Circuit forms industrial-grade digital potentiometer

Both ac and dc motors in modern industrial systems often receive their control from PLCs (programmable-logic controllers) in a control room safely away from the process. If an operator must manually set the motor speed while observing the process, the component of choice is usually an industrial-grade potentiometer. The wiper of this potentiometer produces a signal of 0 to 10V that feeds back to a motor controller in the control room. Such potentiometers are expensive and prone to wear, however. Because of wear, they can open the control loop, allowing the motor to ramp up uncontrollably. With a few components, you can implement a reliable, low-cost, surface-mount digital potentiometer for industrial applications (Figure 1). The result is a direct drop-in replacement for the wear-prone mechanical potentiometer. The digital potentiometer occupies the same space as a mechanical unit within an enclosure. It takes power from the 10V that is formerly supplied to the mechanical potentiometer from the motor controller. The solid-state unit provides a similar output of 0 to 10V and delivers as much as 15 mA to the controller.

The key to the circuit is the low-power, digital-potentiometer 100-kΩ IC₃. Configured as a voltage divider, this IC provides an output of 32 discrete voltage steps between its minimum and maximum settings (0 and 5V). A low-power linear regulator, IC₁, provides a 5V supply rail for IC₂, IC₃, and a resistor ladder internal to IC₃. PB₁ and PB₂ constitute a double-pushbutton industrial switch. Each high-to-low transition that PB₁ produces increments the digital potentiometer's "wiper" by one step. Depressing PB₂ while toggling PB₁ decrements the wiper by one step. IC₂ is a switch debouncer that provides (in addition to the debouncing) a 40-msec fixed delay between its outputs and the switch action. To provide 0 to 10V outputs as required by the motor controller, a single-supply, rail-to-rail op amp, IC₄, amplifies IC₃'s output by a factor of two. The input common-mode range for this op amp—250 mV beyond either supply rail—allows it to generate 0 to 10V outputs like a mechanical potentiometer. The circuit's low quiescent current ranges from 86 µA for a 0V output to 186 µA for a 10V output. To even further lower this quiescent current, you can choose the 200-kΩ version of IC₃. Because the op amp is stable with any capacitive load, it easily drives long, shielded, multiconductor cable lines back to the control room.