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noise over control circuit

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khaled ragab

Junior Member level 1
Feb 15, 2012
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I'm making a controller to control a milling machine , the machine has a box where the control buttons should be arranged , I have the electronic control circuits , and I also have some buttons that control some 220v ac contactors , the problem is that the magnetic field caused by the currents in the contactors wires cause the electronics to misbehave , sometimes completely halt , unfortunately it would be impractical to seperate the contactors buttons from the electronic control buttons .
The electronic circuit is enclosed in a metal box , my question is to what should I connect the metal box to have a shielding effect , I connected it to the machine body and there was no effect , I connected it to the neutral line , and there was no effect either , there is no difference in effect between when the metal box is closed , and when it is open and the circuits are exposed .
Is there something I'm doing wrong ?
I forgot to mention that these malfunctions happens only when I change the state of the contactor , particularly when I switch a contactor off , I thought it must be that the current in the coil is stopped suddenly , so the magnetic filed rate of change got high and induced high voltages in the control circuits , I don't know if I'm right about that.

First of all, the metallic enclosure should never be connected to any live voltage such as neutral for safety reasons.
Metallic enclosures must be connected to the protective earth conductor (typically yellow/green cable) so the ground fault contactor in the installation protects the user against electric shock :!:.
A metal box does not protect against all kind of interferences, especially the ones produced inside the machine!
Having the schematics would be usefull to see if there is any mistake. Maybe you are inducing some conducted noise or you control circuit is weak against interference. You may connect a Y type safety/suppression capacitor between the electronic circuit ground and chassis, this may help...

If as I understand it you have put your electronic controls inside a contactor box, the you have committed a cardinal sin. The only way out of this is to fully screen all the electronic switches from the high power mains stuff (metal screen across the box) and run all the leads to the electronic stuff in screened cable with the outer sheath earthed at the electronics end. if the problem persists, then try .1 MF caps between the live control cables and earth.

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