Experiment with a uc3842 flyback convertor using an EE-42 transformer. Coil the power to wrap 42 turns, coil the bais, wrap 5 turns, coil the power out, wrap 5 turns, use the zd11v serial R820 to control the power out to be stable. by sending voltage through opto pc817 comes to control uc3842 at pin 1 test load 1.6 ohm, power drop a little The tested current is about 7a. probably up to 10a