RF High Voltage Shielding

[N2DTS]

All my stuff is in locked cabinets with lights and meters, and I have shorting clips.
I clip things to ground because I worry about some odd failure that might key the trans while I was back there.
I have got it a number of times with low voltage, 120vac, the high voltage scares the s%$t out of me.

When my son was young, I always took the keys with me on my key ring.
He was not interested anyway.

[Opcom]

A small 20-30A  old style 'fuse box' painted red on the wall near the station door and with an "emergency power cut off" sign above it would be something almost anyone would see, not have to think about, and know how to turn 'off'.

You can only do so much and some people still won't have a clue.

Such an old fusebox might be too old for 'code' but OK as a switch for one or more 24V contactors that control power to the station and any workbenches.  24v is nice because it is low voltage and there are HVAC contactors in large sizes that are inexpensive. Just be sure to buy them that break both sides of the 240V. Some cheapies only break one side.

The whole system could be free:
No one wants the old type fuse boxes any more.
No HVAC technician would -ever- consider using salvaged parts on a job, so any 24V transformers and usable two-pole contactors from worn out furnaces should be free or for pennies.

What would others do or suggest for a power shut off that could be recognized and used by anyone no matter whom?

[K6JEK]

You guys are making a killer case for MOSFETs.

[K1JJ]

You guys are making a killer case for MOSFETs.

Heheheheh... very good point. 

As a future pansy station I can envision an Apache SDR rig driving a monster homebrew solid state linear using the latest pre-distortion software for a super clean signal.  Then add in a class E MOSFET rig to keep our fingers in the AM homebrew "efficiency" arena. 

We could still get hurt with that setup, but probably because of getting too lax.

T

[ka1tdq]

My 10,020,000 ohm resistor string to measure B+.  You know what they say... If it can't kill ya, what good is it?

Jon

[w4bfs]

I think it was Neitzche who said "what doesn't kill us makes us stronger"

check Conan the Barbarian next time it is on ....

[Opcom]

My 10,020,000 ohm resistor string to measure B+.  You know what they say... If it can't kill ya, what good is it?

Jon

A length of glass tubing could be helpful to prevent sagging and add beauty to the installation.

[KJ4OLL]

Thread makes me recall the recent event that taught me respect for HV.
Built the PS for my first homebrew.
Had read warnings about working around HV, believed I had a safe solution.
Put the Fluke HV probe on the end of a 3ft fiberglass rod.
Thought that fiberglass was not conductive. Well, possibly it is not, but @ 5kv...
I grabbed the rod to move the probe...
After I moved the probe to the next test point, released the rod.
Noted fingers now numb.........did not feel anything weird while manipulating the probe.
Possibly just the 99 % humidity in the room was enough to let a bit of HV over the fiberglass.
Luck saves another foole.
 

[WD5JKO]

  Back in about 1981 I joined a company that made particle accelerators. One of the older designs ran the "source" at 400,000 volts above ground. The picture attached is from one of those machines that is still in service today.

  It took a great deal of work to make 400KV. First the power supply diodes were prone to breakdown. We used to test these in a special room with a grounded metal ball tied to the rafters swinging past the supply. After a while the power supply would stop dying, so we went to the machine with it. The bullet shaped machine actually was full of electronics. A pair of 4cx250b's were used to excite a cold cathode rf source. The rf was a self excited oscillator somewhere around 2 meters. The white nylon rods all controlled a variac inside for special purposes. The green diagonal rod was the drive shaft for a 400hz 115v generator.

  There used to be a practice of suiting up a person with a rubber suit. That person would squat in the corner and hold a grounded rod. The object was to find all the corona points while in pitch dark as someone else ran up the high voltage. At say 311 Kv, if a corona spot illuminated, the person inside had to hell out to shut it off, and keep his eyes on that exact spot. Usually just a mere burr in the polished aluminum would cause a corona glow. These corona points would pull current, and subtract from the power supply output. The supply was rated at 400KV @ 1 ma...that is 400 watts power.

  One day there was a guy inside that high voltage room when a team of engineers came back from break. Hmmm, why is the door opened one asked? He closed the door, which locked the other guy inside. Then they proceeded to run up the voltage, 100kv, 200kv, 300 kv, then SNAP SNAP, 100KV, 200KV, 300KV, then again SNAP SNAP! The guy inside was the recipient of multiple lighting bolts. He was yelling his head off, but with other equipment running, nobody heard his screams. Finally he got smart and after getting nailed he grabbed hold of that aluminum bullet. With only 1ma of current available, the high voltage would not go beyond about 5KV (dialectric strength of leather sole shoes). Eventually the engineers upset at another failure decided to shut it down, and open the door. The guy inside busted out hollering, and with fists flying...He was all right.

  About a year later at the same company I got nailed by a 40KV 50ma supply. That one could have easily killed me. I think I saw god during that ordeal.

Jim
Wd5JKO

[ka1tdq]

I mounted the resistors vertically and banjo-strung tight. I'm using a regular voltmeter hard wired in with the battery removed. It works pretty good! I can see the voltage drop to zero.
Although, I must admit, I had quite a bit of trepidation sticking my hand back in there again. Without that meter, I wouldn't do it. For those of you who haven't felt a good >1kv shock, they hurt like hell!

I was actually dozing off in the car the other day while the wife was in the store, and I had a daydream of the same sensation in my other arm with the same thing happening. I woke up abruptly with a quick scream. Shell shock or something.

Jon

[Opcom]

 Back in about 1981 I joined a company that made particle accelerators. One of the older designs ran the "source" at 400,000 volts above ground. The picture attached is from one of those machines that is still in service today.

  It took a great deal of work to make 400KV. First the power supply diodes were prone to breakdown. We used to test these in a special room with a grounded metal ball tied to the rafters swinging past the supply. After a while the power supply would stop dying, so we went to the machine with it. The bullet shaped machine actually was full of electronics. A pair of 4cx250b's were used to excite a cold cathode rf source. The rf was a self excited oscillator somewhere around 2 meters. The white nylon rods all controlled a variac inside for special purposes. The green diagonal rod was the drive shaft for a 400hz 115v generator.

  There used to be a practice of suiting up a person with a rubber suit. That person would squat in the corner and hold a grounded rod. The object was to find all the corona points while in pitch dark as someone else ran up the high voltage. At say 311 Kv, if a corona spot illuminated, the person inside had to hell out to shut it off, and keep his eyes on that exact spot. Usually just a mere burr in the polished aluminum would cause a corona glow. These corona points would pull current, and subtract from the power supply output. The supply was rated at 400KV @ 1 ma...that is 400 watts power.

  One day there was a guy inside that high voltage room when a team of engineers came back from break. Hmmm, why is the door opened one asked? He closed the door, which locked the other guy inside. Then they proceeded to run up the voltage, 100kv, 200kv, 300 kv, then SNAP SNAP, 100KV, 200KV, 300KV, then again SNAP SNAP! The guy inside was the recipient of multiple lighting bolts. He was yelling his head off, but with other equipment running, nobody heard his screams. Finally he got smart and after getting nailed he grabbed hold of that aluminum bullet. With only 1ma of current available, the high voltage would not go beyond about 5KV (dialectric strength of leather sole shoes). Eventually the engineers upset at another failure decided to shut it down, and open the door. The guy inside busted out hollering, and with fists flying...He was all right.

  About a year later at the same company I got nailed by a 40KV 50ma supply. That one could have easily killed me. I think I saw god during that ordeal.

Jim
Wd5JKO

That's an amazing account. I would not have wanted to be the first guy outside the door. Was the RF source cathode heated by the RF from the 4CX250Bs, like some CFAs are heated by the drive?

[John K5PRO]

Indeed, quite a test wearing the rubber suit in there! OSHA would not like that one.
At our accelerator we are still using Cockroft-Walton DC accelerators for injectors from our ion sources into an RF linac. These monsters run at 690 kV DC, and I remember one was featured on Bill Nye the science guy back in the 1990s. Here is a photo of the ion source room for one at Fermilab near Chicago, and of ours. You would not be allowed to stay inside when it is on, as the capacitance is quite respectable just from the sizes of the components and the multiplier stack. When it sparks, its like a thunder clap.

[Opcom]

That's an impressive setup, John. I have to admire you, you get to play with all the big toys, and the particle beams, which are made of all the little toys.