# Inputs & outputs & relays ohmic

28 Apr 2019 04:23 - 28 Apr 2019 04:26 #132157 by JTknives
So this is my thought. If you ran a resistive voltage divider circuit like this then it would technically only output 10v max. You would use the aproporate ratio to achieve the proper voltage division and adjust the resistance of both to control the max load depending on what the thcad10 needs amp wise.
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Last edit: 28 Apr 2019 04:26 by JTknives.

28 Apr 2019 04:27 #132158 by rodw
I strongly recommend you don't use your own divider and just use the technique described in the THCAD manual.

From experience, the THCAD is likely to see some of the scaling resistors as being part of its own network and then will deliver erroneous results. Something I learnt the very hard way. Mesa give you some instructions so just follow them and you will be fine.

28 Apr 2019 21:22 - 28 Apr 2019 21:33 #132196 by JTknives
OMG it just hit me looking at the THCAD10 and their paperwork. This thing does not read voltage it’s reading micro amps “ua”. This is why you can get funky scaling with voltage dividers. And also why thy don’t say you have to balance the resistance on each leg. It makes perfect sense now and why their calculation is so simple. Your just adding to their resistance and you back calculate the needed resistance to have your max voltage give 100ua. Because 100ua=1mhz. So thy are not picking off any voltage like I thought. That’s why you take your voltage and subtract the 10v that it’s allready compensated for becaus 10v/100,000k=.0001 which is 100 micro amps. Take 300v subtrack 10v and that’s 290v/.0001a=2,900,000omes or 2.9 meg ohms. This also means that if you put the resistors in the plasma unit and ran the line to the THCAD in you control box you would only have 300v@100ua. Current over 10ma will create a painful shock but 100-200ma is required for it to be lethal. So the current in that line is .1a(minimum lethal)/.0001a=1000x under the minimum lethal amperage.
My day just got a whole lot better.
Last edit: 28 Apr 2019 21:33 by JTknives.

28 Apr 2019 23:42 #132208 by rodw

JTknives wrote: . This also means that if you put the resistors in the plasma unit and ran the line to the THCAD in you control box you would only have 300v@100ua. Current over 10ma will create a painful shock but 100-200ma is required for it to be lethal. So the current in that line is .1a(minimum lethal)/.0001a=1000x under the minimum lethal amperage.
My day just got a whole lot better.

I suspect this is why if you look at Everlast, Hypertherm and Thermal Dynamics all put a 100k resistor on each side of their raw arc voltage outputs. It does not really matter if the resistance is on one side or both, as long as you count it all, but it was the safety thing that made me suggest you split the resistance to both sides inside the cabinet.

28 Apr 2019 23:58 #132210
Well it's been a long time but he last time time (1991) I was involved in designing personnel safety circuits (not he machinery protection one you are quoting) we had to interrupt and quench in < 5ms and the trip current was 5mA. This also involved rather long cables with significant capacitive storage ability. Please put 1/2 of the resistance in each leg inside of your power supply to be safe.

John
The following user(s) said Thank You: rodw

29 Apr 2019 01:24 #132222
As Rod And John said, use 2 resistors, one on each line.
The following user(s) said Thank You: rodw

29 Apr 2019 04:09 #132237 by phillc54
I will be interested as to how this turns out as I am also going to use a PM30XP on my system if it ever gets to be a system.

Cheers, Phill.

29 Apr 2019 09:36 #132260 by rodw

phillc54 wrote: I will be interested as to how this turns out as I am also going to use a PM30XP on my system if it ever gets to be a system.

Cheers, Phill.

Phill, I know from bitter experience if you use raw arc voltage with the THCAD-10, you can't go wrong. But you can if there is a crappy voltage divider (eg the Everlast machines, not so Thermal Dynamics, dunno about Hypertherm)

Seems like JTk knows a bit about electronics. Maybe you do to! Lets see if we can take your HT 30 amp machines to the next level and add an ArcOK signal!.

My other plasma nerd mate Beefy in Melbourne (not Canberra) told me he fitted an ACS758 current sensor to his plasma cutter. (he has a HT 85amp machine from memory. He built a circuit board but you can get these modules on eBay. (Phill, you don't need to buy one, just email me your shipping address and I'll send you one by Toll Priority satchel)

I've attached the documentation. When I first looked at these units, I thought they output 2.5 volts at 0 amps and 4.5 volts at 50 amps so I also purchased a LM358 weak signal amplifier but when I look at the current sensing module so I could apply 24 volts which would step the 5 volt signal up to 24volts - 1 volt = 23 volts full scale). This is more than enough to get a reasonably accurate amperage via the &i76e/d's analog inputs.

But when I've looked at the specs again tonight, I think the ACS758 module will allow you to power it with 8-35 volts on the VCC pin. I think it still outputs a 5 volt analog signal though.

So Beefy reckoned he could set an amperage threshold and synthesise an ArcOK signal when the amps exceeded that value. He also said he could do very fine accurate centre punches becasue he could turn off the arc before the ArcOK signal was raised by his machine.
Anyway, here is the attachment. I reckin JTk will be selling circuit boards for this as well as knife blanks before long!

Put me down for a 150 amp version
!
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29 Apr 2019 10:24 #132265 by phillc54

rodw wrote:
Phill, you don't need to buy one, just email me your shipping address and I'll send you one by Toll Priority satchel

Rod,
Thank you, I really appreciate it. At the rate my build is going snail mail would be more appropriate.

Cheers, Phill.
The following user(s) said Thank You: rodw