DE1170042B - Electrical switchgear working with contacts - Google Patents

Description

Working with contacts electrical switching device addition to the application: S 72450 VIII d / 21c -Auslegeschrift 1154555 The invention relates to the main Patent Application S 72450 VIII d / 21 c (Auslegeschrift 1,154,555) to a working electrical contacts electrical switching means, for . B. Snap-action switch for pulsed switching of the full heating current of an electrical heating device, in which the contacts to avoid the risk of welding and radio interference are bridged by a so-called PTC thermistor, the resistance value of which increases with increasing temperature in a certain temperature range almost abruptly, preferably by several powers of ten.

According to the invention, there is a further improvement of this switching device achieved in that in series with the parallel connection of contact and PTC thermistor at least an NTC thermistor is connected.

The invention is described below with reference to the FIG. 1 shown Embodiment explained.

The switching device consisting of the two contacts 1 and 2 with the terminals 3 and 4 is via an intermediate piece 5 of the in thermal contact with a heating device standing bimetallic strip 6 actuated pulse-wise. The switching device shown in a manner known per se as a so-called snap switch be trained. Parallel to terminals 3 and 4, to which the circuit to be switched is connected (e.g. the heating circuit of the heating device with the heating resistor 10 and on terminals 11 and 12 for connecting the mains voltage), the PTC thermistor is located 7, the Curie point of which is selected so that when contacts 1 and 2 are open, it is through the current flowing through it reached or exceeded after a relatively short time will.

As already stated in the main patent application, the Resistance value of the PTC thermistor 7 after reaching or exceeding the Curie point so strong that the current without arcing at contacts 1 and 2 drops to a very small value reduced, d. H. is almost switched off.

The advantageous effect of the thermistor 13 additionally provided according to the invention becomes apparent when the contacts 1 and 2 are closed: When the contacts 1 and 2 touch, the current flowing through the contacts 1 and 2 is initially limited by the cold resistance of the thermistor 13. However, this is heated by the current flowing through it, so that after a certain time its resistance drops considerably and the current flowing through the switch reaches the nominal value. These relationships are shown in FIG. 2, the current flowing through the circuit being plotted as a function of time. When the contacts 1, 2 (FIG. 1) are open, the current limited by the hot resistance of the PTC thermistor 7 and the thermistor 13 has the relatively small residual value Ix. If the contacts are now closed at time t 1, the current is only limited by the cold resistance of the thermistor 13. So it rises to the value 1F. As the thermistor 13 heats up, the current rises to its normal value IN, which it reaches at time t. achieved. After opening the contacts at time t. the current initially drops to a slightly reduced value 1A; which is determined by the cold resistance of the PTC thermistor 7 and the hot resistance of the thermistor 13. With increasing heating of the PTC thermistor 7, the current goes back to the original value IR.

Ner advantage of the series connection of a thermistor according to the invention can therefore be seen in the fact that the current at the time the contacts close initially has a relatively small value and only increases gradually, what happens in terms of reducing the risk of welding, especially when closing the contacts have a beneficial effect.

With such switching devices it can often be sufficient a thermistor alone (i.e. without being connected in parallel to the switch contacts PTC thermistors in series with the switching device. Such a thermistor can by itself largely reduce the risk of contact welding.

Claims (1)

Claim: Electrical switching device working with contacts, e.g. B. Snap switch for pulsed switching of the full heating current of an electrical heating device, in which the contacts according to main patent application S 72450 VIII d! 21 c (Auslegeschrift 1 154 555) are bridged by a so-called PTC thermistor to avoid the danger of welding and radio interference, the resistance value of which rises almost abruptly with increasing temperature in a certain temperature range, preferably by several powers of ten, characterized in that in series with the parallel connection of contact and PTC thermistor is connected to at least one thermistor.