high voltage" fuse, vs. a standard type AG or AGC fuse???

Collins uses this same Bussman high voltage fuse in the HV section of the Collins KWS-1; these fuses are also becoming difficult to source.

Thanks & Best Regards,

Bruce

As I understand it... the Bussman HVA and HVB fuses are filled with something akin to sand. The idea is to quench any potential plasma.  A standard 1" fuse will actually support a plasma in the event the elements melts and the voltage is high enuf!

I was able to get Bussman fuses for my KWS-1 at a local electrical supply - I think they were $8.  But that was 8 or 10 years ago.

73, Bill  N2BC

Hi Bill,

Thanks for the explanation; it makes a lot of sense! BTW, Graybar Electric sells the Bussman HVA/HVB fuses over-the-counter, but the pricing is certainly not around $8.00 each as you paid a number of years ago!!! Even still, I think I'll pick-up a few as spares for my KWS-1, and add one of these HV fuses to protect the HV supply, the modulation xfmr, and the modulation reactor in my homebrew AM rig, just to be safe.

73,

Bruce

Being a cheapskate I have used the 3AG band-aid for years. I don't see plasma as being an issue at 2500-3500V. The only issue I have had is cleaning up the  shrapnel (glass) out of the bottom of the cabinet when the fuse blows. Perhaps putting 2 fuses in series would be a benefit.
I also like the W2DTC method -- Ken included a fusable link of #22 wire in his amp. 2 ceramic standoffs with wire between them. Cheap, simple , effective. See his website and look at the 3CX3000F7 amp article. Look up the wire tables in your handbook and select a wire that will handle the current you need.
Skip

#22 can carry serious current. Try a strand of RG8 shield or wire wrap wire

Hey we aren't talking about a pair of 6AQ5's here!
Like I said, select wire to suit the current. I have taken the buss fuses apart and they have a fine wire inside. Your particular application might need #40 wire or a piece of wet dental floss for all I know .

Yes, a single strand of RG/8 shield is FB for 2-3KV supplies. For the bigger stuff, use 2 or 3 strands in parallel. Mount the standoffs at least 1.5" apart, as there definately is a bad arc when it goes and may sustain itself if the standoffs are too close together.

I've saved my diode stacks many times with this method. It's usually a result of  leaving the Jesus shorting stick on the supply and spacing out.

I didn't comment earlier cuz, I know Bruce. He builds mil-spec type gear and wants to use the correct fuses and holders... no standoffs and wire JS's...  ;)

T

That is correct, those type of fuses have sand, chalk or some sort of silica inside. We use them here for the DC drives on some of the equipment that pulls allot of current they've been known to go off like an M-60 firecracker.. Then they spray sand everywhere inside the cabinet.. It's pretty funny when one goes. One blew on a fork truck the other day and almost set of a shell shock episode with the poor Nam-Vet driving it!

This otta get me flamed BIG time..........
Seems to me if your are trying to protect perhaps hundreds of dollars if not thousands of dollars of parts and equipment....the price to pay for HV fuses is insignificant. Some ppl will pay big bucks for a toob......then piss and mone about the price for a specialized fuse??????? I dont get it.
As my dear old departed Dad used to say.........some ppl would not pay 25 cents to see the statue of liberty lift her leg and pee in NY harbor!!!!
So there!!!!

10 ohm large carbon resistor works around a couple thousand volts if you can find one.
Those precision gold plated wirewounds blow off like firecrackers. Stay away from those.

Would this fuse typically be installed in the HV supply or in the RF deck?

I use a # 37 wire on two standoffs about 2" apart at the base of the final plate choke of the homebrew 813's. A few turns, about 1/4" in diameter. Works great.
GAry...WZ1M

Once I had to built a monitor that would detect an explosive squib was present as part of a bit test. This squib was set off by current so I had to be sure not to blow it when I sent the test current through it. I needed a resistor that would never fail short and always open up. We tehsted a number of types and all but one reduced in value as you let the smoke out and light begins to come out. The only resistor that increased in value then opened was a wire wound. All other types film and carbon lit up as they decreased to short.
My thing was part of a bit test for an aircraft weapon system and user didn't want to light squibs off if they ran a bit test. fc

N3RLX is CORRECT!!!
cheap is much different than frugal

In Australia we have an electronics supplier that sells "Microwave oven fuses". These things are rated at 5Kv and come in ratings up to 2 amps. They are really safe and are only about $6.00 AU. Maybe there is a similar distributor in USA!?

Yep, its difficult to break a DC circuit when fault current is high, it forms a plasma across the fuse ends. If you try and open a switch with high DC current, it may continue to arc as you open it. 3AG fuses work much better breaking AC circuit. HV fuses are made to extinguish the DC arc when the fuse wire blows open. At work we use some HV fuses for 90 kV DC, 25 Amp service. They are about 3 feet long, 6 inch diameter, and filled with sand. We have them rebuilt when they blow....

HMMM

WU2D Mike

Reminds me of a chick I used to see back in the 70's at the Cambridge Common Sunday afternoon concerts selling jewellery she had made from resistors. I suppose she thought the little banded cylinder shaped objects were unique and pretty, but to me they just looked like resistors, and I thought it looked pretty lame to be walking around with resistors hanging off your ears.

But this has been used as a form of political protest.  I recall stories back in the late 80's, when the Iron Curtain was crumbling and the Solidarity Union movement was gaining momentum in Poland, the Communist government was cracking down on leafleting and other forms of public dissent.  So people who had sympathy for the Solidarity movement went underground and began to wear resistors on their clothing to advertise the fact that they were "resistors", resisting the government crackdown. This was sort of an "in-crowd" protest.  The authorities never figured it out until the whole Communist system in Poland was collapsing.  Within a year or so, East Germany had fallen and the USSR was in the process of imploding.

BTW, the collapse of USSR has an AM amateur radio connection.  Reportedly, when the  hard-liners were attempting the coup and Boris Yeltsin was showing his defiance by sitting on top of the tank in Red Square, there were several pirate radio stations in operation in the vicinity of Moscow, hastily put together by hams who modified amateur AM transmitters to work on the broadcast bands (not sure if it was MW or Shortwave).  This was the only news source of what was happening, since the state-run radio was under control of the hard-liners.

Anyway, to get back on topic, I have used ordinary fuses in HV lines.  They usually explode like firecrackers, but with me  they always protected the equipment.

I have found them very useful in troubleshooting HV power supplies when the a.c. line fuse randomly lets go.  It could be a short in the transformer, a bad diode, a flash-over in MV rectifiers, an intermittent short in the filter choke, an intermittent short in the filter cap.  You get the idea.  That sort of thing can drive you crazy trying to isolate the trouble.

To start off, I temporarily JS wire a couple of ordinary fuses between each end of the transformer HV winding and the rectifier, and another between the rectifier output and filter circuit, and a third one at the output of the power supply.

If the one at the output of the PS blows, you know it is something in the transmitter itself, not the power supply.  If it is the one between rectifier and filter, you know it is in the filter circuit.  If it is the one(s) between transformer winding and rectifier, but none of the others, then it is most probably the rectifier.

I have chased down intermittent problems (which accounts for about 99% of my transmitter failures), by leaving the test fuse in line sometimes weeks at a time until the problem re-occurs.  I keep one fuse in the circuit until the failure happens, and then I go on to place another one elsewhere until the problem is tracked down.

The most serious cause is indicated by no blown fuse except the main power fuse.  That indicates that the problem is probably the plate transformer itself.  Over the years, my HF-300 rig has eaten one main HV plate transformer, at least two medium-voltage plate transformers and one high voltage swinging choke.

For testing purpose, it doesn't really matter if the fuse actually disconnects the circuit or not, since the power supply is still protected by the main a.c. line fuse.  But there is no problem figuring out which fuse blew!