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Load Cell Amplifiers, Indicators and Transmitters Explained Simply
A load cell on its own produces only a tiny electrical signal, often just a few thousandths of a volt, so you need something to boost it and make it useful. That something is an amplifier, a signal conditioner, a transmitter or an indicator, depending on what you want to do with the weight. If you only want to see the weight on a display, you need an indicator. If a PLC or control system needs the weight, you need a transmitter or converter with a standard output such as 4 to 20 mA. If you are a hobbyist or building a prototype, a small amplifier chip may be enough. This guide explains each option in plain language, with simple examples and a checklist for choosing.
Why a load cell needs an amplifier
A load cell’s strength is its precision, not its power. Its output is rated in millivolts per volt (mV/V). Take a cell rated at 2 mV/V and powered with 10 V. At full load it gives 2 × 10 = 20 millivolts, which is 0.02 volts. At half load it gives just 10 millivolts. A normal PLC input expects something like 0 to 10 volts or 4 to 20 mA, so that 20 mV signal is far too small to use directly. An amplifier takes the tiny signal and scales it up cleanly, without adding noise. For the maths behind this, see load cell output explained.
The four main choices
1. Weighing indicator (the display)
An indicator powers the load cell, amplifies the signal, converts it to a number and shows it on a screen. It usually lets you set zero, tare, calibrate and sometimes set alarms or print tickets. Think of the display on a shop scale or a weighbridge office. Choose an indicator when a person needs to read the weight.
2. Weight transmitter (analogue output)
A transmitter amplifies the signal and converts it to a standard output that control systems understand. The most common are:
- 4 to 20 mA: a current signal that travels long distances without losing accuracy and tolerates electrical noise. A reading of 4 mA means zero weight, and 20 mA means full scale.
- 0 to 10 V: a voltage signal for shorter runs.
Choose a transmitter when a PLC, a control panel or a variable speed drive needs the weight.
3. Digital transmitter or converter
These convert the load cell signal into digital data and send it over a network or serial link such as RS-485 or Ethernet-based protocols. They can provide very stable readings and let you read many cells in one system. They suit modern control systems and data logging.
4. Small amplifier or ADC module
These are small boards used by makers and engineers. A popular example is the HX711, a chip that amplifies and converts the load cell signal into digital numbers that a microcontroller such as an Arduino can read. It is useful for prototypes, learning and non-critical projects, but it is not a substitute for approved equipment in a trade or safety application.
Which one do I need? A quick table
| Your goal | Choose |
|---|---|
| Show the weight to an operator | Weighing indicator |
| Send weight to a PLC or SCADA | 4 to 20 mA or digital transmitter |
| Control filling or batching | Indicator or controller with relay or network outputs |
| Log force or weight in a lab | Data acquisition device |
| Build a prototype or learning project | HX711-type board |
| Legal-for-trade weighing | Approved indicator that is part of an approved scale |
Example 1: a tank with a display
A brewery weighs a 2,000 kg ingredient tank on four load cells. A junction box joins the cells, and a panel-mount indicator shows the weight. The operator reads the display and fills to a target. Cost and complexity are low. The indicator also has two relay outputs that stop the pump near the target weight.
Example 2: a hopper feeding a PLC
A packing line has a hopper on three load cells. The plant uses a PLC to control everything. A weight transmitter converts the combined signal to 4 to 20 mA, and the PLC reads it on an analogue input. The PLC decides when to open the discharge gate. A separate indicator mounted near the hopper shows the weight for operators.
Example 3: a lab test rig
A university lab pulls samples on a test frame. An S-type cell measures the pulling force. A data acquisition unit reads the cell many times a second and sends it to a laptop, which records a graph of force against time. See load cell data acquisition for how this is set up.
Example 4: a maker project
A hobbyist builds a smart bin scale. A small load cell connects to an HX711 board, which talks to an Arduino. The Arduino calculates the weight and shows it on a small screen. It is a great way to learn, but the result is not accurate or stable enough to charge customers.
Key specifications to check
- Excitation voltage: the amplifier must supply the voltage your load cell expects, often 5 V or 10 V.
- Number of cells it can drive: each 350 ohm cell draws a certain current, so an indicator may be limited to, for example, four 350 ohm cells.
- Input sensitivity: the lowest signal in mV/V that it can read. This must be lower than your cell’s rated output.
- Resolution and sampling speed: how fine the steps are and how fast it reads. Fast filling and check weighing need fast sampling.
- Output type: display only, 4 to 20 mA, 0 to 10 V, relays, serial or network.
- Four-wire or six-wire input: six-wire is better for long cables.
- Environment: panel, wall or field mounting, enclosure rating and temperature range.
- Approvals: needed for legal-for-trade use. See legal for trade weighing in Australia.
Analogue or digital load cells?
Traditional (analogue) cells send a tiny voltage that needs an amplifier. Digital load cells have the electronics inside the cell and send digital numbers over a network. Both are common. Our guide to digital and analogue load cells compares them, and smart load cell technology covers the newest designs.
Wiring basics
- Match the colours or labels on the cell cable to the terminals on the amplifier, using the load cell datasheet. Colours differ between brands, so never guess.
- Connect the excitation wires and the signal wires to the correct terminals.
- If the cell has six wires, connect the sense wires as shown. If you use a four-wire input, link the sense wires to the matching excitation wires at the terminal.
- Use shielded cable and connect the shield at one end only, as the instructions say.
- Keep load cell cables away from motor and mains cables.
- Seal the cable entries so water cannot get in.
Common problems and simple fixes
| Problem | Likely cause |
|---|---|
| Display jumps around | Electrical noise, poor shielding or loose wires |
| Display reads negative when weight is added | Signal wires swapped |
| No reading at all | No excitation, broken wire or wrong terminals |
| Reading drifts | Moisture in a cable or junction box, or a failing cell |
| Reading maxes out | Cell overloaded, or amplifier gain set too high |
See load cell troubleshooting for a full checklist.
Filtering, averaging and sampling in plain English
Real weighing is noisy. Machines shake, people lean on platforms and liquid sloshes. Indicators and transmitters therefore average the readings over a short time to give a steady number. The more averaging, the calmer the display, but the slower it reacts. For a slow tank, heavy filtering is ideal. For a fast filling machine, you want a quick response and light filtering. Sampling rate, measured in readings per second, tells you how often the instrument looks at the load cell. Fast jobs such as check weighing need a high rate. Slow jobs such as stock monitoring do not.
Calibration happens at the indicator
Once the cells are wired up, the indicator or transmitter is calibrated so that a known weight shows as the right number. The usual routine is to set zero with the scale empty, place a known test weight, and tell the instrument what the weight is. The instrument then works out the scale factor. See load cell calibration for the full procedure and how often to repeat it.
Frequently asked questions
Can I connect a load cell directly to a PLC?
Only to a PLC input designed for load cells. A standard analogue input cannot read millivolt signals, so you need a transmitter or a dedicated weighing module.
Can I use any amplifier with any load cell?
Not always. Check excitation voltage, input sensitivity and the number of cells it can power.
What is the difference between an indicator and a transmitter?
An indicator shows the weight. A transmitter sends it as a signal to another device. Many products do both.
Is the HX711 accurate enough for professional use?
It is good for prototypes and learning. For industrial or trade use, choose proper weighing electronics.
Do you supply amplifiers and indicators?
Contact us with your application through the contact page and we will advise on compatible electronics for the cells in our shop.
Where to go next
Start with our complete load cell guide, then see the load cell glossary for unfamiliar terms and five real weighing system examples to see how it all fits together.
