using a current sensor to energize a soleniod

[2571]

Good morning,
In our Tig welding process we are currently using a micro switch located on the operators handle of the torch to activate a soleniod that supplies our welding assist gas.
I would like to purchase design and build a current sensor that would eliminate the need to run a pair of wires and the micro switch to the torch handle. I'm thinking of a switch that can sence the current flow in the power cable and then close energizing the soleniod to open and gas to flow. I am hoping to keep it this simple. My question is can it be done and how.

respectfully
2571

 

[cbarn24050]

A current transformer, rectifier, sensitive relay, might work. Put it on the mains input live wire, you may need a few turns through the core.

 

[2571]

The power cable on these tigs about .500 in diameter with a cooling line running through the center. the hole in te current trans would need to be very large to prevent a tight radius and a pinching action of the cooling line. Not to mention these are purchase power cables and manufactured with large threaded connectors (.875) on each end. We could attempt to remove but reconecting them would be tough. I was looking for something that could fasten to the outside of the cable. As the current was traveling through the cable the magnetic line of flux would pull the switch closed and allow the soleniod to energize.

 

[danw2]

What turns the current to the torch handle on/off?
Why not sense off that circuit when the current is on/off?

 

[RajeevCell]

How large a current do you have here? There are clamp on CTs that operate exactly like you requierment.

 

[cbarn24050]

You should read my post more carefully, you put the current transformer on the MAINS lead. The lead that plugs in the wall.

 

[ScottyUK]

Well, strictly on either the line or neutral core, not over the mains power cable outer sheath. You'll probably have to open the unit up to get to them.


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Sometimes I only open my mouth to swap feet...

 

[itsmoked]

I do not see how any of this is going to work.. The inerting atmosphere must be present before the arc is initiated! Turning on the gas after the arc has started is not usually acceptable.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Leo01]

One thing you must take into consideration is that the switch is what starts the PRE-purge of the gas so that when the arc starts, there is already a shielding gas present in the arc zone. If this does not occur, it will affect the quality of your welds.

Lee

 

[itsmoked]

I guess I beat you by a few seconds Leo.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[cbarn24050]

I guess you need to read more carefully as well scotty .

 

[Leo01]

Itsmoked, Looks as though you got me on this one. I didn't look back after I left the message to see if anyone was a faster typer than myself. {:- ]
Lee

 

[ScottyUK]

Hi cbarn,

Depends which of your posts you think I answered. I've always understood the 'mains lead' is the cable containing 2 or more cores in one outer sheath. Your first description was fine, but the second was careless. Using a CT on the input supply is certainly a good solution though, and if the OP needs the level of explanation that we're bickering about then it's a lost cause anyway!


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Sometimes I only open my mouth to swap feet...

 

[wrsharper]

How about a CT (current transformer). These are round donuts that only one hot wire passes through that are used commonly to detect AC current flow. There is a small amount of current draw before the arc is initiated but with a little trial and error you could find a relay to fill the bill. This would only be a little slower than the micro-switch. Off the cuff guessing I would say half a second to start the gas flow once the arc is started. This was already mentioned in the second reply to this post.

Or a cheap solution is a manual shut off valve where the welder operator would turn on the valve before he starts welding. He would have to remember when to turn off the gas when he stops. Should be simple since he already knows to turn on the gas regulator.

 

[OperaHouse]

I designed similar circuit for an automated welder to verify that an arc had started and sold several hundred of them. It sensed a voltage on the current shunt of the meter. However, I fail to see the need for circuit as requsted and see many problems. The trigger not only starts the gas flow, but starts the HV HF arc as well as the main power source. Having a torch constantly live seem very dangerous. It could never be set down. If the switch had to be eliminated a contact detect circuit could made to sense the tip hit to ground. Then the gas could start and the trig arc a short while later. Pulling the torch away would stop the current and reset the system. This all seems bad welding form and the nature of the original question seems to be from someone not familiar with welding.

 

[2571]

Thanks to everyone who had taken the time to reply, I will take each sugestion and apply what I have learned, For those of you that have to throw in your negetive remarks, well let just say I hope when I'm at your level someday I don't lose sight of where I came from and how I got there. I ask these questions to learn and everything I have learned up to this point I'm very grateful for.

respectfully
2571