4–20mA Absolute Encoders: How They Work and When to Use Them
The 4–20mA absolute encoder is one of the simplest position sensors to install, test and maintain. It reports shaft angle as a plain analog current: 4mA at one end of the measuring range, 20mA at the other.
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Many absolute encoders instead report position over a fieldbus such as EtherNet/IP, CANopen, EtherCAT or PROFIBUS. These networks are powerful, but testing or fault-finding them takes a laptop, configuration software, the right cables and some training. A 4–20mA encoder can be checked by anyone with a multimeter.
Why choose a 4–20mA encoder?
Easy to test. Put a multimeter in the loop, turn the shaft, and watch the current change. There is no software or protocol to learn.
Built-in fault detection. Because the signal starts at 4mA rather than zero (a "live zero"), a reading near 0mA can only mean a problem: a broken wire, a loose connector or a lost supply. A 0–10V signal can't tell you the difference between a fault and a genuine zero position.
Noise-resistant over long runs. A current signal is far less affected by electrical noise and cable resistance than a voltage signal. That makes it well suited to industrial sites, with cable runs of a few hundred metres.
No protocol licensing costs. Some fieldbus protocols carry licensing or membership fees that add to the cost of devices. 4–20mA is an open, universal standard that any PLC, display or controller can read.
Limitations to consider
One input per encoder. Each encoder needs its own analog input channel on the PLC, and analog input cards aren't cheap. A single fieldbus port, by contrast, can read many encoders on one cable.
Resolution depends on the PLC. Position is only as fine as the analog input card that reads it. Spreading 360° over a 12-bit input (4,096 steps) gives roughly 0.09° per step. Spread a 16-turn range (5,760°) over the same input and each step becomes about 1.4°. If you need fine resolution over many turns, a fieldbus or SSI encoder is usually the better choice.
The output wraps around. When the shaft passes the end of the measuring range, the signal drops straight from 20mA back to 4mA. Set the range, or use the zero-set pin, so this jump falls outside the machine's normal travel.
4–20mA vs 0–10V vs fieldbus at a glance
| 4-20mA | 0-10V / 0-5V | |
| Testing | Multimeter | Multimeter |
Broken-wire detection | Yes (live zero) | No |
| Cable length | Long runs, noise resistant | Short runs |
| Inputs needed | One analogue channel per encoder | One analogue channel per encoder |
| Resolution | Limited by PLC input card | Limited by PLC input card |
| Licensing costs | None | None |
POSITAL's single-turn 4–20mA encoders ship with a default range of 0–360° = 4–20mA. Because they're programmable, PCA can reconfigure them locally to any range within that span before dispatch. For example, a valve that only travels a quarter turn can be set to 0–90° = 4–20mA, so the full signal is used across its actual movement.
Direction and zero-set pins
Two connector pins make on-site setup quick:
- Direction pin: apply the supply voltage to this pin to reverse the counting direction, so the current rises with clockwise or anticlockwise rotation as required.
- Zero-set pin: move the shaft to the home or top-dead-centre (TDC) position, apply the supply voltage to this pin, and that position becomes 4mA. Check the connection diagram in the encoder's datasheet before wiring.
Single-turn, multiturn and voltage options
- Single-turn: the most common and cost-effective choice, covering up to 360°.
- Multiturn: for shafts that rotate more than once, with ranges up to 16 revolutions (5,760°).
- Voltage outputs: 0–10V and 0–5V versions suit short cable runs and controllers without current inputs. For long runs or electrically noisy sites, 4–20mA remains the better choice.
Typical applications
4–20mA absolute encoders are a natural fit wherever a position needs to feed an existing analog input: valve and damper position, crane slewing and boom angle, solar tracker angle, gate and door position, and retrofits to older PLC systems.
Talk to PCA
PCA is the Australian distributor for POSITAL encoders, and we program 4–20mA encoders to your range locally before dispatch. [Link: view our 4–20mA absolute encoder range] or call 1300 236 467 to speak with our team.
