So I purchased an inexpensive TIG welder a few years ago. An Eastwood 2000DC for around $250. Overall I have been happy with it. I can do thin walled stainless on its lowest settings and it has plenty of penetration on its higher settings even on 115V. But, one thing has really bothered me. It has no way to control the post gas timeout. The post gas is the amount of time the shielding gas continues to flow after the arc switch has been released and the weld has finished. There are a few reasons for the gas to continue flowing.

TIG welding has no slag to protect the hot metal from the oxygen in the air so the hot metal needs additional shielding while it is cooling. Titanium and aluminum(aluminium) need more as they are highly reactive at higher temps.
If the torch is air cooled, the gas carries away heat that could travel up the torch handle and cause problems, especially if you have just finished a long weld on a high heat setting.

According to the manual, The 2000DC is factory set to 5 seconds. According to the manual... In reality, according to my timer, it is set to 11 seconds. This is an outrageous time for a machine that is mostly used for material under 1/8 inch. I was burning through Argon way too fast.

To break it down:

Argon cylinder - 80 cubic feet
Gas flow - 15 cubic feet per minute
Post - 11 seconds
Let's say I make 20 tack welds
20 x 11 sec = 220 sec / 3.6 minutes
15 Cfm x 3.6 minutes = 55 Cf

I just used 68% of my tank on NOT welding. Even if the factory setting was correct, that would be almost 35%. I set out to correct this.

Step 1: A Little Bit of Theory

So, we need to understand what we are looking for.

There are a couple ways the timing of the Post gas could be controlled. The two that came immediately to mind was either a microcontroller or an analog resistor/capacitor/transistor decay circuit. Thankfully, the second method was used on this machine. If it was a microcontroller it would involve reprogramming the chip, and I am not to keen on making a $250 doorstop.

A quick overview of the decay circuit

In the first diagram, I show a common layout of the circuit.

RY1 - solenoid valve
S1 - arc switch
Q1 - transistor powering solenoid
C1, D1, and R1 form the decay circuit

when S1 closes it pulls the base of Q1 negative through diode D1, allowing current to flow through Q1 to the solenoid. This also charges the capacitor C1. When S1 opens, the current still flows to the solenoid because C1 is still charged and holding Q1's base negative. This is where R1 comes into play. The resistor slowly drains the charge from the capacitor until the voltage comes up to a level that turns off the transistor. If the transistor is a bipolar transistor the gate will also allow the voltage to rise, but using a resistor allows us to control the time. The rise time is a function of the value of the resistor and capacitor. The higher the resistance and the higher the capacitance, the slower the voltage will change. D1 in this circuit is just to isolate the capacitor from the rest of the board to keep other components from draining it prematurely

The second diagram shows the circuit in the welder as designed by the factory.

Notice the addition of R2. R1 and R2 form a voltage divider. R1 is 100k Ohm and R2 is 51k. This, as far as I can tell is because the total system voltage is 24 volts. By dividing the voltage the result is the Q1's base never sees more than 16 volts. For this reason, it is important to choose a potentiometer or varistor with a max resistance that is equal or less than R1 for this modification. It is this resistor that will be replaced by the varistor.

This takes us to the third diagram. For my setup, I had a 50k Ohm potentiometer laying around. The only difference in the third diagram is R1 is now a potentiometer with the center tap connected to the transistor side of the circuit and one side connected to positive.

Step 2: See Whats Up

Before we start, a little safety and disclaimer.

What you are about to do will most likely oid any warranty you may have on your machine. If done improperly it could cause the machine to malfunction.

Also, your welder has a lot of angry pixies running through the wires. There is a high voltage start circuit on many machines, as well as an inverter, capacitor bank, possible voltage doubler etc etc. Make sure the machine is powered off, unplugged and let to sit for a minute after unplugging before you touch anything. On my machine the fan runs for about 30 seconds after I unplug it. That means there is a considerable amount of stored energy that the machine needs to drain. Please do not perform this modification without understanding the risks and know how to take the proper safety precautions.

Now, let's start.

Unplug your machine, let it chill for a minute then remove the cover. Mine has 14 chromed screws around the perimeter that releases the cover. Most machines are of similar design.

Look to where the gas line runs into the machine, on the 2000DC it is on the rear lower right side of the machine. You should be able to find the solenoid valve connected to the gas line. trace the wires back to where they connect to the mainboard. On this machine there are 3 main boards. The solenoid is wired to the bottom board. This is the board you need to remove from the machine.

Step 3: Remove Mainboard

At this point, it is a good idea to take a picture of your machine. If you forget how the wires should be connected during reassembly, you can use the picture as a reference. Disconnect the white connectors on the board. It is not necessary to remove all of them, only the row on the left and the wide connector at the rear right. Also, there are 2 pairs of wires going to other boards, It is easier to disconnect the wires from the upper boards than to disconnect from the bottom boards as the connectors are hard to reach. All of the smaller connectors were glued in with red Loctite. So be careful not to break anything. I had to use a small screwdriver to scrape the Loctite off. When disconnecting, make sure to use pliers or a screwdriver to grip or pry the connectors, don't pull with the wire. This puts a strain on the crimp connections. Some connectors have a small tab that needs to be pushed in while lifting to unclip the connector. Finally, unscrew the nuts on the screw posts holding the board down.

Once the board is unscrewed and the wires are disconnected, pull the board from the machine. There might still be wires connected, but as long as they don't get pulled or get in the way, they can be left. I only needed to get the board out far enough to lay beside the welder.

Step 4: Tracing the Circuit

So now we have the board in front of us. Start following the traces on the board away from the connector for the solenoid. If you can track down the schematic for the machine that is even more helpful, but it is highly doubtful they are published. You are looking for the trace to terminate at the output of a transistor. Once you have found the switching transistor, look at the surrounding components and the power planes around it. Large wide traces with multiple connections to many components are most likely a power plane of either positive or negative. There are a few ways to figure out which is which. Sometimes they are labeled, you could also look at the diodes in the area, and see what way they are oriented. The Cathode(negative) terminal is marked. You could also look up the transistor model to get the pin layout. With a multimeter, the voltages could be checked. This requires extreme care as the welder needs to be powered up with the cover off, so not really recommended for novice tinkerers.

The three pins of the transistor are Collector, Emitter, and Base. The power flows through the collector and emitter of the transistor to the solenoid. The Base, which is the middle pin turns on the transistor. Following the trace from pin 2 leads to the capacitor and resistor circuit which I have labeled in the picture on this step.

Step 5: Modify!

Remove R2 from the board. Using a soldering iron, heat the leads of the resistor while grasping the body of the resistor with needle nose pliers. Be careful not to overheat the board or damage other components surrounding the resistor. this could also be done with a hot-air reflow tool.

Now that you have an empty spot, solder two lengths of wire in place of the resistor. then solder the other ends to the potentiometer. The Potentiometer has 3 leads. Connect one wire to the center lead, and the other wire to either of the remaining leads. For the function of the machine, it doesn't matter which of the remaining leads you choose so long as the center lead is used. However, for aesthetics, you want the knob to turn clockwise for more gas flow time and counter-clockwise for less. The simplest way to do this is to choose one, and if it is backward, desolder and reconnect to the other. If you want to get it right the first time, Turn the knob all the way clockwise as far as it will go. Then measure the resistance between the center lead and each of the outer leads. Whichever has the low resistance is the lead to choose.

Now that the potentiometer is connected, we have to calibrate one thing. Reinstall the board and reconnect all the electrical connectors. leave the potentiometer out of the machine and power on the welder. Please be careful during this part. Don't go reaching your fingers inside the welder. If you feel safer putting the cover on, feel free, but you will have to remove it again after this step.

With the welder powered on, trigger the arc switch. If you selected the correct leads on the potentiometer, the solenoid should turn on for the full length of time when the knob is turned all the way clockwise. Now play with the potentiometer and find the knob position the gives the minimum gas flow time you would like for your machine. I chose .5 seconds. Now, keep the knob at that position, turn off the machine and measure the resistance across the potentiometer. Mine read 12.5 ohms. Now turn the knob all the way to the lowest gas flow setting to get the minimum resistance the potentiometer can provide. Take another resistance reading. I got 10 ohms. Choose a resistor equal to the difference of those two numbers. Desolder one of the wires from the potentiometer and solder the resistor in between. In the second photograph on this step, I show what this looks like.

Test everything out before proceeding. I was able to retain the factory time on the highest setting and go down under a second. I do a lot of small gauge welding, so this is great for me.

Step 6: Button Up

Now that everything is working, find a place to mount the potentiometer. I found a spot on the front panel that would work. Drill a hole through the panel slightly larger than the threaded part of the potentiometer. Be careful not to drill into the electronics behind it. Also be sure to choose a spot with ample clearance around the body of the potentiometer so it is not interfering with the other mounted parts. Once you have the potentiometer mounted, tidy everything up with zip ties. Re-assemble the machine, hopefully, there are no spare parts left over.

I haven't had a chance to use the welder yet, as I was prompted to do this because I had run out of gas again.

Let me know if you have any feedback