Need a Simple Current Sensor

[I'mClueless]

Hello,

I have a 30kV high frequency arc starter and I need to sense the presence of current to switch 12 Volt DC current to turn on the welding shield gas. The high frequency voltage ionizes the argon gas shield which creates a path for the lower Voltage welding current.

Your help is appreciated and snickering is allowed, but please keep it down so that I can't hear it.

Yep,
I'mClueless

 

[crutschow]

So why can't you use the switch that controls the arc to control the gas flow?

 

[I'mClueless]

So why can't you use the switch that controls the arc to control the gas flow?

There is no switch for the high frequency arc starter/stabilizer. When the tungsten is placed within approximately 3/32" from the work piece then the spark will jump the gap and there is current. The objective is to eliminate the need for a switch on the torch body.

Removing the immediate application from the question I would like to know how sense current on a power line and use the detection for switching purposes.

Yep,
I'mClueless

 

[MrAl]

Hi,


One of the easiest ways to sense current is to place a small value resistor in series with the power line and measure the voltage across it with an op amp, then use the output as a signal to detect which will trip some other circuit to turn something on or off.
The basic idea is to use the op amp to amplify the signal across the small value resistor so that the output voltage is easier to detect. If you have used op amps in the past this will be easy to do. The op amp has to have fast enough response to fit your application too though so the circuit can detect fast enough. If on the other hand it doesnt have to be too fast, this is going to be very very easy to do.
You also have to consider a driver for whatever you have to turn on or off, so maybe you can provide a few more details and possibly a schematic.

The small value resistor is usually called the 'sense' resistor, and it is made small enough so that it doesnt cause any significant loss of power nor heat up too much, yet still provide enough voltage drop so that the op amp can amplify that and provide a better signal to the detect circuit. Sometimes even a current shunt can be used that has very low resistance like 0.010 ohms or even as low as 0.001 ohms.

There are also hall effect current measuring devices out on the market that are cheap and provide a signal output in response to a current. These things take little effort to use really.

How much current are we talking about here anyway?

 

[Reloadron]

Hello,

I have a 30kV high frequency arc starter and I need to sense the presence of current to switch 12 Volt DC current to turn on the welding shield gas. The high frequency voltage ionizes the argon gas shield which creates a path for the lower Voltage welding current.

Your help is appreciated and snickering is allowed, but please keep it down so that I can't hear it.

Yep,
I'mClueless

You lost me on a few notes but especially the arc initiation sequence. Generally the torch head is lowered typically with an open circuit DC voltage between 90 and 130 volts (depending on the system). The gas is turned on to the torch head cone to create the shield. Then you apply the HF only to initiate the arc. The gas is generally on with the gas flow rate established before the arc is initiated. Once the arc is struck the arc voltage drops to whatever the arc voltage is set for (in the case of an automated machine weld).

Most of this depends on the system and type of weld but generally the weld schedule should reflect gas prior to arc, then RF or HF to start (initiate) the arc, then HF off. What system do you have? Is this a hand held torch or automated weld head?

Ron

 

[Lighty]

I assume you are talking about TIG - Tungsten Inert Gas

The problem with your idea is that the argon gas will only start flowing after the torch has established an arc, this is NOT good practice, normally the gas is purged before the HV arc is started, removing the oxygen from the weld area preventing any contamination, especially when welding Alu.

Most modern TIG machines have a flip-flop circuit so that you don't have to hold the button, push - on, push off.

 

[I'mClueless]

MrA,

Thank you for your information. I vaguely know what OPAMPS are and how they work so I will research OPAMP circuits.

Ron,

My application is TIG or more technical GTAW welding. The process typically uses a hand held torch that facilitates delivery of shield gas, creates an arc that generates the heat and in my case uses coolant to maintain the temperature of the hand piece.

The high frequency arc starter/stabilizer is high voltage low amperage current that is used basically as a carrier for the welding current. The welding current does not sufficient Voltage to jump a spark without first shorting the tungsten on the work piece. That is referred to as 'Scratch Starting'. Without the presence of shielding gas the spark will cease to occur.

I do not wish to use the scratch start method nor do I want to burden the torch with clumsy switches or buttons. That leaves only two options: 1) tie the switching for the gas to the power control foot pedal. 2) sense current and use detection to activate relays.

The problem with the first method is that I don't always use the power foot pedal.

Lighty,

The high frequency arc starter/stabilizer does not have enough Amps to melt material. I guess that it is possible that the welding current could arrive prior to the gas shield. If that is the case then I think that the gas will finish a close second in the race. (like before the puddle occurs)

Yep,
I'mClueless

 

[Reloadron]

Hi Ya

Though not a welder type I am somewhat familiar with the machines we use where I work, both manual and automatic. However, when it comes to actual welding I would fare out better using super glue.

I am familiar with TIG (Tungsten Inert Gas) or as it is also called GTAW (Gas Tungsten Arc Welding)

Even the small machines where I work that use a hand held torch as you describe have an internal gas solenoid. The gas (Argon in our case for mostly 316 SS welding) is initiated before the arc is struck. I do understand the difference between a touch or scratch start and a RF start. However, in the processes I have seen and worked with the gas is always initiated before the arc is struck. Thus I don't quite get your desire to initiate gas flow:

I have a 30kV high frequency arc starter and I need to sense the presence of current to switch 12 Volt DC current to turn on the welding shield gas. The high frequency voltage ionizes the argon gas shield which creates a path for the lower Voltage welding current.

After the arc has been initiated? Generally in what I have seen they initiate the gas and then strike the arc using either a touch (scratch) or HF RF burst.

Even the smaller systems I have worked with (Hobart, Miller and Lincoln) have a current and voltage meter on them. Current is generally measured using a current shunt on older systems and the newer systems use a Hall Effect sensor. The voltage measuring can be a little tricky as the arc voltage (voltage across the arc gap) is passed through a choke (large inductor) prior to being measured. This precludes the RF from toasting the electronics in the voltage measuring plane when a HF RF start is used.

Now if your system uses a current meter with a signal derived from either a shunt or Hall Effect device that signal can be used for a variety of purposes. That signal is normally a proportional mV equivalent of the actual weld current. Does your system have voltage and current meters?

If your system is anything like the models we use I can likely help you out with some suggestions. We actually record all our welds on production parts, so I have somewhat of a handle on things. Like I mentioned I am not a welder or weld engineer but there are times I work closely with those guys.

Ron

 

[I'mClueless]

Ron,

It's not that I want to initiate the gas after the arc, I'm just looking for a way to automate the flow of shield gas. The HF arc starter is the first function so it seemed only logical to use that current as a trigger. In reality I would actually be sensing the welding cable because that is the only outlet for the HF current. There's always current in the HF primary windings of the induction coil so it could not be used.

It's probably not a viable idea because the welding current travels much faster than the shield gas which would allow oxidation of the material to occur.

Yep,
I'mClueless, aka Ron

 

[Reloadron]

Yep,
I'mClueless, aka Ron

LMAO at that. All this time I was wondering why you would want to initiate gas flow after starting the arc. Yep, call me clueless.

Yeah, on the automated systems I have worked with the sequence begins with the gas solenoid turning on then a delay of a few seconds then the HF arc starter. When the arc voltage drops to a normal limit the system assumes an arc and continues to run, however, if within a timed period the system does not detect an arc it aborts and the gas is turned off. Like a giant reset.

Now in your case, beyond the manual method you are using I don't know of an easy way to turn on the gas, pause, turn on the HF and strike the arc. Not easily looking at what there is to work with anyway. Was the HF an add on unit? There may be a way to place a delay in there so when the HF is triggered the gas comes on and then a delay before HF initiates? I'm just not sure.

Ron

 

[I'mClueless]

Ron,

I was not implying that you are clueless, I was simply stating that my name is also Ron. I made my own high frequency arc starter box. It started out with a France 15030 P5G-2E neon lighting transformer which was replaced with a 3uF / 400 Volt motor start capacitor and an automotive ignition coil. Originally I had planned on using a 15HP / 3PH electric motor to power the Leece Neville 270 Amp alternator. The electric motor turned out to be a total piece of crap so my only other option was to use a 25HP Kohler gas engine. That changed the welder from a stationary to a portable so I did not want to have any dependence on commercial power.

The HF is the first function so in order to automate the shield gas I will use a split second delay for activating the welding current. As I previously stated the HF can only be sensed in the final because it is always present in the primary side of the HF induction coil. I just purchased a half dozen Honetwell CSLA2DK linear current sensors through eBay so hopefully that will provide the current sense. If it does not pick up on the HF current then it will for sure sense the welding current and that detection will be used to terminate the HF. While I was shopping I also picked up some neat Texas Instrument ICs that I will use for constant voltage control when performing FCAW and MGAW.

Thank God, not Al Gore, for the Internet and eBay.

Yep,
I'mClueless

 

[kinarfi]

Put a reed switch next to one of your input power leads or wrap the lead around the reed switch, depending on how much current you draw, and when you strike or draw an arc, the increased current will cause the reed switch to close and you can use that to turn on a relay or maybe even open the gas valve.
Kinarfi

 

[brianjil]

Csla2dk

Hi Ron,
Got here googling the CSLA2DK and found this thread. The absence on the internet of info about using this device is annoying and Micro Switch has the same problem.. the big question I have is given the "-" , "0", and "+" pins on the unit, how do you hook-up the voltage to operate the Hall Effect sensor, and how do you read the output voltage that is supposed to vary with the current passing thru the device. "3 pins, how to connect to them ?" is my question. If you can help I'd really appreciate it. I have designed a microcontroller based unit to control and monitor a solar panel system I am building for my house and want to use the CSLA2DK to monitor AC current being distributed to some appliances on a round robin schedule. This looks like the device to use, if I can figure out how to hook it up.
Thanks

 

[I'mClueless]

Hi Ron,
Got here googling the CSLA2DK and found this thread. The absence on the internet of info about using this device is annoying and Micro Switch has the same problem.. the big question I have is given the "-" , "0", and "+" pins on the unit, how do you hook-up the voltage to operate the Hall Effect sensor, and how do you read the output voltage that is supposed to vary with the current passing thru the device. "3 pins, how to connect to them ?" is my question. If you can help I'd really appreciate it. I have designed a microcontroller based unit to control and monitor a solar panel system I am building for my house and want to use the CSLA2DK to monitor AC current being distributed to some appliances on a round robin schedule. This looks like the device to use, if I can figure out how to hook it up.
Thanks

Hello,

I have the .pdf file for the CSLA2DK and it explains the connection. I have been working 6:00AM to 9:00PM so it will be sometime Sunday before I will have a chance to send it to you. To expedite the delivery please send an e-mail to ron_becker@cox.net

 

[Reloadron]

Hi Ya brianjil

Attached is an image of an AC current sensor. I believe this is what you are getting at with the + and - sign. If that is correct sensors like this are loop powered and output a current that is proportional to the mains power current. Depending on make/model they can be powered with between 12 and 24 VDC. They output for example 4 to 20 mA, so you can place a 250 or 500 Ohm resistor in series with their - side and derive 1 to 5 or 2 to 10 VDC. For example if you use a 0 to 50 AAC sensor 0 amps is 4 mA and 50 amps is 20 mA.

I like this line of sensors of this type. How much current maximum do you want to measure?

<EDIT>

Attached is also a basic drawing of how such a sensor is configured. Additionally sensors of this type can also be purchased with voltage outputs of 0 to 5 or 0 to 10 VDC for measuring various currents. This works well for inputs to assorted devices including micro controllers.

</EDIT>

The Other Ron