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041

Polyswitch Thermal Protection

J.-M. Brassart

RL1

R4

A Polyswitch is an electronic fuse, which interrupts a pro-

tected circuit when the nominal current has been

exceeded, but returns to its original state once it has

cooled down.

In reality, the Polyswitch performs the same function as a

fuse, namely interrupting the current when this has reached

an excessive level. The classical fuse simply gives up the

ghost — it is either ‘ok’ or ‘blown’. There is no other solution

then to replace it, once the electrical problem has been

resolved. The Polyswitch however, may be returned to its

serviceable state by switching off the power supply. It is no

longer necessary to replace the fuse.

As yet, the most conventional areas of application for a

Polyswitch are loudspeakers, batteries, motors, power sup-

plies, chargers and other equipment with a coil.

Investigating the operation of the Polyswitch a little

closer, it can be seen that the resistance (which is of the

order of a few ohms at room temperature) rapidly increases

above about 110 °C.

It may be assumed that everyone knows that the heating

element of an electric kettle or coffee maker will quickly

burn out if it is allowed to boil dry. Putting one and two

together, the author had the idea to use a circuit with a Poly-

switch, as is shown here, for the previously mentioned

household appliances that are not protected or, potentially,

as a replacement of the original protection.

The Polyswitch depicted here is in thermal contact with

CHARGE

R3

R5

230V

R2

1W

TRI1

A2

DI1

R6

G

A1

TRIAC

R1

DIAC

VARISTOR

C1

C2

100n

100V

100n

250V

014063 - 11

the heating element. Together with the triac, which drives

the load, it acts like a kind of thermostat that limits the tem-

perature to about 120 °C. The size of the Polyswitch is not

important, but because of its small dimensions an RXE 010

was selected.

The Polyswitch from Raychem used here has the follow-

ing characteristics:

Holding current:

0.1 A

Trip current:

0.2 A

Ω

Visiting the Internet sites www.raychem.com and

www.bourns.com respectively is highly recommended. You

have a good chance of finding these parts in the catalogues

of the well-known mail order supply houses.

Initial resistance:

4.5

(014063-1)

12/2001

Elektor Electronics

79

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