I was studying single phase rectifier circuit with RL load. There I found that they are using a freewheeling diode in parallel to the RL load. So is a freewheeling diode different from a power diode ? Or it is just the same with a little higher rating?
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2\$\begingroup\$ en.wikipedia.org/wiki/Flyback_diode \$\endgroup\$– got trolled too much this weekCommented Nov 12, 2015 at 12:54
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4\$\begingroup\$ Question should be closed - lack of research. \$\endgroup\$– Leon HellerCommented Nov 12, 2015 at 13:04
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\$\begingroup\$ Protection is the key word. But please investigate harder before asking here \$\endgroup\$– cventuCommented Nov 12, 2015 at 13:29
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3 Answers
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A free wheeling or flyback diode is used to prevent damage to circuits typically with any load that has an inductor and any switching potential. An inductor can't change current immediately. Attempting to change the current rapidly such as when a switch opens after an inductor has built up energy, will cause the inductor to generate immense back EMF's.
If a flyback diode isn't present, the immense voltage will build across the switch potentially damaging it. The flyback diode gives the current through the inductor a path to continue until the energy in the inductor has dissipated (in the inductors internal resistance).
answered Nov 12, 2015 at 14:50
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\$\begingroup\$ what is CO? Also I don't think you get an immense charge, you can get an immense EMF. \$\endgroup\$– IcyCommented Nov 12, 2015 at 15:23
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\$\begingroup\$ CO is a typo from phone's auto Predict. If a switch is involved it definitely produces an immense voltage. EMF is Electromotive force and is measured in Volts. If you're interested in some simulations, here is one online. Any switch in a practical circuit has some capacitance value. When the inductor gets disconnected with no freewheeling path, it begins driving current through the capacitance of the switch, which builds up a massive voltage, as you can see in the simulation it reaches 2.2kV. \$\endgroup\$ Commented Nov 12, 2015 at 22:50
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\$\begingroup\$ Sorry I now realize you may have been confused with the terminology. It produces both a charge and a voltage. Voltage is just the measure of potential energy between 2 points per unit of charge. Capacitance is measured in the ratio of charge to voltage, so by building up a voltage you also build up a charge. \$\endgroup\$ Commented Nov 12, 2015 at 23:10
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\$\begingroup\$ I'm not confused by the terminology. The voltage (EMF) produced by the inductor is proportional to the rate of change of current through it. The energy stored by the inductor is proportional to the current through it. Current flowing into the switch capacitance will cause the capacitor to charge, and the voltage across it to increase, but the charge in Coulombs will not be immense, it will be proportional to the energy originally stored in the inductor, it will be distinctly finite - and would be the same if the current through the inductor is reduced slowly or rapidly. \$\endgroup\$– IcyCommented Nov 13, 2015 at 8:20
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\$\begingroup\$ It is distinctly finite, but the voltage generated on the capacitor does reach much higher levels than originally in the inductor. This is due to the current not being able to change instantly and being driven through the switch when it's in the off state, now being modelled by a capacitor. Capacitors themselves have something known as Equivalent Series Resistance (ESR). At any given instant, current is constant, therefore the voltage across the switch is then V = I*ESR. If the ESR of the switch is large, the switch will experience a very large voltage applied to it. \$\endgroup\$ Commented Nov 14, 2015 at 3:08
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The freewheeling diode, for power rectifier applications, usually is a diode of the same type as used in the rectifier. This improves manufacturability.
The function of this diode is to provide a path for the current generated by the back emf when the rectifier is loaded with an RL circuit. Thus, damage to components of recitificador are avoided.
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\$\begingroup\$ Freewheel diodes are applied to the inductive loads not really to do with power rectifiers. Why do you get a back EMF? \$\endgroup\$– IcyCommented Nov 12, 2015 at 15:26
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\$\begingroup\$ @Icy The answer is about a rectifier. I think that "rectifier" here is used as general term. Of course, the freewheeling diode aplies on controlled rectifiers than basic rectifiers. \$\endgroup\$ Commented Nov 12, 2015 at 15:38
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Whenever there is an RL load, it's always good to have a diode connected in reverse biased across the inductor. This diode is called freewheeling diode. It's purpose is to block the back emf generated by a coil whenever power is disconnected from this. This back emf could damage other components in the circuit especially solid state components.
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2\$\begingroup\$ It doesn't block the back EMF - it provides a path by which the inductor current can continue to flow when the original source is disconnected. \$\endgroup\$– IcyCommented Nov 12, 2015 at 12:52