welder current adjustment and measurement

The best way to control the amps of your secondary is to control the voltage of your primary. For this you could use a triac or even a commercial light dimmer control of sufficient current handling capacity. I do not know how many amps you are talking about and I do not know what your circuit looks like but since you are talking transformers I will assume you are trying to measure AC amps. You can make your own ampmeter by winding a coil around one of the transformer secondary wires and using the induced voltage to drive a milli or micro volt meter with a shunt resistor. Set up a resistive load on your secondary so you have a known current, say 10 amps, flowing. Add or subtract winds of the coil around the wire until your meter reads 10 accross the shunt resistor. Then you will have a correspondence between the Amps of your secondary and the millivolts of your meter. As I am fairly old school I will not be the least bit surprised to find out there is a better way to do this but this should get the discussion going.

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Thank you so much for your help sir, and what I was wondering about was a back lit digital AC amp meter that could handle up to 300 amps to measure the current if you know of one that meets those specifications, though building one is also a something I would consider. A commercial light dimmer is designed to be sufficient for maximum mains current correct if so where would I purchase the device? I really appreciate your help.

The simplest thing to do might be to use a current-sense resistor, perhaps 1000 Siemens, and measure the drop across it.

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Hi Hampton,

Check out and take the information from Don seriously. He is very smart and intuitive. Sometimes I don't understand his advice right away but it is usually very good. I had another idea for the control of the current. You could use a Variac. I do not know where you are located but almost every older Dental X-Ray machine has a variac built in for controlling the Kvp of the x-ray. If you went to a dental supply house they may have an old x-ray unit you could salvage. The variac will allow you to control the voltage to the primary of your transformer and even boost it. I know they are capable of 15 amps at 120 VAC for short time periods. I am not sure what they can handle continuous duty. The x-ray will draw upwards of 15 amps but then for only 1/2 second. My other idea with the light dimmer would not control the voltage but rather the duty cycle of the power to the primary. I don't understand your application well enough to know which would be better. In my area the dimmer controls are available from hardware stores or electrical equipment suppliers. Don's Idea for using a precision .01 ohm resistor in your secondary would allow you to use a digital volt meter to read the voltage drop across it.  For example if you measured 3 volts across a .01 ohm resistor you would know that there was 300 amps flowing. If you don't need the precision of of a commercial resistor you could build your own by looking at the specs on wire gauge, length and resistance and make your own .01 ohm resistor.

Maybe one of these would work:

https://www.google.com/shopping/suppliers/search?source=cunit&group=Sensors+and+Transducers&gclid=CKrwxdPg3r4CFeJDMgodxk…

Hello Mr. Wiltrout, I like your variac idea but I doubt I would have the funds or the access to the machine due to the fact that I am only fourteen. I will try the resistor method for measuring the current from the secondary, but I was wondering whether the resistor is meant to be in parallel or series with the output I assume parallel because Mr. Hersey said to measure the drop across the resistor. And I believe I understand the relationship between the voltage and the amps in that method based on Ohm's law the volts relate directly to the amps so measuring the volts will allow you to calculate the amps. Also would it work to use a 1 ohm resistor instead of a .01 ohm so the volt meter would display the exact current? Thank you for your explanation of the volt meter method.:)

Thank you also Mr. Hersey for your idea I believe I will use it in my project.:)

Hi Hampton,

The resistor that Don was talking about would be put in SERIES with your load. If this is a welder it would be in series with the Arc. If you put it across the secondary voltage it would short it out.  You have to keep the resistance very small when you are working with large currents or the test resistor will have to dissipate too much power. At the 300 amps you mentioned a 1 ohm ohm resistor would have to dissipate 90,000 watts. We are obviously in fantasy land here. Good luck on your project and be very careful of the danger from high voltages. It may be better to work on some low voltage projects.

John

Guys

Conventional Arc welders usually use about 50-60v to strike and then a lot lower to maintain the arc.

The newer inverter welders are often higher at 90v strike, which makes them much easier to use.

In a conventional welder the air gap limits the current available to the stick, (so its basically an transformer with adjustable efficiency).

IMO you would be much better trying to control the input side of the welder, than the load side.

I've always found the current to be a guideline, as the arc eventually dictates the current and weld quality obtained.

Bear in mind that you still need to maintain a certain voltage to keep the arc, and this also depends on the type of rod you use.

I would have thought that conventional welders were so cheap. Here in NZ you can pickup a lower quality brand new for $NZ160 or second hand for $50-60

Inverter welders are probably more worth the effort due to their higher cost, however these have relatively expensive parts that can go bang quite easily.

Also with a single phase welder, you would be lucky to get much more than 150-160A as it would overrate the mains supply.

For a 300A welder, you're either talking about a very low duty cycle (ie weld time v cooling time) or a serious piece of kit.

Mark