# AC Dimmer (FL5150) Controlled by Low-Power MCU – Grounding and Isolation

**URL:** <https://sierraconnect.protoexpress.com/t/ac-dimmer-fl5150-controlled-by-low-power-mcu-grounding-and-isolation/3949>\
**Category:** Beginner’s corner\
**Tags:** schematic-diagram\
**Created:** [March 30, 2026, 2:10pm UTC](https://sierraconnect.protoexpress.com/t/ac-dimmer-fl5150-controlled-by-low-power-mcu-grounding-and-isolation/3949 "2026-03-30T14:10:55Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![rayansayedpd](https://avatars.discourse-cdn.com/v4/letter/r/82dd89/32.png) [@rayansayedpd](https://sierraconnect.protoexpress.com/u/rayansayedpd)\
**Post date:** [March 30, 2026, 2:10pm UTC](https://sierraconnect.protoexpress.com/t/ac-dimmer-fl5150-controlled-by-low-power-mcu-grounding-and-isolation/3949/1 "2026-03-30T14:10:55Z")

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I am trying to design an **AC dimmer using the FL5150** , based on the datasheet reference circuit.

I want to control it using a **3.3V MCU** :

- PWM → for dimming control

- GPIO → for mode selection

* * *

### **My main doubts**

### **1. Ground connection**

In the datasheet circuit, the IC ground is connected to the **source/common node (mains side)**.

Can I connect my **MCU ground directly to this node**?  
Or will this be unsafe / incorrect?  
do i have to separate IC FL5150 ground and mcu ground

* * *

### **2. Optocoupler requirement**

Since MCU is low voltage (3.3V) and the IC is on **AC mains side** :

👉 Do I need **optocouplers for PWM and GPIO signals**?  
👉 Or can I interface directly?

* * *

### **3. Basic feasibility**

👉 Will this design work reliably with a 3.3V MCU controlling FL5150?

 ![image](https://global.discourse-cdn.com/sierraconnect/original/1X/4c6451c2fb356e150a36770be36171fa60fce8ba.png)

* * *

Datasheet does not clearly explain MCU interfacing, so looking for guidance.

* * *

Thanks in advance.

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<div class="post-metadata">

**Author:** ![abhishek](https://sea2.discourse-cdn.com/sierraconnect/user_avatar/sierraconnect.protoexpress.com/abhishek/32/45_2.png) [@abhishek](https://sierraconnect.protoexpress.com/u/abhishek)\
**Post date:** [April 14, 2026, 2:26pm UTC](https://sierraconnect.protoexpress.com/t/ac-dimmer-fl5150-controlled-by-low-power-mcu-grounding-and-isolation/3949/2 "2026-04-14T14:26:22Z")

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As per datasheet the VDD pin is internal 5V supply for digital logic on **FL5150** , which means the IC works on a 5V logic. The microcontroller works on a 3.3V logic, now a days most of the ICs are compatible with 3.3V and 5V but better to not assume that. The simple solution would be an optocoupler which is to provide isolation between microcontroller DC side and high power AC side. 3.3V will easily drive the photo diode of optocoupler and on the photo transistor side the VDD can be used to operate the digital pins of **FL5150**. Do not use direct connections between controller and AC side, isolation is must. In your circuit do not connect nets DIMC and DIMM to the microcontroller directly. they should be pulled up to VDD pin of **FL5150** and then connect photo transistor of optocoupler between GND pin of **FL5150** and the respective pin 2 or 4. Two optocpuplers will be needed. I assumed by default the two pins should be High, you can do the opposite also as per the application.