what is digital potentiometer?
Separate a digitally adjusted reference from a resistor IC
Hello friends. In electronic circuits, a digital potentiometer is an IC that adjusts a resistive divider through digital commands. In industrial automation, the same name is also used for electronic reference instruments adjusted with keys. These products operate differently and require different connections. Replacing the mechanical adjustment of a conventional potentiometer with electronic control does not mean the output must be digital data: the controlled quantity can still be an analog signal.
Editor’s note: A resistance-adjusting digipot IC is different from a panel instrument producing 0–10 V or 4–20 mA. A similar product name is not enough to justify connecting an IC to a drive’s potentiometer terminals. Check the model documentation for its output type, voltage limits and wiring diagram.
How does the electronic wiper inside an IC work?
A common design contains a resistor string and electronic switches. A and B identify the ends of the resistance track, while W selects an intermediate point. Changing the command changes the connection to W. In three-terminal divider operation, the total A–B resistance remains essentially fixed; the resistance ratio on either side of W changes. Some devices connect one end internally to a common supply rail or provide only a two-terminal variable-resistor arrangement. Check the actual pin configuration.
Depending on the device, adjustment uses I²C, SPI or up/down commands. For example, the Texas Instruments TPL0401A-10 is a 128-position, 10 kΩ I²C device with its low resistance terminal internally connected to GND. The Microchip MCP41HV51 is a 256-position SPI example. Interface type and position count are not universal digipot specifications.
What does an industrial panel instrument do?
With a panel-mounted electronic potentiometer, the operator enters a setpoint and the instrument converts it into an appropriate reference output. Variable resistance, voltage or current output depends on the model. For example, a panel signal generator such as the SENECA S311G can provide a manually set value through its selected voltage or current output. A panel instrument does not necessarily contain a digipot IC; a suitable digital-to-analog converter and output circuit can perform the task.
Front-panel size is only one selection criterion. For the 48 × 48 mm, 37 × 72 mm and 72 × 72 mm product groups, also compare supply voltage, panel cutout, output loading and control method. Do not assume every model supports external buttons, communication, adjustment locking or stored settings.
How does a percentage setting become voltage or current?
The example below shows a linear, forward-acting panel reference. With a 0–10 V voltage output selected, a 50% setting gives 5 V. With a 4–20 mA current output selected, the same setting gives 12 mA. These are alternative output types; the illustration does not imply that one instrument provides both simultaneously.
For a setpoint s expressed as a percentage from 0 to 100, ideal scaling is V = 10 × s / 100 or I = 4 + 16 × s / 100 mA. A zero-percent setting in a 4–20 mA system therefore means the 4 mA lower endpoint, not 0 mA. Do not confuse this percentage scale with an IC’s raw command code: an 8-bit setting field, for example, can use codes 0–255.
How is it used with a drive or PLC?
A compatible reference connected to a variable frequency drive analog input lets the operator change the speed command from the panel. If the input is linearly scaled from 0–10 V to 0–50 Hz, for example, 5 V represents a 25 Hz frequency reference. Actual operation depends on the command source, limits and acceleration ramp among other drive parameters. The motor’s mechanical speed is also different from the reference itself.
Displaying a number does not, by itself, provide automatic regulation. In PID speed control, feedback and the control calculation are separate functions; an electronic potentiometer may simply supply the requested setpoint. When connecting to an analog controller, establish whether the output is 0–10 V, 4–20 mA or variable resistance.
Resolution, accuracy and memory describe different properties
Resolution describes the number of available settings; accuracy describes how closely the actual result matches the target. The TPL0401A-10 has 128 positions but an end-to-end resistance tolerance of ±20%. Ratio error in a voltage divider and absolute resistance error in a two-terminal arrangement must also be assessed differently. Digital control does not guarantee greater accuracy than every mechanical potentiometer.
Retaining a setting after power loss is model-dependent. A nonvolatile device may start with its stored setting; a volatile device has a power-up state defined in its datasheet. Nonvolatile memory can also have a limited write endurance. Instead of unnecessarily writing every adjustment to EEPROM, follow the manufacturer’s recommended storage method. Establish which reference will appear when power returns.
Check voltage, current and load limits
A digipot’s digital-interface voltage is different from its permitted analog-terminal voltages. Some devices are restricted to low voltages, while specialised parts such as the MCP41HV51 support an analog voltage range up to 36 V using a separate analog supply. This does not make every digipot suitable for direct connection to a 24 V or 10 V circuit. Keep wiper current, wiper resistance, power dissipation and supply sequencing within the model’s requirements.
Panel outputs require compatible loading too: check the minimum permitted load resistance for a voltage output and the maximum total resistance a current output can drive. Determine signal-common connections, active/passive current wiring and any isolation requirements from the documentation. A digipot output is not intended to power a motor or another power load directly.
Where is it useful, and what does it replace?
In electronic circuits, digipots can adjust audio volume, amplifier gain and calibration. In a power supply, a suitably designed feedback network can use one to adjust the target voltage; the load current is not routed through the digipot itself. Industrial panel instruments can provide speed references, process setpoints and test signals. Suitability depends on the complete circuit design.
A rotary control may instead use an incremental encoder, which produces pulses and performs a different electrical function. Before replacing an existing control, identify its connection type and then choose a suitable potentiometer assembly or electronic reference instrument. Selection should follow the input/output specifications and operating requirements, rather than the product name alone.