Yes! Relay kickback voltage is real, and it can be high enough to damage a transistor or MOSFET if there is no protection added to the circuit.
The reason behind this is simple – a relay coil is inductive in nature. Inductor naturally resists to any sudden change in current by producing a counter emf or the so-called kick-back voltage.
What Happens When the Relay Is Turned On?
Consider a relay is driven by an NMOS transistor in a typical low-side switching circuit.
When the NMOS turns ON, current flows from the supply, through the relay coil, and into the MOSFET and to the circuit ground.
If the MOSFET is saturated, its drain-to-source voltage is negligible, the voltage across the relay coil is approximately equal to the supply voltage (Vdd2 in the sample circuit).
Supposing, Vdd2 is 12V, then the coil will have approximately 12 V across it while the MOSFET is on. At this point, energy is being stored in the magnetic field of the relay coil. The energy stored in an inductor is characterized by this equation.
E = ½LI²
where:
- E = energy stored in the inductor
- L = inductance
- I = current through the coil
What Happens When the NMOS Turns Off?
The kickback voltage is not happening when the relay is turned on. It is going to occur when the NMOS suddenly turns OFF. Thus, the analysis is focused during this time.
The current through the relay coil cannot instantly fall to zero as the inductor try to resist it. An inductor will generate whatever voltage is necessary to try to keep its current flowing. This voltage is what called a kick-back voltage.
The behavior of the voltage is described by the inductor equation:
VL= L × di/dt
where:
- VL = voltage generated across the inductor
- L = inductance
- di/dt = rate of change of current
The important point is that the faster the current is forced to change, the larger the induced voltage can become.
When the NMOS turns off, the relay coil reverses the polarity of its voltage in order to maintain the current direction through an available current path.
If there is no protection circuit, the drain voltage of the NMOS can rise to a very high value.
For a simple circuit where the coil’s generated voltage appears in series with the supply:
VDrain ≈ Vsupply + VL
or:
VDrain NMOS ≈ Vdd2 + L × di/dt
This means that a 12 V relay circuit does not necessarily produce only 12 V at the MOSFET drain when the relay is switched off. The drain voltage can rise to tens, hundreds, or even higher voltages depending on the coil, switching speed, parasitic elements, and the available current path.
Why Does the Kickback Voltage Become So High?
Suppose a relay coil is carrying 1A when the MOSFET turns off.
If the circuit provides no path for the coil current, the current attempts to fall extremely quickly. Since:
VL= L × di/dt
a very large di/dt can produce a very large voltage.
The coil essentially says:
“The current was flowing, and I want to keep it flowing.”
It therefore generates a voltage that can rise until the current finds another path.
In a real circuit, this path may be provided by parasitic capacitance, transistor breakdown, an arc, or some other unintended mechanism. This is why an unprotected relay coil can produce a surprisingly high voltage spike.
Can Relay Kickback Damage the NMOS?
The answer in general is yes. But if a MOSFET voltage rating is much higher than the kickback voltage level, it will not. Though calculating the exact level of the kickback voltage is not an easy task.
If the MOSFET has a maximum drain-to-source voltage rating of, for example, 60 V and the relay produces a voltage spike greater than 60 V, the MOSFET can enter avalanche breakdown.
A single event may or may not immediately destroy the MOSFET, but repeated avalanche events can cause excessive stress and eventually lead to MOSFET failure.
Therefore, the MOSFET must be protected from the relay’s inductive kickback.
How to Calculate the Kickback Voltage Level?
The peak drain voltage on the MOSFET when it is turned off is given by the equation below.
VDrain NMOS ≈ Vdd2 + L × di/dt
Vdd2, L and di are given. The trickiest part is the dt. dt is can be derived through below illustration.
By the time the current is interrupted, the voltage will start to rise and it will peak once the current reach zero. The current waveform is forming a sinusoidal shape. As illustrated, dt is ¼ of the period.
The period of the waveform is the inverse of the frequency. The frequency is not readily provided. It can be determined by using below equation.
Frequency = 1 / [2 x pi x sqrt (L x C)]
Where L and C are the relay coil inductance and MOSFET drain capacitance respectively. The coil inductance is given in the datasheet of the relay and the MOSFET drain capacitance is also provided in the MOSFET datasheet.
Supposing these are the circuit givens:
Vdd2 = 12V, current = 1A, L = 10uF, MOSFET drain capacitance = 232pF
Solving for the frequency:
Frequency = 1 / [2 x pi x squirt (L x C)] = 1 / [2 x pi x squirt (10uF x 232pF)] = 3.304 MHz
Solving for the dt:
dt = ¼ x Period = ¼ x 1/Frequency = ¼ x 1 / (3.304 MHz) = 75.67nsec
Solving the peak drain voltage:
Peak drain voltage = Vdd2 + L x di / dt = 12V + 10uH x 1A / 75.67nsec = 144.15V
If the MOSFET voltage is lower than this, consider adding protection. There are some protection topologies in the next topic.
If the MOSFET voltage rating is much higher than 144.15V, no need to provide protection.
How Is Relay Kickback Controlled?
The most common protection method for a DC relay coil is a flyback diode connected across the coil.
When the MOSFET turns off, the diode becomes forward biased and provides a safe path for the coil current. This greatly reduces the voltage appearing across the MOSFET.
Other protection methods can also be used depending on the application, including:
- Flyback diode
- Zener or TVS clamp
- RC snubber
- RCD clamp
- Other dedicated transient-suppression circuits
The choice of protection depends on the required switching speed, relay characteristics, MOSFET rating, and application requirements.
The Important Lesson
It is not a theoretical phenomenon or a measurement error. It is a direct consequence of the fundamental behavior of an inductor.
Whenever the current through a relay coil is interrupted, the coil generates a voltage that opposes the sudden change in current. If the circuit does not provide a controlled path for the stored magnetic energy, the resulting voltage can become high enough to damage the switching transistor.
That is why a relay driven by a MOSFET should normally include an appropriate kickback or flyback protection circuit.
The relay coil stores energy when energized—and when the current is interrupted, that stored energy has to go somewhere.