2

Varistor Products

28

www .littelfuse .com

Thermally Protected

1

2

3

3

2

1

Thermal

Fuse

Element

MOV

Thermal

Fuse

Element

MOV

Monitor Lead

i

TMOV Varistor Application Examples

The application examples below show how the indicator lead on the

i

TMOV can be used to indicate that the thermal element has been

opened. This signifies that the circuit is no longer protected from

transients by the MOV.

Application Example 1 (Figure 1)

In this case, the LED is normally on, and is off when the thermal

element opens.

Application Example 2 (Figure 2)

This circuit utilizes an optocoupler to provide galvanic isolations

between the iTMOV varistor and the indicating or alarm circuitry.

Application Example 3 (Figure 3)

This circuit illustrates the use of the monitoring lead of the iTMOV

varistor to ensure that equipment is only operated when overvoltage

protection present. In normal operation the load switch relay solenoid is

powered via the indicator lead of the iTMOV varistor. In the event of the

thermal element being activated, the relay will de-activate, cutting power

to the protected circuit and the fault LED will illuminate.

Please note: Indicator circuits are provided as a guideline only.

Verification of actual indicator circuitry is the responsibility of the end

user. Component values selected must be appropriate for the specific

AC line voltage service and application.

R

LED

Line Fuse

i

TMOV Varistor

Neutral

Line

Figure 1. Application example 1

R

Line Fuse

i

TMOV Varistor

Neutral

Figure 2. Application example 2

AC OPTOCOUPLER

TO STATUS

ANNUNCIATOR

LIGHT/ALARM

Line Fuse

i

TMOV Varistor

Neutral

Figure 3. Application example 3

Fault LED

(Normallyoff)

"Load Powered"

neon lamp

Load Switch Relay

(loss of protection)

R

Line

Line

Lead Configurations

TMOV Varistor

i

TMOV Varistor

Note: MOVs are non-polarized passive elements