A method of trimming down the volume of a semiconductor resistor element using electrical resistance feedback. The central core region preferably has an alpha profile with an alpha value preferably less than about 2. Such PU-systems preferably further comprise at least one phosphorus-containing compound, most preferably a phosphorus-containing plasticizer, which is preferably halogen-free. The temperature dependent resistor is thermally and electrically coupled to the shunt regulator, wherein the first variable resistor limits current flowing through the shunt regulator if the current reaches a predetermined level less than that which would cause failure of the regulator, due to ohmic heating of the regulator.