How do I protect an electronic circuit
Simple Ohm’s Law solution  duplicated many times in this EE forum.

(V drop) / If = Rs    

(12 - 9.9 )/0.3A= 7 Ohms

Pd=V drop * If  = 2.1* 0.3A = 0.63W so to reduce 100’C temp rise use 2W part or two 1W 14 ohm parts in parallel or 8 * 1/4W resistors in parallel , each 8x 7 Ohms =56 Ohms 1/4W







simulate this circuit &ndash; Schematic created using CircuitLab