The main function of the diode connected in parallel with the relay coil is to eliminate the back electromotive force (reverse voltage) to protect other components in the circuit. When the relay coil is energized, a magnetic field is generated in the coil; when the power is off, the magnetic field disappears rapidly, and a back electromotive force is generated in the coil. This back electromotive force may damage other sensitive components in the circuit (such as transistors, microcontrollers, etc.).
By connecting a diode in parallel across the relay coil, the diode conducts when power is off, eliminating the back electromotive force and thus protecting other components in the circuit. This diode is commonly referred to as a "protection diode" or "freewheeling diode."
In practical applications, the anode of the diode is connected to the cathode of the relay coil, and the cathode is connected to the anode of the coil. Thus, during normal operation, the diode is reverse-biased and does not affect the coil's operation. When the coil is de-energized and generates a reverse electromotive force, the diode conducts in the forward direction, eliminating the reverse electromotive force.
01 Inductors and Inductors An inductor is a coil of wire wound on a magnetic material. When an inductor carries an electric current, it generates a magnetic field. Magnetic fields are lazy and do not like to change. As a result, the inductor becomes a component that opposes changes in the current.
If the current flowing through the inductor is constant, the inductor is happy because it does not need to exert any force on the electrons; in this case, the inductor coil is just an ordinary wire.
If we want to interrupt the current in an inductor, the inductor will exert an electromotive force (EMF) to try to maintain the current. If the inductor forms a loop and there is no resistance in the circuit, then theoretically, the electron flow will always be circulating. However, unless we use a superconductor, all wires impede the current, and eventually the inductor current will decay to zero; the higher the resistance, the faster the decay. Conversely, the higher the inductance, the slower the decay. (See Figure 1.)
Once the current becomes zero, the inductor, which always tries to impede the change in current, then tries to keep the circuit current at zero.
Therefore, when we connect an inductor to a circuit, the inductor immediately tries to resist the increase in current, but the current still increases slowly. The larger the inductance reactance, the slower the current increases. Once the current stops increasing and reaches a steady-state value, the inductor is happy to stop trying!
When we cut off the current in an inductor, the inductor tries to maintain a steady-state current value. If the inductor is connected to a resistor at this time, the voltage across the resistor is the product of its resistance and the current. Since the inductor's primary function is to prevent sudden changes in current, regardless of the resistance value, the current in the inductor is the same immediately after the circuit is cut off as before. If the resistance value is very large, the product of the current and resistance will also be very large, resulting in a momentary high voltage across the inductor.
Because the current in an inductor cannot change abruptly, a current release path must always be provided to disconnect the inductor circuit. Without such a path, the inductor current will find its own way, for example, through the air, through switch contacts, or other non-conductive components. A short period of high voltage can severely damage the circuit. The ability of an inductor to generate high voltage is extremely useful in power supply design, but it also means that an inductor circuit should not be disconnected without a pre-prepared release path.
02 Freewheeling diode
As can be seen from the diagram, the instantaneous high voltage generated by the EMF during a power outage (several times or even tens of times the power supply voltage) will damage other components of the circuit if it has nowhere to be released. If a release circuit is provided, how can it be connected in time? That is, when the inductor circuit is connected, the release circuit is not connected, and when the inductor circuit is disconnected, the release circuit is connected.
Resistors conduct electricity bidirectionally, while diodes conduct electricity unidirectionally. Therefore, we use the circuit shown in Figure 5, where the diode connected in parallel across the inductor is called a freewheeling diode (flyback diode or flywheel diode).
03. The Function of a Freewheeling Diode: A freewheeling diode is typically used in conjunction with energy storage components. Its function is to prevent sudden changes in voltage and current in a circuit and to provide a path for dissipating power through the reverse electromotive force. An inductor coil can use it to provide a continuous current to the load, preventing sudden changes in load current and smoothing the current flow. In switching power supplies, a freewheeling circuit consisting of a diode and a resistor connected in series can be seen. This circuit is connected in parallel with the primary winding of the transformer. When the switching transistor is turned off, the freewheeling circuit can release the energy stored in the transformer coil.
Selection of freewheeling diode in BUCK circuit
In BUCK circuits, fast recovery diodes or Schottky diodes are generally chosen as "freewheeling diodes." They are typically used to protect components from breakdown or burnout by induced voltage. They are connected in parallel across the components that generate induced electromotive force and form a loop with them. The high electromotive force generated is dissipated in the loop as a freewheeling current, thus protecting the components in the circuit from damage.
Theoretically, the diode should be selected with a current rating at least twice the maximum current. In practical applications, due to the strong instantaneous overload resistance of diodes, ultra-fast diodes with a maximum current of 50A can also be used. With a suitable heatsink, damage is rare in actual use. The total impedance when conducting is the motor's internal resistance plus the equivalent internal resistance of the driver transistor. The total impedance when freewheeling is the motor's internal resistance plus the equivalent internal resistance of the freewheeling diode. Generally, since the AC equivalent internal resistance of the freewheeling diode is smaller than that of the driver transistor, in conventional designs, the maximum current of the freewheeling diode is usually twice the maximum current of the motor.
Transient current is only momentary, and surface-contact diodes have decent overload resistance, as long as the voltage isn't too high. If necessary, a small resistor can be connected in series for current limiting. Freewheeling diodes protect switching devices. The transient current during freewheeling is related to the motor's operating voltage and winding resistance, but not the motor's power. To calculate the peak transient current, the reverse self-inductance voltage minus the diode junction voltage drop, divided by the loop resistance, is used. The reason for using a diode with a certain current rating is that low-voltage, high-power motors have low winding resistance, resulting in a relatively large transient current. A small resistor in series can suppress the peak current. The slight increase in transient voltage across the switching transistor caused by this is negligible because the operating voltage is already low; modern transistors have a withstand voltage of at least 50V.
05 Selection of Relay Freewheeling Diode The diode connected in parallel with the relay is not the freewheeling diode used in BUCK circuits. Since the relay coil is an inductive load, its function is to absorb the self-induced voltage of the relay coil when the driving transistor is turned off. According to Lenz's law, when the current in the inductor decreases, a self-induced voltage is generated. The direction of this voltage is negative at the positive power supply terminal and positive at the collector of the driving transistor. This voltage can break down the transistor. Therefore, a snubber diode is connected in parallel with the relay to absorb this self-induced voltage.
First, the influence of circuit time parameters at the millisecond level on mechanical contacts is ignored.
Secondly, even the 1N4000's reverse recovery time is far less than milliseconds, and its forward conduction time is even shorter.
Third, the inter-electrode capacitance of the driver transistor and the parasitic capacitance of the relay are sufficient to render high-speed diodes useless.
Fourth, the energy stored in the inductor is mainly consumed by the winding resistance, and it is generally in an overdamped state.
For the switch shown in the diagram, we often use transistors. A transistor TR1 is used to control the conduction of the relay coil, and the relay contacts then control the load circuit.
Freewheeling circuit of relay coil
When the cathode of a diode is connected to the positive terminal of a DC power supply, and the relay coil is de-energized, the diode provides a path for the high voltage in the coil to dissipate. Without a freewheeling diode, the high voltage generated across the coil when the transistor is disconnected would cause significant damage to the transistor circuit; in this case, the freewheeling diode provides protection.
Therefore, diodes are often integrated directly with relays, as shown in the diagram below. 06. Contact Protection Circuit
Inductive loads are generally more likely to damage contacts than resistive loads. However, if appropriate protection circuits are used, the effect of inductive loads on contacts can be made to be roughly the same as that of resistive loads. But please note that if used incorrectly, it may have the opposite effect.
The table below shows representative examples of contact protection circuits. Please avoid the contact protection circuits listed in the table.
07 Circuit of freewheeling diode
The freewheeling diode should be connected across the inductive load. Here, "inductive" refers to a load with inductive characteristics, not a load with a kinetic charge. The characteristic of an inductive load is that the current cannot change abruptly; it cannot suddenly stop flowing or suddenly increase. There needs to be a process.
Common inductive loads include relay coils and solenoid valves.
08 Why add a freewheeling diode?
Inductive loads generate an induced electromotive force (EMF). The direction of the induced EMF is opposite to the direction of the voltage applied across it. When the inductive load is suddenly de-energized, the induced EMF remains. Since the induced EMF is in the opposite direction to the original voltage, it was canceled out by the original voltage when the power was on. After the power is cut off, there is no voltage to cancel out the induced EMF, which may damage the components in the circuit. Adding a diode forms a closed loop with the inductive load. The current in the loop is forward-conducting with the diode, which can release the current of the induced EMF.
09 is a model that can be used as a freewheeling diode.
A common diode such as 1N4007 can be used as a freewheeling diode, but it is best to use a fast recovery diode or a Schottky diode.
Fast recovery diodes can be made using: FR107, 1N4148. Schottky diodes can be made using: 1N5819.
10. What parameters should you look at in the diode datasheet?
The reverse voltage rating of a diode is the voltage it can withstand in reverse. As you can see, a freewheeling diode is connected in reverse in a circuit. For example, if your circuit has a 12V coil, then your diode's reverse voltage rating must be greater than 12V. However, most diodes have very high reverse voltage ratings.
A diode's maximum forward current, for example, the 1N4148's maximum forward current is 150mA. If your coil current is too high, the freewheeling diode will burn out. Therefore, the 1N4148 is only suitable for low-current coil protection, such as a 5V relay.
11. Practical Experience: A freewheeling diode should be connected to both ends of the relay coil and the solenoid valve interface in any circuit. The connection method is shown in the diagram above: the negative terminal of the diode is connected to the positive terminal of the coil, and the positive terminal of the diode is connected to the negative terminal of the coil. However, it is important to understand that the freewheeling diode does not utilize the reverse voltage withstand characteristic of the diode, but rather its unidirectional forward conduction characteristic.
If you're too lazy to look at the diode's datasheet parameters, just use the FR107; it's suitable for most applications.
