I'm designing a control circuit for a universal (AC) motor (single phase, brushed). I've simulated the control circuit and it works as expected. Circuit below is controlled by a STM32G4 series microcontroller. I'm in the process of adding the current measuring capability to the circuit and I need some help in choosing the best method for my use case.
It is my first time designing a circuit to measure AC current so I'm not sure what the best way in this scenario is.
Requirements and info:
- Maximum RMS current: 10 A (everything above is considered as an overcurrent condition)
- Accurate current measurement is required over the whole current span (from 0 A - 10 A)
- Solution doesn't have to be the cheapest possible (still has to be reasonable)
- Measurement circuit doesn't have to be isolated since the circuit isn't either (but it can be)
- Phase control is used
- RMS current is calculated in software for each period (50 Hz) and can then be averaged over multiple periods if needed
- ICs with serial interface for reading current need to be avoided
Note: I'm not really sure what kind of accuracy is even possible but let's say that I'd like +/- 20 mA from 0 A - 3 A and +/- 100 mA from 3 A to 10 A.
What I've looked at so far:
My first idea was to use a simple shunt resistor and an integrated (STM32G4) operational amplifier. However, high-side sensing isn't suitable due to the very high common-mode voltage (230 VRMS). Low-side sensing could work but this would introduce a variable voltage drop in the triac's gate loop which would in turn affect triac triggering I guess.
Next idea was a hall-effect based current sensor such as TMCS1101 from Texas Instruments. Everything was fine until I've seen the RSS error graph which shows that for measuring currents lower than 2.5 A, error grows significantly.
Next idea is a SMT current sense transformer such as this one here. I've never worked with them before. What kind of accuracy is expected over a large span that I need (0 A - 10 A)?
What would be my best bet and what should I be cautious of?
Triac triggering circuit: