MOV are available in different Diameters, is there advantage using a 7mm vs 10mm Vs 20mm.
Where in the AC line should they be placed?
thanks
Choose the MOV by it's voltage rating. The thickness is determined by the chemistry, and some manufacturers are using different chemistries to achieve thinner or smaller MOVs with the same or higher voltage ratings. It's all about reducing size.
MOV's should be placed just at the line entrance point to your circuit.
Also, you should ideally have a fuse in it at the line entrance point too. The fuse would be placed before the MOVs. Then, when a transient occurs and punches through the MOV you want the fuse to blow. This is especially important with a low-current load, such as in this case with the LED inside the opto-isolator being the load. When a MOV punches through, it creates a low-impedance path that can continue to flow current, bypassing the load (the MOV becomes the new load) and overheating the MOV, causing it to catch fire. To counter this possibility (for low current loads) a fuse is used to open the circuit if the MOV shorts out like this. Since the load is less than 10 mA, I would say perhaps a fuse rated at 100 mA should be ok; if that blows, something is wrong and you likely need to replace the MOV's then.
*note- for higher current loads, a fuse might be rated too high for a MOV event to blow it.. in those cases you would need to use a TCO (thermal cut-off) in series and placed near the MOV, or a single TMOV, a device with a built-in TCO. This detects the overheating of the MOV and takes the MOV out of circuit before it can catch fire.
This AC mains stuff needs to be put in Dave's Wiki... should I do it