and compared with several thermally self

protected

MOVs

(Littelfuse

20mm,

130Vacrms,

TMOV™

varistor

–

TMOV20R130).

Both methods were

subjected to a sustained abnormal over-volt-

age of 240V at 5A. As can be seen in Table

2a and as expected, the TCOs with higher Tf

take longer to clear. The 73ºC TCO proved

difficult to hand solder without clearing the

device despite the use of an appropriate

heat-sink. Table 2b shows the clearing times

for the internally protected MOV.

Clearly,

the times are shorter than for any of the

MOV/TCO combinations tested.

Figure 5. shows the effects of applying a

UL1449

abnormal

over-voltage

test

(240VRMS, 5A) on three devices or combi-

nation of devices - 1) MOV alone (20mm,

130Vacrms – V20E130) 2) MOV/TCO

combination (20mm, 130Vacrms MOV –

V20E130 and TCO with Tf = 94°C), and 3)

TMOV

varistor (20mm, 130Vacrms –

TMOV20R130).

Epoxy surface temperature vs. time was

captured for each method. As can be seen,

the case temperature of a standard MOV

rated for 130VRMS will continue to rise (to

the point of combustion) if no thermal

protection is used. The MOV/TCO combo

performs better reaching temperatures of

220°C before the TCO clears. The internally

protected MOV has a faster response time,

clearing at temperatures of around 150°C in

less than 20 seconds. Note that the temper-

ature continues to rise once the thermal

fuses have cleared. Heat generated within

the zinc oxide disk is at a higher temperature

than the outer epoxy coating. Heat contin-

ues to flow outward to the epoxy for some

time before finally cooling down.

Figures 6a – 6c illustrate the effects of the

temperature rise on each MOV. As can be

seen, the new technology eliminates much of

the charring when compared with a stan-

dard MOV or MOV/TCO combination.

Wave Soldering the

TMOV Varistor

Figure 7 shows a suitable wave solder profile

that can be used for the TMOV varistor. The

profile temperatures are very typical to

those found in general wave solder methods.

In contrast, the solder profile shown for the

TCO shows temperatures much lower than

those found in a typical solder bath. In fact,

the profile shown for the TCO actually

depicts a profile at which the TCO fails

(opens) generally indicating that a TCO

(even one with a high Tf (142°C) cannot be

wave soldered.

Generally, here will be a cost benefit associ-

ated with eliminating the TCO which must

be hand soldered carefully to avoid opening

the element.

Integrity of an Open

Thermal Element

Once the thermal element of a TMOV varis-

tor opens, it is important that the element

stays open and that a reconnection does not

occur. Remember, in order for the thermal

element to have cleared (opened), the varis-

tor disk itself must have heated up due to

thermal run-away and the thermal runaway

began with a failed (shorted) varistor. It is,

therefore, undesirable to have a failed varis-

tor placed back into the circuit, electrically

speaking.

In order to ascertain the integrity of an open

thermal element within a TMOV

varistor,

devices were first subjected to an abnormal

over-voltage limited current event causing

the thermal element to clear. These devices

were then subjected to two tests. First, the

devices were subjected 6kV, 3kA 8x20µsec

pulses. The TMOV

varistors were then

subjected to bias voltage and monitored for

leakage currents indicating a full or partial

reconnection.

None were noted.

Next,

1000Vrms was applied for several hours,

again with no connection as verified by the

leakage test.

Table 2a. MOV/TCO observed clearing times for 5A

limited current test

Table 2b. TMOV varistor observed clearing times for

5A limited current test

0

102030405060708090100

110

0

50

100

150

200

250

300

350

400

450

Time (seconds)

MOV/TCO

TMOV Varistor

MOV

Figure 5. Typical surface temperature vs. Time for several

protection schemes

Figure 6a. Standard MOV

Figure 6b. MOV/TCO combination

Figure 6c. TMOV varistor

Wavesolder trials on TMOV varistor vs. TCO

142 deg TCO

14/20mm TMOV varistor

0

500

1000

1500

2000

2500

Figure 7. Wave solder profile of TMOV varistor vs. TCO

(Tf=142°C)

TCO

Tf (°C)

73

94

121

Mean

30

34

36

Range

11-52

20-46

16-56

Clearing Time (s)

Tf (°C)

TMOV

varistor

Mean

13

Range

2-25

Clearing Time (s)