THE FACTS ON
HIGH-CURRENT MOV CIRCUITRY
This information was developed to dismiss the myth that MOVs (Metal Oxide Varistors) are inherently bad,
and to clarify our implementation of them in our SMP+ surge suppression technology.
MOVs have been (for 50+ years), and continue to be, used by a vast (>95%) majority of companies that provide surge
suppressor equipment - ranging from protection for your stereo to power utility sub-stations.
MOVs are available in a variety of sizes, voltages and surge current ratings from <100 amps to 70,000 amps. Degradation
problems can occur when small under-rated MOVs are employed.
If MOVs are used in a properly designed circuit and within their well-defined design specifications, they DO NOT degrade.
Furman’s SMP+ circuitry uses a high voltage 12,000 amp MOV and a 12,000 amp TVZ (Thermo-Fuse Varistor) in conjunction
with a precisely tuned circuit that utilizes shunt-capacitance, series-inductance, and rectified direct current capacitance surge
suppression.
Statistically, the primary cause of MOV failure is an overvoltage caused by a defective neutral. This is also the most common power
line anomaly.
Furman’s EVS (Extreme Voltage Shutdown) circuit protects the MOV (and your equipment) from damage caused by overvoltage,
and does so at an extraordinarily fast rate. This helps to assure that damage to the primary voltage clamp (our ultra-high current
MOV) is kept to an absolute minimum. In fact, in real-world conditions, our SMP+ protection circuit has had a repair - replacement rate that is well below 0.005%!
Due to the inductance (50uH to 200uH) of the power line, all protection circuits that have MOV, SAD or Zener Diodes wired
across the AC line are technically series protection for short transients. The excess voltage is dropped across the power line
impedance that is in series with the surge protection device.
The Furman SMP+ circuitry consists of the following:
An overvoltage circuit to disconnect power if the input voltage exceeds 137VAC (within less that 1/2 cycle).
A HIGH-VOLTAGE MOV to protect the overvoltage circuit from extreme transient voltages.
A TVZ (Thermo-Fuse Varistor) adds additional clamping to the circuit (but is not essential for protection).
Three capacitors, two inductors and two resistors form the critically tuned and dampened low pass network.
The series inductor is an ultra-high current design with a carefully selected core material.
This stage also attenuates AC noise and RFI WITHOUT being affected by input or output impedance.
The last stage is a robust bridge rectifier with a large capacitor (and resistor) from + to -.
Any transients 1.5 to 2V above the peak line voltage will be shunted to the capacitor.
The low pass filter / bridge / capacitor attenuate the typical lightning voltage surge even without the MOV or TVZ!
Bottom line - Furman’s SMP+ circuitry provides the best protection available.
Tests have been performed at World Products Laboratory with a Keytec #587 generator, measured with an Agilent Infiniium oscilloscope and associated current and voltage probes. Applied signal = 120VAC with a 6kV/3kA B3 pulse applied at the peak of the
sine wave, direct connection to the SMP board.
Complete circuit suppressed voltage = 183V to 188V peak. (Equivalent RMS voltage = 133V)
Robert Cavin, V.P. of Engineering, Furman Sound, Inc
FURMAN SOUND • 1997 S. McDowell Blvd. • Petaluma, CA 94954 USA • PH:707.763.1010 • FAX: 707.763.1310 • www.furmansound.com