Xmt remote contactor problems

[jjohn76]

Awesome! That's a big cap. I see the NTC to keep startup surge down, but it's also worth putting in some type of fuse ahead of the rectifier and cap (XMT does have a breaker though). With your voltage doubler, you'll either need a rail-to-rail opamp or provide +/- . Also be sure to put a low-pass filter either ahead of or as part of the voltage doubler/amplifier. Frequency should be at least a decade below the DAC frequency.

Are you using spi for the DACs?

[Chadv]

The first DAC I was trying was 8 digital bit pins the one in the mail is i2c. Adafruit has a library and example code for it

[Chadv]

What do you mean by
Frequency should be at least a decade below the DAC frequency

[jjohn76]

You need a low pass filter on the output of your DAC. You usually use an RV filter - resistor in series with output, then capacitor in parallel (hopefully that makes sense). Whatever the DAC put frequency is (usually listed in kilo or mega samples per second - KSPS, MSPS), use resistor (pretty high ohm, depends on what the DAC can output current wise) and capacitor so that 1/(2*pi*R*C) is less than 1/10th of the DAC frequency. This is called the filter cutoff frequency. It should also be at least a decade above (10x higher) than whatever frequency you want your pulser to be (don't need to worry too much about the harmonics for Tig pulser). Some multiply the DAC frequency with the pulsing frequency, then take the square root of that (geometric mean if you see it in a filters handbook) to figure out the cutoff frequency, you can try both or experiment with different resistors and ceramic capacitors. You should be able to find example low pass filter circuit examples for Arduino projects with that DAC.

Hope this helps. Awesome project!
Jon

[jjohn76]

The 8-bit will probably be faster and give you more flexibility in what shapes you want. I'll take a look at the datasheet. There should be a port write library or example for the Arduino.

[Chadv]

I liked the 8 bit one better because I could understand how I thought it worked. I made a r2r ladder dac with the arduino but the quality resistors where twice as much as the IC. I think maybe I couldn’t understand the relationship between the dac and the op amp resister feedback

[jjohn76]

Chad, I am having trouble visualizing your power circuits. How are you powering the Arduino? What's the Arduino common referenced to on the XMT?

I can't remember if pin D and G/K are common on the XMT (my 250 GTSW is and I think I the XMT 350 is, I will check in the morning). If they are, it's worth referencing all commons (Arduino, DAC) to D/G/K. You may need to adjust your power circuit - connect G to the - on the capacitor and keep the + of your rectifier connected to the + of your capacitor (half wave rectified). This also lets you get a negative voltage rail too, which you'll need for your output buffers. To get the negative rail, you need another capacitor and the complement to whatever voltage regulator you are using (it's worth having a +15V/-15V and a +5V or -3.3V depending on your Arduino's operating voltage.

Did you connect the TLC 7524 like page 5 or page 7 unipolar in the datasheet? If you have it connected like page 7, you'll get an inverted output - you will need to add an inverting opamp to get the polarity right for pin E (put a zener voltage clamp on the output and connect to pin E). You shouldn't need a low pass filter for the R-2R DAC, but definitely should be sure it's clamped to a max 10V. You can connect it like page 5 and add a voltage follower on the output (Vref in this case). If you're using the circuit on page 5 (probably the way I would go), here's a good example: just use his MK I version, but.you don't need the voltage divider between Vref and the opamp - just connect Vref directly to the opamp as a voltage follower.

http://www.waveguide.se/?article=a-s...ng-the-tlc7524

It sounds like you figured out the timing for CS, WR and how to write all 8-bits to Port D.

[Chadv]

here is a drawing of how i did the power supply the arduino can accept 0-12v on vin

i didnt figure the timing on CS, WS I connected them to ground

[jjohn76]

Got it, so your Arduino has the onboard voltage regulator, and that's the 5V you're using for the optocoupler and relay drive circuits?

I'll draw something up in a bit, but it looks like your Arduino common/"ground" wil be somewhere other than the level of pin D, so you may have issues communicating with the DAC. The schematics say pin G/K and pin D are connected to earth ground, but it's worth checking continuity between the pins at your remote socket.

The TLC7524 datasheet doesn't give too much info regarding CS and WR, but keeping them low should work. It's probably worth initializing the data port for the DAC 11111111 in your setup function so you have the XMT panel reference as your default.

[jjohn76]

I need to find time much later to draw any circuits while still staying married... Instead of connecting G to the rectifier, connect it to Arduino and capacitor common/ground. This should put your Arduino, any opamp circuits, and your DAC on the same common reference. Confirm D and G are connected (304 schematic says they are, my XMT 350 and 250GTSW both have it that way, but best to confirm on your XMT). If you're sticking with a single power supply, be sure to use rail to rail (input and output)/RRIO unity gain stable opamps for your input and output buffers. Be sure to fuse between pin A and the rectifier (.5 amp should be good).

[jjohn76]

Chad, hopefully these are legible. The top one uses a dual supply for the opamps. The bottom/middle one is closer to what you have. I would caution you're at the upper end of input voltage range for your voltage regulator. The 24VAC is closer to 35VDC, minus a diode drop. You'll definitely want to heatsink the Voltage Regulator, or add in a switching buck regulator. I will probably use a 18-0-18 transformer when I put together a unit for the 250 GTSW. They're about $12 from mouser (241-6-36 if I remember right).

[Chadv]

Thanks that clears up a few things. That should keep me busy for a few weeks

[jjohn76]

Sounds good. I think it's worth replacing your ~33VDC to 10.1VDC voltage regulator circuit. It has a 23V drop across it, and for every 100ma it provides to the pulser circuit, it's dissipating 1/4W. It looks like it's heat sinked to your project box, which is good. Adding a buck converter (lots on Amazon) down to about 20V output between your big cap and NTE1960 lets you keep the linear regulator for low noise without having it dissipate so much heat.

[Chadv]

Will this work it meets the specs I understand

[Chadv]

TLV237x
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