In the last few weeks I built a tiny TIG welder for using it on a 3D Powder Printer, which I want to test in this project.
While testing today, I figured out that the HF Start loaded the rectifier capacitor, even when I put a MOV or capacitor in parallel. That caused a decrease in arc strenght and after loading the capacitor to a certain voltage an high power discharge on the tungsten electrode.
I tested it again with nothing but the HF Start coupling coil connected to the rectifier and its capacitor still gets loaded, so it is definitely caused by the HF Starter.
By connecting a capacitor in parallel together with the 400ohm resistor in series to the rectifier + reducing the electrode to workpiece distance, I could reduce the loading of the rectifier capacitor to a minimum. Thanks to @Bharbour for the great advice.
The next problem was that as soon as there was a voltage from the welding source on the welding lines (open circuit), what also included a charged rectifier capacitor, the HF Starter arc did no longer work for some reason.
But at the end I was able to get it to work with a simple workaround - Maybe no permanent solution, but for now it's good because it works.
I added a 15W lightbulp as load to the rectifier to discharge its capacitor as soon as there is no longer power from the welding source, what starts the HF arc again. The lightbulp also indicates that there is power from the welding source on the lines what is also a good thing, so maybe I will keep it.
I think there will be a lot to do in terms of emi/rfi protection, what could end up to be really complicated. I will try many things out and hope that I can find a solution for it over the time.
I spent the weekend testing stuff to destruction - not intended but I figured some things out.
- I tested a DC SSR which either never worked or failed at the very beginning.
- An AC SSR before the rectifier seems to work for now.
- The four 100W 100ohm resistors seem to get too hot (even with the active cooling) for continuous operation, so I maybe will try something different for current limiting, like an inverter circuit or a saturable reactor.
But the part I want to get right first is the HF Start Circuit.
Initially I had planed to pulse the HF Start and Work Current together to keep the temperature on the powder not higher than needed and to put less stress on the electronics.
But while testing this I figured out that the arc ignited not every time and the HF HV ignition arc got weaker over time until at some point the HF Starter stopped working completely.
After that I spent some time building an arc detecting circuit which could detect whether the arc has ignited and turn on the HF Start if it would extinguish. (Which I will likely never use).
I stopped working for a day because I had the false assumption that the Flyback transformer was the part that had failed and I ordered a new one and some parts to build a different flyback driver, until I tried building a new simple single mosfet driver (IRFZ44N + 470ohm resistor on the gate + 6+6 turns flyback/primary coil) for testing the transformer and... the transformer still worked. So the driver must have failed which I think was caused by turnung it on and off at a high frequency. I tested the same driver (6 IRFZ44N in parallel) with the transformer for half an hour and it did not fail, so I think the problem was not caused by mosfet overheating, but by the high frequency on and off switching.
With the driver I built yesterday I tested out the HF Starter again and the HF HV iginition arc got still weaker over time until even the spark gap stopped working. I disconnected the transformer from the circuit and it still worked and so it could only be the capacitor which must have had failed and this was the case. After replacing the capacitor with a new one it worked again, but after a short time the arc got weaker again until the capacitor failed again.
I think what happened was the capacitor (a 20kV 1nF Ceramic one) failed over time due to the high frequency of loading and unloading through the spark gap.
So I ordered 10 pieces WIMA FKP1 15nF 1600V DC capacitors which I'm planning to connect in series to increase the rating to 16000V DC 1.5nF. I read in a tesla coil forum that they are better for the high frequency and I hope they will not fail like the ceramic ones.
I think if I can get the hf hv ignition arc to work continuously I can just pulse the work current whit the SSR and everything (maybe) will be fine. Until the next problem appears :)
I figured out the reason for the death of many ceramic capacitor beside that they are not suited for HF. It was the spark gap. For some dumb reason I set it 3 times as wide as planned what leaded to the premature dead of the capacitor after under 2 minutes.
I set it to 2mm what I think should give me around 6000V and with a fresh capacitor it is enough for a 5mm spark at the tungsten electrode. I will test out how long the ceramic capacitor will last until it gets destoyed (they are not expensive).
The capacitor (still the ceramic one) lasted over an hour and would likely continue working for a long time without failing completely, but it was clearly visible on the arc that it was no longer working at "full power". I think/hope the WIMA capacitor will perform even better, but an hour is not bad and enough for testing some things out.
After letting it run for about 1.5 hours the spark gap was covered in an unknown (by me) yellow dust.... maybe it could be vaporised metal from the screws.
Does anyone know what that dust is?
After turning the machine off for a moment and on again, the...Read more »
In this project I want to try melting metal powder with an arc in an inert gas chamber to create 3D objects. Inspired by HomoFasciens video https://youtu.be/Y-yjsyJQGfI. in which he burned holes into a piece of paper to print with an hacked arc lighter, I started this project.
First I connected an arc lighter module to a 3D printer and printed on steel powder which changed its color and bond toghether a bit.
After a short time the arc lighter module stopped working, so that I thought about building a more capable power source.
So I spent some time of research, to learn how to build a HF starter to start the arc without touching the powder and built one.
I tried different voltages, currents, AC and DC and figured out, that it worked the best with rectified mains voltage which is for safety isolated from ground via a 1:1.09 transformer and gives 340V DC. I added 4x 100ohm 100W power resistors to limit the current to 0.85A, what seems to be enough for melting the powder.
With 340V DC the arc ignited every time and kept on going on the last try until I shut it off. I tested it on a distance >1mm.
I tried to test the welding system on the printer, but it creates so much emi/rfi noise that the printer controller keeps rebooting as long as it is active.
So I'm currently working on a way to shield the printer controller from noise. For doing so I'm planning to send the trigger signal for the arc via an optical toslink cable, I'm also planning to use shielded cables for the stepper motors, place the controller in a grounded metall box and use Trinamic's StallGuard feature for homing without limit switches. Hopefully that will make printing with an HF start welder possible.
I'm currently working on a test stand for printer designs, which will contain the welding system and printer controller.
It will be a bit like a shelf on which the printer (mechanics + motors) will be placed.
The printer will later be covered with a transparent enclosure which will then purged with shielding gas.
Maybe the dimensions of 1000*600mm are a bit too large, but I don't know the dimensions of the later printer so I built the test stand a bit larger.
I will try to find a good place for all parts and complete the test stand in the next days.
I wired the welding circuit, what's left is the trigger circuit and the printer controller.
Only the printer controller left...