I'm looking for cheaper ways to measure current in a load switched by optoisolated TRIACs, without losing galvanic isolation (no breakdown below a couple kV) between the microcontroller and the load. The rms current can be anything between 100mA and 5A. A Hall effect sensor would work (say, ACS722), but it would cost me around $5.
Here's what I've got so far:
- Regular optocouplers (eg 4n25, LOC110): bad choice because I can't just put them in series since my currents are too high. Maybe there is some way to bias the LED and use it in parallel with a shunt resistor? Would be dirt cheap if it worked.
- Shunt resistor + isolation amplifier (eg TLP7820, LIA130S, AMC1100, ADUM3190): with these I can amplify the voltage on a shunt resistor (say, 0.05R). Problem is, I now need a low voltage supply on the mains side. Also, isolation amplifiers aren't much cheaper than Hall sensors. Isolation can be achieved by optical coupling, thick gates (eg AMC1100) or even magnetic isolation (ADUM3190), but they all seem pretty much the same to me.
- Current transformers (eg 56200C): these seem like the ideal solution, but everything I find seems designed for SMPSs that operate way above 60Hz. I'm not too sure how to get them or to specify them. The examples I find at digital retailers are all ferrite cores and don't even bother to specify primary inductance or saturation current. Shouldn't I use silicon steel cores instead of ferrite? Should I buy the cores and have someone wind them in bulk? Anyway, it should cost me about $3.50 assuming I get the 56200C, make a primary with 2 turns (that's 7.2µH on the primary I guess?) and amplify the output of the secondary with a cheapo opamp.