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Here is a listing study them and choose, try out, learn and then build the one right for you.
PWM Motor Speed Controller / DC Light Dimmer - Here is a listing study them and choose, try out, learn and then build the one right for you.
DC Controlled Dimmer Velleman kit - K8064
Simple brightness adjustment by means of a DC voltage, optically isolated adjustment input. Pre-set adjustment for full brightness, suitable for incandescent lamps, mains voltage halogen lighting and low voltage halogen lighting in combination with a conventional transformer. "Soft start" feature to ensure lamp longevity, transformer protection in case of defective light sources
Electronic Transformer for 12V Halogen Lamp - STMicro
The topology of the circuit is the classic half-bridge. The control circuit could be realised using an IC (so fixing the operating frequency), but there is a more economical solution which consists of a selfoscillating circuit where the two transistors are driven in opposing phase by feedback from the output circuit.