Indication of an Open

Thermal Element:

iTMOV Varistor

The benefits of the TMOV

varistor have

been thoroughly discussed, but one question

remains: How do I know when the thermal

element has cleared? By design, MOVs

exhibit a very high impedance when

subjected to voltages below its MCOV

(Maximum Continuous Operating Voltage).

So, once installed into an end product, how

do you know if the TMOV varistor is still

operational? Enter the iTMOV varistor.

The iTMOV

varistor

adds an additional

third indicator lead that provides access to

the connection between the thermal

element and the MOV electrode. Having

access to this point of the circuit makes indi-

cation of the thermal element a simple

procedure. Figures 8 shows a simple appli-

cation circuit with indication.

In Figure 8, an iTMOV varistor is used to

protect the L-N connection of a typical U.S.

120Vac line. An AC rated LED in placed

across the iTMOV varistor’s indicator lead

and the Neutral line. A series resistor, R1, is

added to limit current through the LED. A

47k

Ω, 0.5W resistor is shown, but the

designer should review the LED’s ratings to

choose the correct value. A series diode,

D1, may be needed if the reverse voltage

rating of the LED is insufficient to handle

negative

voltages

from

the AC

line.

Additionally, a Littelfuse 3AG, 10A (313010)

fuse is shown to protect against excessive

over-current into the load, but the designer

should choose a value specific to his/her

design..

Under normal conditions, the LED is forward

biased from the line voltage through the

thermal element to Neutral. If the thermal

element opens current will be interrupted

and the LED will go off. The LED will also go

off if the Line Fuse opens indicating a loss of

power.

When paralleling TMOV

varistors, the

iTMOV varistor can be used for several or

all parallel devices. That is, one may wish to

indicate when a cer tain percentage or

TMOV varistors fail. Generally, once some

of the MOVs in parallel begin to fail, all begin

to fail.

Conclusion

The UL1449 standard includes a require-

ment that was created to protect the end

product and users from a loss of neutral situ-

ation

where

an

abnormal

over-voltage/limited current condition could

be applied to Metal Oxide Varistors. This

event would cause an MOV to have a

sustained voltage applied in excess of its

maximum working voltage, which in turn

would cause the MOV to enter a thermal

runaway condition.

Several methods exist to prevent the MOV

from reaching combustible temperatures -

the most common of which is to use TCOs.

While TCOs perform adequately in limiting

MOVs from reaching very high tempera-

tures, there are limitations. Out-gassing and

some charring are evident when the test is

applied. Additionally, the assembly process is

difficult to automate, as wave soldering is

typically not an option.

Overall, the new integrated TMOV varistor

thermal fuse technology reduces part count,

saves space and is UL1449 recognized. It

performs better than other methods of

protection when subjected to a limited

current over-voltage condition, by clearing

more quickly at a lower temperature to

reduce the potential for out-gassing or char-

ring. It has all the performance capability of

a standard MOV, including peak pulse

current capability, energy rating and clamping

voltage. The new device can also be wave

soldered which saves on assembly costs and

simplifies the assembly process by eliminating

most of the hand assembly required with

other methods.

Note:

All data was taken with a limited sample size.

Results may vary due to normal variations in

electrical

and

mechanical

parameters.

Designers are encouraged to evaluate their end

design with a large enough sample size to

ensure consistent results. In some instances

TMOV varistors may exhibit substantial heating

and venting prior to opening. Module design

should be such as to contain this possibility.

Application testing is strongly recommended.

References:

1. Transient Voltage Surge Suppressors -

UL1449, June 25, 1998

2. Littelfuse Datasheet, Thermally Protected

Metal Oxide Varistor (TMOV Varistor),

March 2001

3. Littelfuse Datasheet, High Surge Current

Radial

Lead

Metal

Oxide

Varistor

(UltraMOV Varistor Series), March 2001

4. Paul Traynham and Pat Bellew, Using

Thermally Protected MOVS in TVSS or

Power

Supply

Applications,

Power

Systems

World,

Inter tec

Exhibition

Proceedings, September 2001

1 In Table 1., “device” is defined as the end

TVSS product - example: UPS,TVSS Strip

etc.

Littelfuse, Inc.

800 E. Northwest Highway

Des Plaines, IL 60016

www.littelfuse.com

Specifications descriptions and illustrative material in this literature are as accurate as known at time of publication, but are

subject to change without notice. Please visit www.littelfuse.com for the most up-to-date product and application literature.

Copyright © 2001 Littelfuse, Inc., All Rights Reserved

EC635

Fuse

Line

TMOV™

Varistor

TMOV20R130M

Neutral

120VAC

D1

LED Normally On

R1

47k,0.5W

Figure 8. Indicator circuit using the iTMOV varistor (LED

normally on)