**1. what “AC current control” really means**

A [microcontroller](https://www.ampheo.com/c/microcontrollers) cannot directly control AC current.
Instead, it controls switching, phase, or amplitude, and the external power circuit converts that into a regulated RMS current.


At system level, AC current control always has three functional blocks:

1. AC current sensing (feedback)
2. Power modulation element (how you change current)
3. Control algorithm in the MCU (how you decide how much current)

This forms a closed-loop current control system.

**2. Ways to control AC current (conceptual methods)**
**Method A: Phase-angle control (TRIAC-based)**

You change when the AC waveform turns on each half-cycle.

* Later firing angle → lower RMS current
* Earlier firing angle → higher RMS current

✔ Simple
✔ Cheap
✖ Distorts waveform
✖ EMI, poor for precision mA control

**Method B: Burst / cycle control (integral cycle)**

You turn whole AC cycles on and off.

* 10 cycles ON, 10 cycles OFF → ~50% RMS current
* Works best with thermal loads

✔ Low EMI
✖ Slow response
✖ Poor for fast or inductive loads

**Method C: High-frequency PWM + isolation (best for precision)**

You rectify AC, then use:

* [MOSFET](https://www.onzuu.com/category/fets-mosfets) / [IGBT](https://www.onzuu.com/category/igbts)
* High-frequency PWM
* Current feedback

The output current is reconstructed as controlled AC current.

✔ Accurate mA control
✔ Clean waveform
✔ Works with inductive loads
✖ More complex

**3. Current sensing (how the MCU “knows” the current)**
**Option 1: Shunt resistor (low-value resistor)**

* Measure voltage drop: V = I × R
* Use an isolated amplifier for safety

✔ Accurate
✖ Needs isolation for AC mains

**Option 2: Current transformer (CT)**

* Naturally isolated
* Good for AC only

✔ Safe
✖ Not good at very low mA unless designed carefully

**Option 3: Hall-effect current sensor**

* Measures magnetic field
* Isolated, works for AC

✔ Easy to use
✖ Offset drift at low mA

**4. Control loop in the MCU (core idea)**

The MCU does not control voltage directly — it controls current error.

Typical loop:

1. Measure AC current → convert to RMS value
2. Compare to target current (e.g. 20 mA)
3. Calculate error
4. Adjust:

* TRIAC firing angle, or
* PWM duty cycle

5. Repeat every cycle

This is often implemented as a PI controller.

**5. Safety requirement (non-negotiable)**

If this involves mains AC:

* You MUST use galvanic isolation
* Use optocouplers, isolated amplifiers, or transformers
* MCU ground must never touch live AC

**Practical Examples (with real MCU + circuit choices)**
**Example 1: 0–20 mA AC current control using TRIAC (basic, low cost)**

Use case: AC lamp, heater, resistive load

**Components**

* MCU: [STM32F103C8T6](https://www.ampheo.com/product/stm32f103c8t6-131876)
* TRIAC: [BTA16-600](https://www.onzuu.com/search/BTA16-600)
* Optotriac driver: [MOC3021](https://www.onzuu.com/product/texas-instruments-moc3021-3462257) (non-zero-cross)
* Current sense: Small CT or shunt + isolated amplifier
* Zero-cross detector: Optocoupler (e.g. H11AA1-type)

**How it works**

* MCU detects zero crossing
* Waits a calculated delay
* Triggers TRIAC gate
* Adjusts firing angle to control RMS current

⚠ Accuracy below ~10 mA is limited.

**Example 2: Precision mA-level AC current source (recommended)**

Use case: [Sensors](https://www.ampheo.com/c/sensors), calibration equipment, medical / industrial signals

**Architecture**

AC → Rectifier → DC bus → PWM current regulator → Reconstructed AC

**Components**

* MCU: [STM32G030F6P6](https://www.ampheo.com/product/stm32g030f6p6-129893)
* Power stage: MOSFET (e.g. logic-level N-MOSFET)
* Current sense: 0.1 Ω shunt + isolated amplifier
* Isolation: Gate driver or isolated DC/DC
* Output: Controlled sinusoidal current

**How control works**

* MCU generates sine reference
* Uses PWM to regulate current
* Current loop ensures exact mA output

✔ Accuracy down to <1 mA
✔ Low distortion

**Example 3: Isolated AC current loop (4–20 mA AC / industrial)**

Use case: Industrial signaling over AC

**Components**

* MCU: [STM32F407](https://www.ampheo.com/search/STM32F407)
* Isolated amplifier: precision current sense amplifier
* Power switch: IGBT or MOSFET
* Isolation: Transformer or isolated gate driver

**Control**

* Digital setpoint → RMS current
* MCU computes waveform and regulates current

**6. MCU features that matter for AC current control**

| Feature | Why it matters |
| -------------------------- | -------------------------- |
| ADC with good resolution | Accurate current sensing |
| Timer with capture/compare | Phase control / PWM |
| DMA | Stable waveform generation |
| Math capability | RMS calculation |
| Isolation support | Safety |

**7. Summary (engineering takeaway)**

A [microcontroller](https://www.ampheoelec.de/c/microcontrollers) never controls AC current directly.
It controls switching behavior, and a closed-loop circuit turns that into regulated AC current.

**Quick selection guide**

* Cheap, rough control → TRIAC phase control
* Thermal loads → Burst control
* Precise mA current → Rectify + PWM + current feedback