GB2028594A - Miniature potentiometer - Google Patents

Abstract

A miniature potentiometer having a third terminal assembly 13 with a hollow spigot 15a turned over onto a conductive track 7, the rotor 19 having snap-in legs 27, 28 which snap fit into the spigot to locate the rotor. <IMAGE>

Description

SPECIFICATION Miniature potentiometer This invention relates to variable electrical resistance devices, particularly to devices known as potentiometers or rheostats.

Variable resistance devices comprise a resistance element and a contact which can be moved across the surface of the element in order to vary the voltage drop occurring across part of the element. A potentiometer comprises the resistance element and a pair of terminals for making external connection to opposite ends of the element, with a movable contact which can be moved over the surface of the element to vary the voltage drop between the terminal connections of the element and the movable contact.

Trimmer potentiometers act as voltage adjusting devices. It is not often they require adjustment and in many cases the movable contact (wiper contact) can be actuated by a screwdriver.

The demand for smaller and smaller electrical components has required that variable resistance devices also have smaller dimensions. UK patent specification 1,399,542 discloses an advance in miniaturising Trimmer potentiometers and discloses a base member carrying a resistive track and a collector track bridged by a wiping contact member, in which the collector track is secured to the base member and is electrically connected to a collector or centre termination. The termination can be a terminal for connection to conventional wiring or a pin for connection to printed circuitry.

This specification shows the formation of the collector track and the associated termination each as part of a metallic one-piece unit having a spigot passing through an aperture in the base member and turned over to form the collector track and secure the one-piece member to the base member.

According to the present invention there is provided a variable resistance device comprising an insulating substrate having a hole and carrying on the first side an electrically conductive track around the hole and an arcuate electrically resistive track radially outwardly of the conductive track, two electrical connection terminals, one connected to each end of the arcuate track, and a third terminal comprising a plate seated adjacent the other side of the substrate with a hollow spigot projecting through the hole in the substrate and connected to the conductive track on the first side, and a rotor comprising an insulating member carrying a wiper which radially electrically bridges across the two tracks, the rotor having an axial resilient extension which is snap fitted in the spigot, the spigot acting as a bush for the rotor extension.

In order that the invention can be more clearly understood reference will now be made to the accompanying drawings in which Figure 1 is a perspective view of a miniature trimmer potentiometer according to an embodiment of the invention, Figure 2 is an exploded view of the embodiment of Figure 1 and Figure 3 is a section of Figure 1 in the directions of the arrows X-X and Figures 4A and 48 are a longitudinal section through and a plan view of, respectively a collector terminal in an alternative embodiment.

Referring to the drawings the trimmer potentiometer comprises an insulating substrate 1 made of 96% alumina and formed with a large hole 2 and three smaller holes 3, 4 and 5.

Preferably the substrate is formed from a larger mother board commonly called a snapstrate which is scribed and from which the substrate 1 is snapped off along the scribed lines.

An arcuate resistive track 6 made of thick film resistive ink is screen printed onto the substrate, the screen preferably being made of stainless steel. Likewise a circular conductive track 7 along with conductive terminations 8 and 9 making connection with the ends of the arcuate resistive track 6, are screen printed onto the substrate.

Then the deposited inks are fired according to known techniques in the thick film art, the conductive track being printed and fired first and then the resistive track is printed and fired.

Two terminals pins 10 and 11 made of tin plated brass are inserted into respective holes 3 and 5 and hollow end spigot portions 1 Oa and 1 a are thermally swaged to make good electrical contact with the conductive tracks 9 and 8 respectively.

A collector terminal 12 forms part of a one piece metallic collector terminal indicated generally by the reference 13 and comprises a circular flange 14 having an upstanding spigot 1 5 and oppositely extending portions 1 6 and 17, the portion 1 7 having an up-turned tail 1 8 for locating in the hole 4 in the substrate. The upturned tail 18 locates in the hole 4 to prevent the collector terminal turning. It also acts as a mechanical.stop for the rotor 1 9 where there is a corresponding moulded stop 30, thus limiting the anguish rotation to 2400. The portion 1 6 has a downturned terminal pin 12, already mentioned.The spigot 1 5 of this one-piece assembly is then introduced into the large hole 2 in the substrate with the up-turned tail 18 fitting into the hole 4 to angularly located the assembly 1 3.

The rim 1 5a of the spigot 15 is then spun over to make good electrical contact with the circular conductive track 7 and also mechanically locks the assembly 13 to the substrate, the flange 1 4 being held against the underside of the substrate as viewed in Figure 2.

A rotor indicated generally by the reference 1 9 is formed rather like a wheel having a knurled outer rim 20 and a central screwdriver slot 1 9a. It has an annular rim portion 21 which, as shown in figure 3, acts as a spacer against the upper surface of the substrate 1 to maintain a circular hollow interior 22 when the rotor is assembled onto the substrate. The rotor carries a multi-fingered wiper or brush 23 formed from individual wires welded to a bar 24 and made of highly conductive metal, such as a nickel silver alloy. Bar 24 fits into a slot in the underside of the rotor and is "heat staked" in position. This involves locally melting the material of the rotor along side the slot in which the brush is inserted and this is effective to hold the wiper in position in the slot.

The preferred material of the rotor is glass filled nylon which is particularly robust and resilient The rotor has an axle-like part indicated generally by the reference 25 and which comprises a neck portion 26 and two springy legs 27 and 28 having a slot 29 between them. Each springy leg 27 and 28 has an associated annular rim portion 27a and 28a respectively.

In assembling the rotor onto the substrate the part 25 is merely forced through the spigot 1 5 until the shoulders 27a and 28a snap back against the lower surface of the annular flange 14 of the assembly 13.

We have found that this method of turning the rim 1 5a of the spigot 1 5 onto the conductive track 7 in combination with the springy legs of the rotor, enables a simple and effective manufacturing technique to be accomplished. In particular the dimensional tolerances on the assembly 13, particularly the spigot 1 5 and the rim 15a need not be very close and the spun over rim 1 5a produces a smooth entrance rim for the springy legs of the rotor 19 when it is snapped into position.

The annular rim portion 21 of the rotor maintains the correct spacing from the conductive and resistive tracks for the wiper 23 so that this is resiliently biased by the correct amount against the tracks. This rim 21 also provides an effective dust cover against ingress of dirt or other foreign matter onto the resistive and conductive tracks and the wiDer.

An annular shoulder 26a is moulded on the rotor and prevents undue distortion of the rotor when pressure is applied with a screwdriver in slot 19a. Shoulder 26a will abut the turned-over rim 1 5a when pressure is applied, thus protecting the wiper and the relatively thin rim 21 from undue forces.

Where the potentiometer is mounted on a printed circuit board with a hole aligned with the slot 29 it would also be possible to adjust the potentiometer by means of a screwdriver in the slot 29.

Although not shown in detail, it would be possible for the potentiometer to be modified so that the substrate is mounted at right angles on a printed circuit board. In this case the pins such as 10 and 11 would be bent at right angles so that the major portion of their length extended parallel to the substrate and the collector terminal 1 3 would, in place of the particular embodiment already shown, have a corresponding terminal 12' not bent through 90 as shown but instead extending continuously parallel with flange 14'.

This is illustrated in Fig. 4 in which only the collector terminal is shown. In this embodiment however the staight terminal 12' extends parallel to and in the same direction as the now bent terminals corresponding to terminals 10 and 11, so that all three terminals project from the front edge of the substrate, as viewed in Figure 2. The locating ear 18' is still present and has been cut out of the flange 14' leaving a hole 18".

In other respects this embodiment corresponds to the previous embodiment and corresponding parts have been given the same reference numeral but suitably indexed.

The collector terminal is shown in its virgin state, that is prior to assembly on the substrate when of course the rim 15a' will be thermally swaged to connect with the conductive track and hold the terminal on the substrate.

Claims (5)

1. A variable resistance device comprising an insulating substrate having a hole and carrying on the first side an electrically conductive track around the hole and an arcuate electrically resistive track radially outwardly of the conductive track, two electrical connection terminals, one connected to each end of the arcuate track, and a third terminal comprising a plate seated adjacent the other side of the substrate with a hollow spigot projecting through the hole in the substrate and connected to the conductive track on the first side, and a rotor comprising an insulating member carrying a wiper which radially electrically bridges across the two tracks, the rotor having an axial resilient extension which is snap fitted in the spigot, the spigot acting as a bush for the rotor extension.

2. A device as claimed in claim 1, wherein the wiper is located within a hollow annular recess in the underside of the rotor, an outer rim around the recess serving as a spacer between the rotor and the substrate and preventing or minimising the ingress of foreign matter onto the resistive and conductive tracks.

3. A device as claimed in claim 1 or claim 2, wherein the axial resilient extension comprises a plurality of springy legs each having a outwardly projecting shoulder which snaps behind the underside of an annular flange of the third terminal assembly.

4. A device as claimed in claim 3, wherein the rotor is made of glass filled plastics material.

5. A miniature trimmer potentiometer substantially as hereinbefore described with reference to and as illustrated in Figs. t to 3 or Figs. 1 to 4, of the accompanying drawings.