Power supplies are the backbone of modern electronics, converting raw AC mains into stable DC voltages. However, they face constant threats from voltage surges, transient spikes, and overload conditions. To ensure reliability and safety, designers employ protective components like Metal Oxide Varistors (MOVs) and fusible resistors. Let’s explore their roles in detail.
Metal Oxide Varistor (MOV)
- Working
Principle: MOVs are voltage-dependent resistors. Under normal
operation, they remain in a high-impedance state, allowing current
to pass unaffected. When a surge or transient spike exceeds their
threshold (varistor voltage), they switch to a low-impedance state,
clamping the voltage and diverting excess energy away from sensitive
circuits.
- Placement
in Power Supply: Typically connected across the AC input line, right
after the mains entry. This ensures the MOV reacts instantly to surges
before they propagate deeper into the circuit.
- Key
Functions:
- Protects
against lightning strikes, switching surges, and utility
disturbances.
- Extends
the lifespan of downstream components like rectifiers, capacitors, and
ICs.
- Works
in tandem with fuses to absorb transient energy while the fuse
disconnects during sustained overvoltage.
- Design
Considerations:
- MOV
selection depends on clamping voltage, energy rating, and response
time.
- Oversized
MOVs may survive longer but can fail catastrophically if not paired with
proper fusing.
Fusible Resistor
- Definition:
A fusible resistor is a resistor with built-in fuse functionality.
It limits inrush current during normal operation and sacrifices itself
under fault conditions by opening the circuit.
- Role
in Power Supply:
- Acts
as a current limiter during startup, preventing stress on
rectifiers and capacitors.
- Provides
overcurrent protection by fusing open when excessive current
flows, thereby isolating the fault.
- Unlike
standard fuses, fusible resistors also contribute to circuit impedance,
improving stability.
- Placement:
Commonly found in the primary side of SMPS (Switched Mode Power
Supplies), often in series with the AC line before rectification.
- Advantages:
- Dual
functionality (resistor + fuse).
- Compact
and cost-effective.
- Prevents
fire hazards by ensuring controlled failure.
MOV + Fusible Resistor: A Protective Duo
When combined, MOVs and fusible resistors form a layered
defense system:
- MOV
clamps high-voltage surges instantly.
- Fusible
resistor limits current and disconnects the circuit during sustained
faults.
- Together,
they protect against both short-duration transients and long-duration
overloads, ensuring the power supply fails safely rather than
dangerously.
Practical Example
In a typical SMPS input stage:
- AC
mains → MOV (parallel) for surge suppression.
- Series
fusible resistor for current limiting and fault isolation.
- Rectifier
+ filter capacitors receive clean, safe power.
This arrangement ensures that even if lightning strikes or a
capacitor shorts, the MOV absorbs the surge and the fusible resistor
disconnects the faulty path, preventing fire or further damage.
Conclusion
The role of MOV and fusible resistor in power supplies
is indispensable. MOVs act as the first line of defense against voltage
surges, while fusible resistors provide controlled circuit isolation
during faults. Their synergy ensures that power supplies remain reliable,
safe, and long-lasting — protecting both the equipment and the user.