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SMT50

The electrical characteristics of a SMT50 device are similar to

that of a self-gated Triac, but the SMT50 is a two terminal

device with no gate. The gate function is achieved by an

internal current controlled mechanism.

Like the T.V.S. diodes, the SMT50 has a standoff voltage (Vrm)

which should be equal to or greater than the operating

voltage of the system to be protected. At this voltage (Vrm)

the current consumption of the SMT50 is negligible and will

not affect the protected system.

When a transient occurs, the voltage across the SMT50 will

increase until the breakdown voltage (Vbr) is reached. At this

point the device will operate in a similar way to a T.V.S.

device and is in avalanche mode.

The voltage of the transient will now be limited and will only

increase by a few volts as the device diverts more current. As

this transient current rises, a level of current through the

device is reached (Ibo) which causes the device to switch to a

fully conductive state such that the voltage across the device

is now only a few volts (Vt). The voltage at which the device

switched from the avalanche mode to the fully conductive

state (Vt) is known as the Breakover voltage (Vbo). When the

device is in the Vt state, high currents can be diverted

without damage to the SMT50 due to the low voltage across

the device, since the limiting factor in such devices is

dissipated power (V x I).

Resetting of the device to the non-conducting state is

controlled by the current flowing through the device. When

the current falls below a certain value, known as the Holding

Current (Ih), the device resets automatically.

As with the avalanche T.V.S. device, if the SMT50 is subjected

to a surge current which is beyond its maximum rating, then

the device will fail in short circuit mode, ensuring that the

equipment is ultimately protected.

SELECTING A SMT50

COMPLIES WITH THE

PEAK SURGE

VOLTAGE

CURRENT

ADMISSIBLE IPP

NECESSARY

FOLLOWING STANDARDS

VOLTAGE

WAVEFORM

WAVEFORM

RESISTOR

(V)

(

µS)

(

µS)

(A)

(

(CCITT) ITU-K20

1000

10/700

5/310

25

-

(CCITT) ITU-K17

1500

10/700

5/310

38

-

-

VDE0433

2000

10/700

5/310

50

VDE0878

2000

1.2/50

1/20

50

-

IEC-1000-4-5

10/700

5/310

50

-

level 4

level 3

1.2/500

8/20

100

-

FCC Part 68, lightning surge

1500

10/160

10/160

75

12.5

type A

800

10/560

10/560

55

6.5

FCC Part 68, lightning surge

1000

9/720

5/320

25

-

type B

Bellcore TR-NWT-001089

2500

2/10

2/10

150

11.5

first level

1000

10/1000

10/1000

50

10

Bellcore TR-NWT-001089

5000

2/10

2/10

150

11.5

second level

CNET I31-24

1000

0.5/700

0.8/310

25

-

I

V

1. When selecting a SMT50 device, it is important that

the Vrm of the device is equal to or greater than the

the operating voltage of the system.

2. The minimum Holding Current (Ih) must be greater

than the current the system is capable of delivering

otherwise the device will remain conducting following

a transient condition.

V-I Graph illustrating symbols

and terms for the SMT50 surge

protection device.

ELECTRICAL CHARACTERISTICS