1. Field of the Invention.
The field of the invention is digital switching circuits and in particular switching circuits for accepting DC signals for input to industrial controls and the like.
2. Background Art
Industrial controllers such as those described in U.S. Pat. Nos. 3,810,118, 3,942,158, 4,165,534 and 4,442,504 are typically connected to industrial equipment such as assembly lines or machine tools to operate processes or equipment in accordance with a stored program. In such industrial controllers the stored program includes instructions which, when executed, examine the condition of selected inputs to the controller from sensing devices on the controlled equipment and energize or de-energize selected outputs from the controller to operate devices on the controlled equipment.
The inputs to the controller may be discrete signals such as those from switches which may detect limits of process variables such as motion, temperature, time, or other quantities, or the inputs may be analog measures of the process variables themselves, which are generally then converted to digital binary form for processing.
The binary signals input to the industrial controller are typically generated by a switching device having two terminals wired in series with an external voltage source (typically, up to 32 volts) so that when the switching device is "closed", a first voltage in a high range is presented to the digital input and when the switching device is "opened" a second voltage in a low range (often equal to zero volts) is presented to the digital input.
The switching device may be a mechanical actuated pair of contacts or may be an electrically actuated relay or solid-state switch associated with active sensing circuitry. In this latter case, it is desirable that the active circuitry be powered from the same voltage source and through the same two terminals as is the relay or solid-state switch.
In order for the active circuitry of a switching device to receive power in either the opened or closed state, a small amount of current must flow through the switching device into the input of the industrial controller when the inputs are presented with either a high or low voltage from the switching device. This current is termed "leakage current" and is typically on the order of two milliamperes at a voltage in the low range of 5 volts under prevailing industrial standards.
One method of ensuring adequate leakage current into what may be otherwise a high impedance input to the controller is to shunt the input of the controller to ground with a resistor. For example, a 2.5 k.OMEGA. resistor shunting a high impedance input will ensure a 2 milliampere current flow at 5 volts with a power dissipation of 10 milliwatts.
Unfortunately, the current flowing through this shunting resistor will increase with the voltage at the controller's input, and for a voltage of 32 volts, within the high range, the power dissipated in this resistor will have increased nearly fifty times.
Often multiple input circuits will be located in a single I/O module of the industrial controller and many such I/O modules may be housed together in a rack. The heat dissipated cumulatively by many such modules employing a resistor to produce the leakage current would produce unacceptable cooling requirements.