This is "part 2" because my original question has only brought up more queries that i am unable to solve. The first part can be seen here for any who are interested: "Hot-knife"/Thermal cutter power supply design
I apologize for making two posts for the same project but my other has seemingly died :(
currently i am trying to construct a thermal knife to cut and seal ropes, both to avoid expensive store bought ones and as an educational project. i have bought a blade online to use that is supposed to be 60W, it can be seen here:
==================No longer relevant, see update======================== To obtain 60W of power im going to use a 60VA transformer to transform 240VAC, to 4V 15A output. so now my speedbumps:
- I understand 15A is a very large current so will a voltage of 4V be low enough to be safe?
- Before building, I wanted to test these conditions using a benchtop supply. When i connect the blade across the two terminals however i cannot get the voltage to go above 0.6V, even pushing the current to the supplies max of 20A. This would only dissipate 12W of power not the 60W i had wanted to test. My theory is that this is due to the extremely low resistance of the blade which according to V=IR would require a very large current to produce a moddest voltage. Any thoughts on another way to test before committing to the transformer?
- I have been told a ceiling fan speed control is a good addition to fine tune the current going to the transformer. using Australian mains of 240VAC, would something like this be appropriate? http://www.ebay.com.au/itm/AC-250V-50-60HZ-Fan-Speed-Rate-Control-Rotary-Button-Wall-Panel-Switch-/301422113787?pt=LH_DefaultDomain_15&hash=item462e2873fb
Thank you for any and all help :)
Edit - Update 1
Request was made for blade datasheet, it can be found here: http://www.saferight.com.au/_literature_167996/Hot_Knife_Datasheet Am i correct in assuming a 60W power draw for the blade? It is possible that some of this power is used by other components in the tool, in which case i think my only option would be a guess and check method as I have been unable to find any other useful datasheets.
I have done some measurements to deduce the resistance of the blade (far too small to measure on a multimeter). Set the power supply to constant current (CC) and then measured the voltage across the blade at different intervals. tabulated V/I using excel and I believe the resistance is 8.39mOhm
Using P=VI and V=IR with the data P=60W and R=8.39mOhm yields values of I=84.57A (im going to need some thicker cable) and V=0.7095V
Being on mains of 240VAC, this would indicate an effective transformer ratio of 338 (using N=240/0.7095) i should be able to achieve this by modifying a transformer as shown in the link provided by Bruce Abbott.
Thank you all, The advice (and scolding) have been a great help thus far, please tell me if this new theory is on the correct path before i start asking around for bigger power supplies to test with.