Industrial 0V to +10V Digital Potentiometer
With a few components, a reliable, low-cost, all-surface-mount, solid-state digital potentiometer can be implemented for industrial applications (Figure 1). The result is a direct drop-in replacement for the wear-prone mechanical potentiometer, and it occupies the same space within an electrical enclosure. It takes power from the +10V formerly supplied to the mechanical pot from the motor controller. The solid-state pot provides a similar 0V to 10V output, and delivers as much as 15mA to the controller.
Figure 1. This solid-state industrial potentiometer simulates an equivalent mechanical potentiometer, and fits in the same space.
The key to this circuit is a low-power digital potentiometer IC, 100kΩ version (U3). Configured as a voltage divider, this IC provides an output of 32 discrete voltage steps between its minimum and maximum settings (0V and 5V). A low-power linear regulator (U1) provides a +5V supply rail for U2, U3, and a resistor ladder internal to U3 (whose maximum rating is +5V).
PB1 and PB2 constitute a double-pushbutton industrial switch. Each high-to-low transition produced by PB1 increments the digital potentiometer's "wiper" by one step (depressing PB2 while toggling PB1 decrements the wiper by one step). U2 is a switch debouncer that provides (in addition to the debouncing) a 40ms fixed delay between its outputs and the switch action.
To provide 0V to +10V outputs as required by the motor controller, a single-supply, rail-to-rail op amp (U4) amplifies the U3 output by a factor of two. The input common-mode range for this op amp, (250mV beyond either supply rail), allows it to generate 0V to +10V outputs like a mechanical potentiometer. The 1% resistors R1 and R2 set the op amp gain to 2V/V.
The circuit's low quiescent current ranges from 86µA for a 0V output to 186µA at +10V. To lower this quiescent current, choose the 200kΩ version of U3. Because the op amp is stable for any capacitive load, it easily drives long, shielded, multi-conductor cable runs back to the control room. Another benefit of the all-surface-mount solid-state construction is important for Class 2 enclosures: its lower power dissipation produces less heat within the electrical enclosure.
A similar version of this article appeared in the September 20, 2001 issue of EDN magazine.