Ferrite filter question

Reecently I helped a neighbor connect a Chinese made CNC plasma table to an American made plasma cutter. After I figured out that the "partial pressure" the Chinese manual mentioned was in reference to a built in jumper selectable voltage divider I was able to get the two machines talking to each other and everything works now. But the Chinese manual shows a filter on a sense wire and I don't know exactly what to do about it. The sense wire is connected to the copper shield on the plasma torch and to a terminal in the CNC control. The machine uses the shield to sense the work. The machine moves the plasma torch down until the shield contacts the work and then moves away a set amount and energizes the plasma torch. I think, but am not sure, that the control can now sense the distance between the work and the torch tip, so that if the work is warped and rises or falls the torch will follow the contour of the work. In any case, the connection diagram for this sense shows it being wrapped a few times through a ring. And somehow the manual alludes to this ring as being some sort of filter. Since the neighbor didn't have any type of ferrite rings I just wrapped a few turns of the sense wire through a few insulated washers just to see if I could get the machine to work. And it did. Well, I dug a ferrite ring out of a bad switching power supply and removed the coiled wire from it. I'm hoping I can just duplicate the washers trick with the ferrite ring but I'm thinking that I need advice on how many turns of wire through the ring I should use. I know, there are many different types of ferrite and I have no idea what kind mine is. And there must be a bunch of other variables as well. But if I can at least not ruin anything it would be great. Thanks, Eric

Reply to
etpm
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Cute trick using some steel washers. If the signal line is coax, or twisted pair, or something else with a shield, then you are making a common mode choke. If a single wire then it's just a choke. Regardless of which it's kinda hard to say how many turns you'll need. The impedance of the choke is working against some other impedance in the circuit. I typically do what you did.. wrap a few turns and see if it works. If you've got a 'scope, you might be able to look at the signal line and see the noise and reduction... but all sorts of caveat's here about floating AC loads and HV. You don't want to blow out your 'scope.

George H.

Reply to
George Herold

Yep, that's a common-mode filter, usually a lossy ferrite bead, sometimes a split bead and plastic housing that clamps over the cable. Washers might work, but (one hopes) a ferrite would be slightly better because steel can magnetize and cores for this purpose ought not to do that.

Since the purpose of such a filter is to deal with unwanted interference, there's no real calculation to selecting the core and number of turns. What works, is starting with the smallest core and least turns, doing trials, and when the bad symptoms die down, use the next larger size or one extra turn.

If you have an emissions test lab available (I did), watch the emissions measured, and omit the 'next larger size' step.

Reply to
whit3rd

You might put your blurb on rec.crafts.metalworking Ignore the bickering that is presently happening, new name popped in and giving one guy a hard time. There are some very bright guys there. Mikek

Reply to
amdx

You did not mention any problems WITHOUT the choke. So it might work anyway and your ring does nothing to it.

A choke is usually wound with five to ten turns around a ferrite. Its performance can be measured, but that was not your question.

w.

Reply to
Helmut Wabnig

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