A flyback diode, also known as a snubber diode or freewheeling diode, is an important electrical component used to prevent voltage and current spikes when controlling inductive loads in circuits.
The key property of inductors and relays is that they resist changes in current flowing through them. When the power applied to an inductor or relay coil is suddenly reduced or interrupted, the magnetic field collapses very quickly, causing a large voltage spike as energy is released. This can damage switching transistors and integrated circuits.
A flyback diode provides a path for current to continue flowing when power to the load is removed. This gradually dissipates the energy stored in inductive components without allowing dangerous voltage spikes.
A flyback diode conducts backward current when power to inductive load is disrupted
https://en.wikipedia.org/wiki/Flyback_diode#:~:text=A%20flyback%20diode%20is%20any,is%20suddenly%20reduced%20or%20interrupted. are almost always placed across the terminals of inductive loads such as relays, solenoids, motors, or transformers. They can be individual diodes wired in parallel or included within the component itself.
Now let’s take a deeper look at the importance of controlling relay switching noise.
Relays are a vital category of electromechanical component used to control power delivery and signals in countless industrial, commercial and consumer electronic systems. From automotive computers to HVAC equipment to vending machines, relays enable automated switching of higher voltages/currents than could be handled by low power circuitry alone.
However, relays do present challenges due to the inductance inherent in their solenoid coils used for actuation along with connections to motors or heaters. Rapid interruption of current through inductors creates reverse EMF spikes and electrical noise which can interfere with microcontrollers and other sensitive digital logic, even causing damage.
Some key negative impacts associated with relay switching noise include:
Reset or spontaneous triggering — Noise glitches may inadvertently trigger logic level changes that reset processors, activate components incorrectly or cause erractic opereration.
False signals — Transients can resemble legitimate data signals, confusing digital systems by simulating buttons being pushed or sensor readings that didn’t actually occur.
Component failures — Excessive flyback voltages beyond transistor or IC voltage tolerances leads to breakdown and permanent damage over time.
Electromagnetic interference (EMI) — High frequency noise may disrupt performance of wireless communication equipment or leak to cause FCC violations.
So suppressing electrical noise generated by relay switching is critical for preventing system malfunctions, instability, premature breakdowns and interference issues across countless applications.
Proper application of flyback diodes helps mitigate such problems by diverting and dissipating inductive kickback safely when interrupting current to relay coils.
In choosing an appropriate flyback diode for protecting relay or solenoid switching circuits, some key parameters need to be considered:
1. Reverse Voltage Rating
A primary factor is ensuring the diode has a peak inverse voltage (PIV) rating well above the worst-case spikes expected from the inductive load when interrupted. Typical relay coil voltages ranges from 12V to 240V. Allowing at least a 2X margin is recommended.
2. Forward Current Rating
The diode needs to handle the full load current flowing when in its normal conducting orientation without overheating. Consider both average and peak forward current. Look for published surge current specs.
3. Switching Speed
Ultrafast diodes with short reverse recovery times are strongly preferred for suppressing fast relay switch-off spikes most effectively. Standard or fast options may still allow some initial flyback.
4. Temperature Range
The diode must perform across the full operating temperature band of the system environment without loss of function. Look for operation from at least -40°C to 100°C.
5. Packaging
Surface mount diode options occupy very little space on PCBs and allow automated assembly. Leaded through-hole diodes work where needed too. Consider heat sinking needs.
Now let’s examine some of the most suitable diode technology options to deploy with relays for robust noise protection.
There are several viable technical routes for integrating flyback diodes into relay driver circuits for control of electrical noise:
Standalone diodes wired in parallel across inductive loads are a simple and low cost approach, provided proper electrical ratings are selected.
Ultrafast diodes like the 1N4148 excel due to very short <4ns reverse recovery times. Other fast switching models using Schottky diode construction may also suffice.
Small size SMD components work well for automated PCB assembly.
Discrete external ultrafast diode placed directly at relay
Many standard electromechanical and solid state relay components have internal suppression diodes integrated directly across the coil inputs for convenience:
Built-in flyback diode inside relay module
This saves some wiring effort and parts cost. However the parameters of internal diodes may not always be fully disclosed or suitable for all applications. External diodes offer more flexibility if needed.
Specialized integrated circuits designed to control relays often also incorporate flyback diodes internally:
Single chip solution integrates control, drive and suppression functions
This provides a compact, self-contained solution when paired with the relay itself. Convenient protection against electrical noise is included by design.
While less effective than diodes, an RC network connected across inductive loads forms a low pass filter to suppress some moderate switch-off spikes. The resistor dissipates energy as current continues through capacitor.
Appropriate component values must be calculated based on the inductance and transient response behaviour. This adds complexity compared to simply using a suitable diode.
Complete integrated PCB assemblies are available which incorporate relay coils, high current contacts, drive electronics and onboard noise suppression:
Smart power relay module with integrated diode protection
These eliminate the need to select flyback components and simplify design effort significantly. However costs may be higher than discrete implementations.
While flyback diodes largely address relay switch-off transients, additional proactive design choices can further minimize disruptive electrical noise:
Gradual power ramp down — Rather than abruptly cutting all power to the relay coil, gradually ramp down the drive current over ~1ms until it reaches zero. This reduces severity of collapse of magnetic field and voltage spikes.
Delayed cut-off — Extend relay drive signal for ~10ms after switching contacts transition before disabling coil. This allows transient to dissipate before removal of drive current.
Snubber capacitor — Small capacitors can be installed along with diodes to form low pass filter networks. Values from 0.1uF to 1uF often used. Dissipates energy gradually. May allow smaller diodes.
Linear drive — Specialized relay driver ICs can modulate coil power linearly instead of simple on/off. Allows gentle make/break ratcheting of magnetic forces for quieter carry current switching.
Opto-isolated control — Fiber optic, digital isolator and optocoupler devices enable electrical separation of low voltage control circuits from relay coil drive. Prevents coupled transients.
So both appropriate flyback diodes as well snubbing components work together with intelligent drive techniques to suppress electrical noise from relay actuation.
To demonstrate practical application of flyback diodes for control relay noise, below are two sample circuit implementations:
In this basic DC load switching design, an NPN bipolar transistor is used to control a 12V standard electromechanical relay. An ultrafast signal diode suppresses coil switch-off spike:
External fast diode suppresses relay noise
High Current Motor Control
For this 120VAC motor load application, a solid state relay allows microcontroller interface. Fast recovery diode and RC snubber dissipate relay turn-off transient:
Snubber network filters SSR switching noise
So in both low and high power circuits, appropriate choice of flyback diodes combined with other noise reduction methods enable clean relay control.
The effective application of flyback diodes across inductive loads like relay coils provides critical suppression of dangerous voltage spikes and electrical noise resulting from abrupt current interruption.
Careful flyback diode selection considering voltage, current and switching parameters alongside techniques like snubbers and intelligent drive control facilitates stable, reliable and safe system operation.
By preventing false signals, interference issues and premature component failures from relay switching transients, flyback diodes constitute a simple and inexpensive design element that is indispensable for controlling relays in industrial equipment, automation systems and many other electronic products.
Q: Why are standard rectifier diodes unsuitable as flyback diodes for relays?
A: Standard rectifier diodes have relatively long reverse recovery times, allowing damaging initial voltage spikes through before they begin conducting. Ultrafast signal diodes with faster switching are necessary to clamp transient instantly.
Q: Is it always necessary to install flyback diodes externally even if relay modules have internal ones?
A: Usually not required. Most quality electromechanical or solid state relay modules integrate internal suppression diodes that sufficiently protect external circuitry. But for very noise sensitive systems, adding another external diode provides extra reliability.
Q: Can flyback diodes be used in AC relay circuits as well?
A: Yes, flyback diodes work with AC relays also but require a slightly different configuration using back-to-back parallel diodes allowing each diode to conduct in opposite polarity direction for full wave transient suppression.
Q: Instead of using a flyback diodes, can a resistor or zener diode provide the suppression needed?
A: Resistors dissipate noise energy but allow higher remaining voltages that may still cause issues. Zeners clamp voltages but must dissipate all transient energy as heat, risking damage. Flyback diodes provide both voltage clamping and current path restarting, handling inductive kickback safely.
Q: What happens if no flyback diode is used with a relay?
A: Operation may seem normal initially but eventually sensitive components will fail prematurely or exhibit functional issues like resetting, false activations, or output signal corruption/interference due to relay switch-off electrical noise. Critical protection against inductive spikes will be lacking.
