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shekhar_dandya

inductors and arching

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I wanted to know that if one uses an inductor in series with a switch
(either a transistor or a mechanical switch)and a DC source without using a freewheeling diode,how will the inductor react when switch is opened?
will it cause arching/sparking or melting of transistor?if so,why?
what happens practically without a freewheeling diode?

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Shekhar,
When a DC current flows in an inductor, a magnetic field is built-up in its core. When the current flow is stopped by opening a series switch, relay or a driver transistor, then a very high reverse voltage is produced across the coil as the magnetic field collapses and induces the high voltage in the coil.
This high voltage can produce an arc at the switch or relay contacts, burning or welding them, because of the high temperature of the arc. The high voltage can cause breakdown damage to a driver transistor.
A freewheeling diode that is connected in reverse polarity across the coil clamps the high voltage so that it is only one diode voltage drop higher than the supply voltage.

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I would not use one of those. My experience has been that many devices are implemented in hopes of prolonging the life of electronics components. You should really not need to do this. I think that the components are hardy enough to withstand negative transients.

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Kevin,
1) Why do transistors have an absolute maximum voltage rating?
2) What happens to a transistor that has a 40V absolute maximum voltage rating and you put 400V or more across it from a non-protected inductor load?
3) Why not use a cheap protection diode instead of an expensive high-voltage transistor?
4) The voltage spike is not only negative. If an NPN transistor is using a normal positive supply and has a non-protected relay coil as a collector load, then when the transistor is turned-off, a positive voltage of 400V or more will appear at the transistor's collector.
Likewise, a negative high voltage will appear at the collector of a PNP transistor/relay circuit. The voltage reverses across the inductor, not the transistor.
That's why a protection diode is connected in reverse polarity across a relay coil, so that when the inductor's voltage rises higher than the supply voltage, then the diode conducts and clamps the voltage to be only 1 diode voltage drop more than the supply voltage.

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Try this.

Use DC Voltage while doing this Experiment.
Lets Use a 12 volt battery and a 12 volt Relay.
Touch you fingers across both the Positive and Negitive of the battery.
Nothing, no fealing.
Now Connect The Relay Coil Across the Positive and Negitive terminals while touching the wires.
Same Thing Until you remove the wires from the battery while holding one wire in each hand.
You can feal what happens to the Transistor when no diode is there.

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Guest Kasamiko

I remember testing the primary of power transformer with an analog VOM set to x1... ;D
You will get a very nasty surprised!!

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