| ??? 02/20/07 16:02 Modified: 02/20/07 16:04 Read: times |
#133334 - Arcing machine Responding to: ???'s previous message |
Andy said:
Remember, a relay coil is a great big solenoid; an inductor - so switching it on and off causes big spikes on the supply This can often be read, but isn't really true. Connecting a really big inductor to a supply voltage results in a current which is increasing very slowly! Let R be the ohmic resistance of the coil and L be its inductance, then the current rises exponentially at a time constant of L/R. The same when the inductor is suddenly switched-off from the supply. Due to the Lenz's rule a voltage is induced to keep the former current flowing. If the switch is opened an arc is generated, which just "eats" this extra voltage. Nevertheless the current smoothes down again following this time constant. When turning-on or -off an inductive load, no current spikes are the result! Just the opposite, smooth rises and falls. The only thing that could suffer at all is the switching element, which has to withstand the induced voltage during the turn-off, unless a free wheeling diode, or another snubber element is installed. With mains voltage situation looks different, though: When an inductive load is switched to mains at the wrong moment a huge current can flow. What Hendryawan did is to design an arcing machine, which causes lots of readiated EMI: During the contact bouncing nasty bursts are generated... Kai |
