[Amps] Glitch resistor + fuse?
Jim Garland
4cx250b at miamioh.edu
Tue Nov 14 19:25:14 EST 2017
Great comments, Jim.
You're right, of course, about a 50ohm/10W glitch resistor being
inadequate. I panicked when I read your comment and checked the
schematic of my latest two power supplies. It turns out I used 10 ohms@
50W, not 50 ohms at 10W. I got things backwards in my comment. Whew! I
realize that 10 ohms may be a bit too low for most power supplies, but
these particular supplies can source a little over two amps, so 10 ohms
isn't unreasonably small in worst case conditions. In addition to the
glitch resistor and HV fuses (two or three in parallel, depending on the
RF deck), I also have have an overcurrent trip circuit and, like you, a
double pole magnetic circuit breaker in the primary line.
Re vacuum relays, I agree that that most any moderately sized vacuum
relay in the HV line can stick if subjected to a DC short to ground.
I've tried to minimize that problem with Gigavac relays that are
designed to hot switch DC voltages of 10kV at 10A, if I"m remembering
right. I had an internal arc in a new GU74B, which tripped everything,
but the Gigavacs worked fine. (The Gigavac relays aren't intended as
protection devices, but just to route the HV around to different RF
decks.) Years ago, I learned the hard way that ordinary vacuum relays
designed for RF switching should NEVER be used to interrupt DC currents.
Here's a tip, I learned at the same time. Sticking vacuum relays can
often be fixed by holding them near a powerful magnet. Once freed, they
seem to work normally, but I imagine they don't meet full specs for
current capacity.
BTW, are you sure that the Drake .82 ohm fuse resistors are 1/2W? I
looked in my resistor bin and found 2W resistors. Probably doesn't make
any difference. Whatever, they sure make a lot of noise when they blow!
73,
Jim W8ZR
On 11/14/2017 4:32 PM, Jim Thomson wrote:
> Date: Mon, 13 Nov 2017 10:09:29 -0700
> From: Jim Garland <4cx250b at miamioh.edu>
> To: amps at contesting.com
> Subject: Re: [Amps] Glitch resistor + fuse?
>
> <Nearly all commercial amplifiers have glitch resistors in the HV line,
> typically about 50 ohms. A 10W resistor is fine, and a larger wattage
> one is fine also, but the larger power rating is unnecessary. The glitch
> fuse is to limit the current pulse in the event of a short in the HV
> line, as might result from an internal tube arc. For example, a short to
> ground in a 2500V power supply will result in a peak current pulse of 50
> Amps. Obviously an HV power supply can't supply that much current, but
> the charge stored in the filter capacitor bank can produce a brief pulse
> with that much current.The glitch resistor is not intended to blow, but
> to absorb the charge stored in the filter capacitors, thus minimizing
> damage to other amplifier components.
>
> ?A fuse in the HV line is also a good idea. As mentioned? in another
> comment, RL Drake provided a 2W low value (less than an ohm) composition
> resistor with the L4B and L7 amps, and these would explode like a
> shotgun shell if there was an HV Short.
>
> Modern fuses designed for the job are a better alternative (and easier
> to find) than the Drake solution, but with a caveat. An ordinary
> 3AG-type fuse won't do the job. It will arc internally and won't
> interrupt the current flow. What I use are microwave oven fuses, rated
> at 5 kV. These look like ordinary 3AG fuses but are a bit longer.They
> cost about $2 each and come in various current ratings. Typically an
> 850mA rating is a normal choice for ham amplifiers, but if more current
> is desired, they can be paralleled. You can buy them from dozens of
> sources and in multiple quantities on Amazon or eBay. Here's a link to
> one supplier:
>
> https://www.amazon.com/Glass-Tube-Fuse-0-85A-Microwave/dp/B00X74LUJ6/ref=pd_day0_469_3?_encoding=UTF8&psc=1&refRID=CSR8VA8Q86YGRD5DJX5W
>
> 73,
> Jim W8ZR
>
> ### A 10 watt rated glitch will not work with a 50 ohm value. 1 A plate current into
> 50 ohms = 50 watts.... so your 10 watt rated resistor is toast right there.
> A 50 watt wire wound, with a 50 ohm value will suffice. Drake uses
> a .82 ohm + .5 watt carbon comp resistor.... for the... HV fuse.
> Drake L4 + L7 amps run at 800 ma into .82 ohm = .525 watts. I use a 50 ohm + 50 watt
> WW in series with the drake .82 ohm.. hv fuse.
>
> ## A 25 watt rated glitch will work, provided its only 25 ohms. 1A into 25 ohms = 25 watts.
> If no room for a 50 watt, 50 ohm glitch, you can series 2 x 25 ohm, 25 watt resistors.
>
> ## The purpose of the 50 ohm glitch is to LIMIT the fault current to a safe value, IF a HARD fault occurs.
> The purpose of a HV fuse, in series with the 50 ohm glitch, is to INTERRUPT the fault current, IF a
> hard fault occurs. Wire the HV fuse inboard of the 50 ohm glitch. IF the glitch blows its brains out,
> and dangles down and touches the chassis, you want the HV fuse on the inboard side of the glitch R.
>
> ## IF you get a short to chassis, etc, inboard of the HV fuse and glitch R, wire a 2nd HV fuse, between
> plate xfmr sec and input to diode bridge, or doubler, or full wave CT. This 2nd HV fuse, can go in EITHER
> leg of the plate xfmr secondary, but only one leg. Then your diode board is protected.
>
> ## IF a tube arc occurs..and say you have 3000 V through a 50 ohm glitch, fault current is just 60 A. 60A will
> open off a 1 A rated fuse asap, like in less than 2 msecs. With a 7700 vdc supply and a 3A rated fuse, and a 50 ohm glitch
> R, the fault current is 154 A. 154 A will open off a 3A rated HV fuse in less than 2 msecs every time.
>
> ## You have to distinguish between a hard fault and a soft fault. A soft fault would be like when plate current is a tiny
> bit above the tubes rated max plate current. Ditto with grid current, esp on tubes with delicate grids like a 3CX-800A7, etc.
> Fast electronic shut down, either opening the key line or shutting down the drive level asap for delicate tubes is required.
>
> ## On tubes like 3-500Z, 3CX-3000A7 etc, a simple FAST grid fuse, like a 3agc will suffice, wired between chassis and
> positive terminal of grid meter...or grid shunt. Suck too much dc grid current, and grid fuse open up. Amp cant be driven, since
> no path for dc grid current. Power output drops to zero watts, and input swr, between xcvr and input of amp rises to near infinity,
> xcvr shuts itself down asap, dure to high swr, event over.
>
> ## I have several of those DPST, sky high V rated open frame relays that are mentioned in marks. K5AM articles. They use
> a 120 vac solenoid coil. Opening off B+ during a soft fault is one thing, but trying to open off the B+ during a hard fault is another matter, esp if
> you have a 5000 to 7700 vdc B+ supply and a few hundred uf filter cap.
> A vac relay was tried for the same idea in the old orr book, using the w8zr 8877 design. Works ok for soft faults, but with a hard fault, you can really fry a
> small vac relay per jennings and kilovac engs. marks other spst vac relay, contactor, was rated at 50 kv and 50A... but they are
> rare..and huge things, at a foot long. The open frame solenoid uses a 120 vac coil. After trying various the above schemes through the years, and robust tubes
> in GG, I came to the conclusion that for my application, the simple, CORRECTLY rated HV fuse, in series with the HD 50 ohm glitch R..and a second
> HV fuse, also correctly rated, between xfmr and diode board... provides for ample and fast protection. The 120 vac solenoid dpst open frame relay will
> not open in less than 2 msecs... like a HV fuse will. I would use the med sized vac relay or 120 vac solenoid open frame relay for soft faults, but not hard faults,
> at least for my particular applications.
>
> ## I also use a magnetic hydrualic two pole breaker in the incoming 240 vac line, inside the HV supply. These are the type that are known as.... instant trip, with
> little to no delay at all. The fluid has been removed from the internal oil reservoir, by the factory, to provide for the instant trip function. These types are much faster than
> the run of the mill heat activated, thermal breakers. Typ thermal breaker, like you have in your main 200A panel, will open off with a hard fault, in typ 35 to 40 msecs.
>
> ## another method is to use a large contactor, with an adjustable current trip threshold. These can be used for a soft fault, but beware of their use for a hard fault.
>
> ## During a hard fault, you not only have a huge amount of energy stored in the HV filter caps, which is only current limited by the internal resistance of the cap, + the Z of
> any HV wiring.... you also have the ..follow on current coming from the plate xfmr and diode board. I have seen paralleled 100 ohm wire wounds, 225 watts each, used for
> a 50 ohm glitch, on a 7800 vdc supply with almost 180 uf filter cap. Incorrect HV fuse used, and the 2 x 225 watt resistors look like you hit them with a propane torch !
> Replaced with 4 x 200 ohm, 225 watt wire wounds... in parallel, to make the 50 ohm, 900 watt rated glitch assy, air cooled by fan, and this time, the correct HV fuse used.
> Crow bar it all you want, nothing happens, blown HV fuse, thats it. Buts thats a one off case.
>
> ## W8ZRs use of simple 5 kv microwave oven HV fuses, rated at 850 ma, is a good one. just one would suffice for most amps, and 2 can be used in parallel for more current.
> I have also tried using them in series , for more B+..and that too works, or 4-8 of em in series parallel for more B+ and more current.
>
> ## HB hv fuses also work, provided the wire fusing element is either inside loose heat shrink, teflon spagetti tubing, or wrapped in 33 or 88 tape..then suspended between
> stand offs. But test em on the bench..1st..with a variable low Vdc supply, like a 0-60 vdc at 5A type supply.
>
> Jim VE7RF
>
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