Selection and application of freewheeling diode

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Selection and application of freewheeling diode [Copy link]

Definition of freewheeling diode
A freewheeling diode is a diode that plays a freewheeling role in a circuit. For example, an ordinary diode 1N4007, if you use it in a circuit to play a freewheeling role, it is a freewheeling diode in the circuit. If it is not in the circuit, it is just an ordinary diode.

Circuit of freewheeling diode

The freewheeling diode should be added to both ends of the inductive load. The inductance here means inductive characteristics, not sexuality. The characteristic of an inductive load is that the current cannot change suddenly, that is, it is impossible for the current to disappear all of a sudden, nor can it appear all of a sudden. It requires a process.
Common inductive loads include relay coils and solenoid valves.

Why add a freewheeling diode?
Inductive loads will generate induced electromotive force, and the direction of the induced electromotive force is opposite to the direction of the voltage applied to it. When the inductive load is suddenly powered off, the induced electromotive force is still there. Since the induced electromotive force is opposite to the original voltage, when the power is not off, there is still the original voltage to offset it. After the power is off, there is no voltage to offset the induced electromotive force. This induced electromotive force may cause damage to the components in the circuit. After adding a diode, this diode just forms a closed loop with the inductive load. The current direction in the loop is just forward-conducting with the diode, and the current of the induced electromotive force can be released.

Models that can be used as freewheeling diodes
Ordinary diodes such as 1N4007 can be used as freewheeling diodes, but it is best to use fast recovery diodes or Schottky diodes.
Fast recovery diodes can be used: FR107, 1N4148.
Schottky diodes can be used: 1N5819.

Look at the parameters of the diode datasheet.
The withstand voltage of the diode is how much voltage can be added in the reverse direction. You can see that the freewheeling diode is connected in the circuit in reverse. For example, if the coil in your circuit is supplied with 12V, then the directional withstand voltage of your diode must be greater than 12V. However, the reverse withstand voltage of general diodes is very high. The
maximum forward conduction current of the diode, such as the maximum forward conduction current of 1N4148 is 150mA, so if your coil current is too large, the freewheeling diode will burn out. Therefore, 1N4148 is only suitable for coil protection of small currents, such as 5V relays.

Practical experience
shows that freewheeling diodes must be connected to both ends of the relay coil and both ends of the solenoid valve interface in the circuit. The connection method is as shown in the figure above, with the negative pole of the diode connected to the positive pole of the coil, and the positive pole of the diode connected to the negative pole of the coil. However, you must be clear that the freewheeling diode does not utilize the reverse withstand voltage characteristics of the diode, but utilizes the unidirectional forward conduction characteristics of the diode.
If you are too lazy to look at the datasheet parameters of the diode, just use FR107, which is suitable for general applications.