The invention relates to trimmable resistors, sometimes simply called "trimmers." Trimming is the technique of setting the resistance of a variable resistor at a fixed value, usually to be maintained over the operating life of the resistor. Trimmer resistance can be changed in successive trimming operations within a narrowed range between the resistance determined by the previous trimming operation and either the upper or lower limit of resistance, depending on whether trimming increases or decreases resistance.
Trimmers are useful in adjusting the parameters of a general purpose circuit to fit a specific application. One of these parameters might be a reference or bias voltage obtained by dividing a supply voltage across two resistive elements and selecting the output across one of the elements. If the output is taken across a trimmable resistor, the reference or bias voltage can be adjusted by trimming the resistor. Usually, trimming increases resistance; however, resistors which are trimmed to decrease resistance from a nominal value are also available.
Trimmers are well known. Perhaps the most common type includes a resistive layer deposited on a base or substrate, and electrically connected between two electrical termination points to form a conductive path. A scriber or other tool is applied to remove a portion of the resistive layer, thereby forming a nonconductive path which changes the shape of the resistive layer between the termination points. This type of trimmer is disclosed in the Seay et al. U.S. Pat. No. 3,594,679, the Brown U.S. Pat. No. 2,759,078, and the Zandman et al. U.S. Pat. No. 3,517,436.
It is well known by those skilled in the art to form the resistive layer of a trimmer in different shapes. In U.S. Pat. No. 1,962,438 to J. A. Flanzer et al. a method for removing a resistive layer from a disc-shaped resistor is shown and in U.S. Pat. No. 2,953,764 a resistive path is formed in a grid having alternate conductive paths which can be severed to adjust resistance.
The prior art trimmers have two common features. First, in these devices the resistive layer must be removed directly from the substrate. If the substrate is alumina and the resistive layer is cermet, the removal involves penetrating one or both of these two relatively hard materials. Second, the removal is accomplished by scribing a continuous nonconductive path in the resistive layer. This type of trimming adjustment cannot easily be made in predetermined regular increments.
Trimming the prior art devices generally requires relatively sophisticated scribing tools and techniques, which many times can only be applied at the factory. An exception is the device disclosed in a U.S. Pat. No. 4,100,525, which is assigned to the assignee of the present application. There a trimmable resistor is disclosed with a relatively simple scribing tool that can be applied in situ, if desired.