On Tuesday 28 October 2014 08:18:30 Viesturs Lācis did opine And Gene did reply: > Hello! > > There is a plasma machine I built and I used THCAD-10 card for thc > purposes, because the plasma source had such an output signal. > Long story short - the machine has new owner and they want to use > JasicCUT plasma source, which AFAIK does not have 0-10V signal output > for thc, so I will have to connect directly to power leads of plasma > cable. > > How do I calculate, what is the resistance of additional resistors > needed to reduce max voltage from 300V to 10V? The unknown element > here is the current that passes through THCAD card. > > Viesturs
That would be a 30/1 ratio, but would need to be a high enough resistance to not draw source or resistor destructive power, and if using carbon film resistors, the higher value resistor connected to the JasicCut should be in the form of several lower value resistors in series so as not to over voltage the carbon grains and destroy them over a log period of tie without their ever getting hot. If the load end load is 10k ohms, then the high end of the string I would make from 11 each 27k resistors, which would total 297,000 ohms and should, if they have sufficient wattage ratings, last nominally until the rapture. Half watt resistors s/b plenty good enough in that case. The resistor tolerances should give a very slightly higher reading, a percent or so, but linuxcnc can probably calibrate that out if it is that critical. I would much prefer to use metal film resistors as their failure mode doesn't seem to have the long term drift in values that bulk carbon or carbon film resistors are prone to. The values I quoted would result in only a red hair north of 1 milliamp of draw at the full 300 volts. That may not be enough, so dividing everything by 10 might be in order. 1000 ohm load, and 11 ea 2.7k's in series says its still under 5 watts total if I am running kcalc correctly. The likely hood of that isn't good, so please check me on that. 11*2700+1000=31700 ohms for the whole string. Then 300 / that=0.0097719869706840390879 amps of current flow, then wattage is usually calculated by me as i^2*r so the i squared is i^2=9.5491729355218623009e-05, and that times the total r of 31700=3.0270878205604303494 which is a red hair north of 3 watts. But since that 3 watts is effectively spread over the 11 2,7k in series, that is then worst case = 0.27518980186913003176 watts per resistor, so half watt resistors in metal film should be fine. 1 or 2 watter's should last forever. Other that striking the arc, do plasma cutters ever work at more than 75 volts? Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
