Kill Switch

This forum is for specialized infomation important to the construction and safe operation of the high voltage electrical supplies and related circuitry needed for fusor operation.
Post Reply
beatlemaniacjng
Posts: 20
Joined: Tue Apr 03, 2012 12:04 am
Real name:

Kill Switch

Post by beatlemaniacjng » Sat Jul 28, 2012 2:48 am

As an added precaution I was thinking of adding a kill switch of sort, but not one that turns to the power off, but one that turns it on. For the fusor to be turned on this button would need to be pressed continually. This is in case the operator is electrically shocked to the point of unconsciousness, or hurt in some way. It would be operated by another person, other than the operator. There would be two switches, one to cut the power BEFORE the charged capacitors, and after the charged capacitors. This would be to make this function available where the operator is.

David D Speck MD
Posts: 129
Joined: Sat Jan 26, 2008 7:05 pm
Real name: David D. Speck MD
Location: Auburn, NY

Re: Kill Switch

Post by David D Speck MD » Sat Jul 28, 2012 3:13 am

John,

What you are describing is more properly (and ominously!) called a "Deadman's Switch". Hopefully, things would never come to that.

A useful precaution would be to have a normally closed high voltage discharge relay that is held open as long as the system is operating properly. In an emergency situation, when you release the deadman's switch, then the discharge relay would close and rapidly discharge your capacitor through an appropriately rated power resistor. Vacuum relays are frequently available on eBay that are suitable in terms of voltage rating and current capacity for this purpose.

While switched bleeder discharge systems are useful precaution, never assume that they have fully discharged the system before touching the HV conductors. Connections can come loose, resistors can go open, wires can break, relays can fail.

Previous postings on the Fusor net indicate that hanging significant capacitance on the HV output of your power supply is unnecessary and significantly dangerous. You may wish to review your power supply design to see if a filter cap on the final output is really necessary.

Dave

Conrad Farnsworth
Posts: 135
Joined: Thu Jul 08, 2010 1:35 am
Real name: Conrad Farnsworth

Re: Kill Switch

Post by Conrad Farnsworth » Mon Jul 30, 2012 1:17 am

I built a similar apparatus for my glassman HV supply, it has a "safety" switch, a voltage control knob, and a deadman's switch (the lever) all of this can be operated with one hand and can easily shut itself off when the operator isn't using it. If you have a "pin-controlled" power supply I would look into something like that.

Best of luck, and safe fusion!

-Conrad
Attachments
378002_1963335223564_485514135_n.jpg
378002_1963335223564_485514135_n.jpg (64.54 KiB) Viewed 3620 times

User avatar
Frank Sanns
Site Admin
Posts: 1725
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: Kill Switch

Post by Frank Sanns » Mon Jul 30, 2012 4:01 pm

At the voltages and currents for running fusion, a kill switch is a false sense of security. You don't get second chances at these voltages and the subsequent currents that will follow. Fusors should be well grounded and all HV leads and connections should be covered. Many people have used plastic pipe to surround their coaxial HV cables. This gives distance as well as some electrical insulation. Lastly, DC energy can be stored in cables, power supplied, insulators and feedthroughs for quite some time. Even when the power is off, there is no guarantee that you can not be bitten by stored charge. Keeping a grounded jumper nearby to connect to the feedthrough when the fusor is not actively running is good practice to provide a path for stored charges to safely be bled away. No second chances with this stuff. Think before doing and think again before doing and have a system that is designed so you can't do something stupid accidentally.

Frank Sanns

beatlemaniacjng
Posts: 20
Joined: Tue Apr 03, 2012 12:04 am
Real name:

Re: Kill Switch

Post by beatlemaniacjng » Mon Jul 30, 2012 5:04 pm

I see. One must be extremely careful. The energy stored on the insulators etc., Is that at the same one-chance voltages? Also I was thinking of the deadman more for if I got hurt and somebody came to help me, they could safely do it (after the deadma switch is lifted, I want it to activate a relay that discharges it all.

beatlemaniacjng
Posts: 20
Joined: Tue Apr 03, 2012 12:04 am
Real name:

Re: Kill Switch

Post by beatlemaniacjng » Mon Jul 30, 2012 5:07 pm

Yeah I am definitely reviewing my design. I have read that unfiltered power may actually help fusion so I am reconsidering based on the ripple magnitude.

User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: Kill Switch

Post by Chris Bradley » Mon Jul 30, 2012 5:19 pm

So you want someone to sit next to a switch with live HV on it, and keep their hand on a button that is connected to electrodes at a very high voltage... hmm....

..and not only is it a false security, as Frank says, a true high voltage switch is a very complex beast. Isolating a voltage big enough to jump across open space is not something a regular switch can handle. Once the gap opens, an arc will strike that will keep on flowing the current even as the contacts open up further.

Even if a switch does have contacts that can open that far, and quickly enough, you then have to have an enclosure and structure to the switch that retains these two electrical contacts in some way, that does not then leak current across its internal parts.

Nor can you switch the 'source' power very easily as there is residual energy stored throughout the system. The higher the frequency you operate your source power at, and the smaller the current output you expect, reduce the energy that is likely to be stored. But there will always be some, else it means your HV source is so soft a supply that it would be likely useless to drive a current.

If you must have a switch, then I suggest it should be some sort of crowbar circuit - viz, you drop a conductor into a slot that then short-circuits the live elements.

I guess what could be done to devise a 'safe' HV supply is to have one that cycles at a high frequency but that for each and every cycle there are electronics that detect the current. If the current jumps above a threshold, it cuts it out on that very cycle. This might allow an 'ac', or 'pulsed' supply that should go some way towards making for safer HV plasma work, that will still do much the job of a dc supply.

But in reality, the MO has to be - Just keep away from the live bits....!

User avatar
Dennis P Brown
Posts: 2030
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Kill Switch

Post by Dennis P Brown » Mon Jul 30, 2012 5:21 pm

I can think of no better device than a plastic rod (long) with a metal probe on one end that has a well insulated wire that runs to a ground. Use this to test all wires, caps, chambers, and anything else to be sure that the power is really off and there is no stored charge. While this will do zero for active (running) systems, the greatest danger, in my opinon, is when someone goes to work on a part thinking it is safe yet it has a stored charge or a 'switch' remains hot.

In any case, all HV systems should have extensive insulation and covers over all caps.

Of course, it is a good idea for all HV caps to have a megaohm resistor to enable it to automaticlly discharge when the system is turned off.

A master kill switch is not critical but a useful idea - that way, someone trying to remove you from a 'hot' contact, can do so safely.

I have used HV for over forty years and have lived by these procedures - apparently, successfully.

As others have said, a deadman's switch is so after the fact, why bother?

George Schmermund
Posts: 262
Joined: Wed Aug 16, 2006 12:51 am
Real name: George Schmermund
Location: Carlsbad, CA

Re: Kill Switch

Post by George Schmermund » Wed Aug 01, 2012 1:32 am

This has to be the most 'swept-up' work area on the planet!
Anything obvious in high vacuum is probably wrong.

Post Reply