Wire-wound resistor

Abstract

Wire-wound resistor covered with a layer of enamel on the basis of a zinc borosilicate having a composition:______________________________________SiO.sub.2 6-18 CaO 5-12B.sub.2 O.sub.3 35-60 Al.sub.2 O.sub.3 5-15Na.sub.2 O 2-12 ZrO.sub.2 5-10ZnO 10-25 CoO 0,1-2______________________________________wherein Al.sub.2 O.sub.3 +ZrO.sub.2 exceeds twice the Na.sub.2 O-content and some maxima for permissible contents of other oxides, including coloring oxides, are indicated.This enamel provides a proper solution of the problem of composing an enamel being adequately alkali-proof and being located in the proper range of coefficients of expansion for Ni-Cr and Ni-Cu-wire.

Description

The invention relates to a wire-wound resistor whose turns are covered with an enamel, and to the relevant enamel.

As a rule, a resistor of such a type is formed from a ceramic tube or rod, for example made of steatite or forsterite, around which the resistive wire made of an alloy on the basis of Cr-Ni or Cu-Ni is wound.

To protect the resistor from atmospheric influences and mechanical damage, the wire turns of the resistor are covered with a vitreous or vitreous crystalline enamel. The originally employed lead oxide-containing enamels which had the advantage of having a low softening point are at present no longer used because during application metallic lead is easily separated out.

U.S. Pat. No. 3,919,682 discloses a vitreous-crystalline enamel, which is obtained by controlled crystallisation of a material which is provided on the resistor body in a vitreous state, so that uniformly distributed micro-crystals are produced. Preferably, the enamel has the following composition in percent by weight

______________________________________SiO.sub.2 10-28 Al.sub.2 O.sub.3 0.01-14B.sub.2 O.sub.3 26-50 CoO 0-5ZnO 28-40 Cr.sub.2 O.sub.3 0-5Na.sub.2 O 0,5-10 TiO.sub.2 0-10CaO 0.01-4______________________________________

However, this enamel has some disadvantages. Firstly, the enamel is not alkali-proof. This is important because, after having been assembled, the resistor is immersed in caustic soda in order to dissolve an oxide skin on the copper leads. This is necessary for a proper solderability.

In addition, the range of coefficients of expansion (around 50.times.10.sup.-7 per .degree.C.) is not adequately adapted to the coefficients of expansion of the customary material of the resistive wire, NiCr or NiCu, namely around 70.times.10.sup.-7 per .degree.C. This may cause small cracks in the enamel.

The invention provides a range of compositions of the enamel for a wire-wound resistor which is located in the proper range of the coefficients of expansion (60-80.times.10.sup.-7 per .degree.C.) which is adequately resistant against the action of caustic soda, which is not hygroscopic and can be deposited in a simple way from a suspension.

According to the invention, a wire-wound resistor having a layer of enamel covering the turns, the enamel being on the basis of a zinc silicoborate, is characterized in that the enamel has a composition in percent by weight within the following range of compositions:

______________________________________SiO.sub.2 6-18 CaO 5-12B.sub.2 O.sub.3 35-60 Al.sub.2 O.sub.3 5-15Na.sub.2 O 2-12 ZrO.sub.2 5-10ZnO 10-25 CoO 0,1-2______________________________________ and with the additional condition, that Al.sub.2 O.sub.3 +ZrO.sub.2 >2 Na.sub.2 O, while ______________________________________BaO < 10 Fe.sub.2 O.sub.3 < 2MgO < 3 Cr.sub.2 O.sub.3 < 2TiO.sub.2 < 5 NiO < 2MnO < 4______________________________________

A number of these last-mentioned oxides colour the enamel, either alone or in certain combinations.

These additions are useful for decorative or for coding purposes. This class also comprises CoO; it appears, however, that this oxide also furnishes an important contribution to the adherence to the Ni-Cr or Ni-Cu. Hence CoO must be present in a quantiy of at least 0.1% by weight.

A preferred composition is within the following limits in % by weight:

______________________________________SiO.sub.2 8-12 CaO 5-10B.sub.2 O.sub.3 35-45 Al.sub.2 O.sub.3 10-15Na.sub.2 O 3-6 ZrO.sub.2 5-10ZnO 15-20 CoO 0,1-2______________________________________ with the additional condition, that ______________________________________BaO < 10 Fe.sub.2 O.sub.3 < 2MgO < 3 Cr.sub.2 O.sub.3 < 2 andTiO.sub.2 < 5 NiO < 2MnO < 4______________________________________

The enamels within the range of composition in accordance with the invention remain vitreous during the usual deposition techniques. A finely dispersed crystalline phase can be formed in these enamels, but this takes place during heating for a longer period of time at an elevated temperature (>1000.degree. C.).

There now follow some examples of enamel compositions which are suitable for use in resistors according to the invention.

______________________________________ 1 (wt %) 2 (wt. %)______________________________________SiO.sub.2 10,2 10,0B.sub.2 O.sub.3 37,7 40,0Na.sub.2 O 4,0 5,0CaO 7,6 7,0Al.sub.2 O.sub.3 13,8 12,5CoO 0,7 0,8MgO 1,5 1,5ZnO 17,0 15,7ZrO.sub.2 7,5 6,5Fe.sub.2 O.sub.3 -- 1,0Coefficient of expansion 70,1 .times. 10.sup.-7 70,3 .times. 10.sup.-7______________________________________

After melting, the said enamels are fritted and thereafter ground to a grain size of less than 60 .mu.m. From this a suspension is prepared having the following composition:

100 g ground enamel 3 g Fe.sub.2 O.sub.3 0.5 g Cr.sub.2 O.sub.3 15 g of a 25% solution of polystyrene in a mixture of hydrocarbons having a boiling range from 100.degree.-200.degree. C. 50 g of a mixture of hydrocarbons (boiling range 100.degree.-200.degree. C.). 2 g butanol.

The suspension is applied by means of immersion if wire-wound resistors of, for example 100 0hm, formed from steatite tubes having a diameter of 4 mm with 150 .mu.m thick wound Ni-Cr wire. The enamel coated resistors are baked in a conveyor oven at a temperature between 680.degree.-780.degree. C. for approximately 10 minutes and thereafter for approximately 10 minutes between 520.degree. and 590.degree. C., the resistors are then allowed to cool down to 100.degree. C. in the oven for a period of 20 minutes and thereafter outside the oven to ambient temperature.

Claims

1. A wire-wound resistor whose turns are covered with a layer of enamel on the basis of a zinc silicoborate, characterized in that the enamel has a composition in percent by weight within the following range of compositions:

______________________________________SiO.sub.2 6-18 CaO 5-12B.sub.2 O.sub.3 35-60 Al.sub.2 O.sub.3 5-15Na.sub.2 O 2-12 ZrO.sub.2 5-10ZnO 10-25 CoO 0,1-2______________________________________

with the additional condition that Al.sub.2 O.sub.3 +ZrO.sub.2 >2 Na.sub.2 O, while,

______________________________________BaO < 10 Fe.sub.2 O.sub.3 < 2MgO < 3 Cr.sub.2 O.sub.3 < 2TiO.sub.2 < 5 NiO < 2MnO < 4______________________________________

2. A wire-wound resistor as claimed in claim 1, characterized in that the enamel has a composition in percent by weight within the following limits:

______________________________________SiO.sub.2 8-12 CaO 5-10B.sub.2 O.sub.3 35-45 Al.sub.2 O.sub.3 10-15Na.sub.2 O 3-6 ZrO.sub.2 5-10ZnO 15-20 CoO 0,1-2______________________________________

with the additional condition, that

______________________________________BaO < 10 Fe.sub.2 O.sub.3 < 2MgO < 3 Cr.sub.2 O.sub.3 < 2TiO.sub.2 < 5 NiO < 2MnO < 4______________________________________