THCAD-10 + hypertherm 45 (nonXP)

Hypertherm CPC (Circular Plastic Connector) components should you wish to make an interface cable of your own (or add additional pins to the plasma cutter CPC connector)

The connector is made by TYCO and all of the components are listed within this manual, and the part numbers are their numbers,
www.te.com/commerce/DocumentDelivery/DDE..._0807.pdf%7F206044-1

www.te.com/commerce/DocumentDelivery/DDE...038_A.pdf%7F206044-1


206044-1 Plug Shell [1 number required]
206070-8 Cable Clamp [1 number required]
164164-1 Pins (male - lead side) [ 7 number required, including ground conn. ]


66101-9 Pins (female - machine side) [only required if you want or need to install additional pins, this should not be required]

Oh, check the quantities at time of order as some of the pins are only sold as singles or packs of 4 or 5 etc


If you search on ebay (or as I did RS components as cheaper in the UK) you can find them, I suspect DigiKey or Mouser in the US will be better but we get hammered on import tax.

RS Components I believe also own CPC [preston] and also farnell in the UK hence having a search of the three for electronic components may bring up different costs + free shipping some times.

The part numbers came mainly from here (Jim Colt post):
www.cnczone.com/forums/hypertherm-plasma...rum.html#post1146019


One of the cleverest things I've seen done is this:
i.ebayimg.com/00/s/MTAzNlgxNjAw/z/yBUAAOSwBLlU56lt/$_1.JPG
this is sold as part of the miniTHC hypertherm kit. minithc.com/
It is clever because once you have an RJ45 connector, extending the lead in the future or using screened + shielded twisted pair may provide better noise immunity especially if you are sampling very small voltages our of the PMX45 with it's 50:1 divided output then noise may affect the signal especially if using unsheilded cable. hence CAT 6E screened + sheulded twised pair ethernet cable can be a good + cheap choice and it's available as an external grade hence will be more abrasion tolerant in a workshop enviroment.

It seems I had been a bit of a Muppet.... Too many projects + work distraction...

I was considering changing the 2k resistor within the pmx45 to change the divided output from 1:50 to 1:20 or something else (1:10 with an offset of -6v)

The problem I was having was offset....

Given the range of the thcad-10 is 10 volts, and anything less than ~70 torch volts and anything greater than ~170 torch volts given these are outside of the cutting range ( by a fair amount ), hence any voltage above and below this should be ignored and the torch (z-axis) should not rise, fall or try to track the voltage ..... When in doubt... Stay put....

Instead of thinking of the problem as a voltage divider change, I realised that this will only change the signal gain (plus I am reluctant to put a soldering iron to a perfectly good low mileage plasma cutter)

I thought about using an opamp, to simply offset the voltage, and then amplify the signal and have come across a few cheap ad620 development boards which offer the opportunity to do just that very simply and require:

- Two ad620 boards (first provide signal offset at a gain of 1, then amplify the signal with a signal gain 2.5)

- An isolated power supply providing dual (+/0/-) 12 volts for the opamp. A dual supply is required to provide the offset stage.

... Hence that is the plan at present ... Just awaiting the slow boat from China with boards to try out.. (more time to get my head around linuxcnc under the hood)

Rob

No eBay APP ID and/or Cert ID defined in Kunena configurationAnyone reading this thread amongst other things may be thinking, once you've distorted the perfectly good divided output voltage from the pmx45 how on earth are you going to know 90 volts within linuxcnc is actually 90v at the torch tip?

I've got good quality multimeters and a scopemeter But none have been calibrated... Hence use them for a check but would not trust their readings for 100% displaying voltage to 1 significant figure

I came across precision voltage reference chips a while ago and picked up an AD584 a while ago, and they are available as cheap PCB's and I'd suggest if you are in need of calibrating anything they are a good cheap source of a reliable accurate voltage for calibration.
www.analog.com/media/en/technical-docume...ata-sheets/AD584.pdf

No eBay APP ID and/or Cert ID defined in Kunena configurationMy quick and dirty methocd was to wire 2 x 48v power supplies in series to give 96 volts and feed that through the plasma divider board and check with a multimeter. I think the output of a 50:1 divider will be enough, but you could scale it with one of these:


I think you will need to calibrate the board in a similar way because I found the plasma voltage divider resistors were counted by the THCAD as being part of the scaling resistor so my 50:1 divider became 75:1.

rodw wrote: I found the plasma voltage divider resistors were counted by the THCAD as being part of the scaling resistor so my 50:1 divider became 75:1.

Hi Rod, thanks for your reply,

Yes it will be... Sort of... It should not however be so pronounced as to make a 50:1 voltage divider into a 75:1 voltage divider

Looking at the math...
The voltage divider is basically 2 resistors... A high resistance (plus power rated) one and a low resistance one (of nominal power rating)... The relationship is 50:1.... So two standard resistance values are selected that have that ratio relationship

In my case with the pmx45, hypertherm chose a 100k and a 2k resistor... Wired in series...

So total resistance is 102k ohm.

350v / 102k = 3.431 mA of current flow

Therefore the voltage across the resistors will be 3.431mA X 100k and 3.431mA X 2k, which will be 343.137 volts and 6.863volts

This is presuming that there are not external impedances .....

Add in the THCAD-10 across the 50:1 output, we now have a 100K resistor in parallel with the 2K voltage divider resistor

Hence the combined resistance in parallel is calculated as 1 / (1 / 100K + 1 / 2K) .... so the 2K resistance actually becomes ~ 1960.784 ohms [note we're not considering resistance tolerances that may vary this number, just the pure lab condition laser trimmed / text book resistances]

So our voltage divider is now 100K+1960.784 ohms, so our current flow is 3.433mA, so the voltage across the 100K resistor becomes 343.269V and across out parallel resistor arrangement (2K+100K) becomes 6.730 Volts

So our "divided" voltage output becomes closer to 50.999:1 (100K / 1960.784) than a "pure" 50:1 (100K / 2K) divided output.

Couple of caveats ...... If your voltage divider uses something greater than a 2K + 100K resistor combination (say 20K + 1M ohm) then the effect of the 100K parallel resistance will become greater [6:1 voltage divider, and not a 50:1!] (but the noise in your signal may also become more pronounced .... because you're not got very much current flowing through your voltage divider hence it becomes easier to effect a 340uA current with noise than a 3.4mA signal .... and bear in mind that this "maximum" current flow does not occur when the machine is cutting .... as its the open circuit voltage (and of no use to us really inTHC) when our cutting range (+THC tracking range [whether target setpoint / sample and hold ] operation) is 90~150V (1/4 to 1/2 of the current flow i.e. easier to affect with noise and interference)

Rob

Bear in mind that the _actual_ voltage isn't that critical as long as you know that your machine cuts material A best at a setting of "100 Rob Units" all is well.