You may have heard the term flyback diode -- also known as, "snubber diode," "suppressor diode," and many other names. Regardless of their name, flyback diodes can be an important tool to prolong the life of your inductive circuits.
Inductor voltage spikes: switch arcing
When you power an inductive load, like a motor or relay, the current across the load will spike, until it drops down to a steady state. This behavior is the reason why such loads initially require a large amount of current, but then level off. Inductors oppose changes in the flow of current, and when this is suddenly cut off by opening a switch, the comfortable supply of steady electron “juice” is instantly removed. The inductor opposes this change in the form of a voltage spike in the opposite direction from its former power source.
The “angry” inductor’s spike in voltage can be so severe, in fact, that it literally pulls electrons out of thin air as an arc to the now very negative inductor terminal. This arcing can damage the switch’s physical contacts, or the transistor used for control, so it needs to be avoided or minimized.
The equation at work is:
Vm = ktω + IRcoil + LdI/dt
Where:
Vm = voltage across the motor (inductor)
ktω = torque constant x the angular velocity of the motor
IRcoil = Current x resistance of the inductor’s coils
LdI/dt = Inductor’s inductance value x change in current/change in time
Neglecting the angular velocity of the motor, if the current is steady, voltage is simply current times the resistance of the coils. Voltage applied to the circuit dictates the steady-state current, but when the switch is opened, the current flow goes to zero, instantly. Therefore, the dI/dt term is infinitely negative, as is the voltage value Vm on the left-hand-side of the equation.
In reality, infinity is a bit of an exaggeration, just as the idea that the circuit is instantly cut off isn’t quite true either. However, a voltage that ramps up (or down, depending on your perspective) to a very high level, isn’t an exaggeration whatsoever.
Flyback circuit design: How does a flyback diode work?
The solution here, is to let the inductor “spin itself down” with a flyback diode. These are normal diodes, which are connected in parallel with an inductor’s (motor’s) two terminals. The diode is oriented in a reverse-biased orientation to the power supply’s voltage, so that current doesn’t flow through it when voltage is applied externally. The motor functions normally, and the diode essentially doesn’t do anything.
When the switch is cut off, however, the flyback diode’s orientation is now forward-biased with respect to the negative-reacting inductor. This forms another circuit, allowing the inductor’s charge to slowly (relatively speaking) dissipate, by looping through the diode and the inductor’s coils, over and over. This avoids the need for electron arcing and the potential damage that comes along with it.
Flyback drawbacks: flyback diode relay delays
Cushioning an inductor’s current change is generally a good thing, however, consider that in the case of a control relay, the goal is normally a near-instant controlled response. As flyback diodes keep the current flowing for longer, this can cause an undesirable delay in the response of the relay itself. An alternative is to place a resistor or reverse-biased Zener diode in series with the diode to hasten its energy dissipation as heat. The drawback to this is that the reverse voltage will be higher than with only the flyback diode.
Flyback diode snubbers: extending circuit life
When properly applied, this type of diode setup is an easy, inexpensive way to prolong the life of your inductive circuits. While many of the terms for this are used interchangeably, Chris McGrady highlights the term “snubber” in his article, as well as some variations on the concept we didn't explicitly discuss here.
