I think Jose's on the money and if we just have half wave rectification(as previously suggested by Jose and Erik) then I think the dissipation is tolerable. Erik - I'm not quite following what you've suggested with the other opto. The major issue with this circuit is keeping an eye on the dissipation and building in enough protection so it will survive most situations. A bit of simulation will help here. To be honest, I haven't put a lot of brain power to the problem - I was just trolling to see if anyone else had a ready-made solution (it's something I'd expect to see in those little circuit ideas columns in the magazines) where I could adapt for my requirements. But, alas, I'm going to have to earn my money!