Light source unit and lighting fixture

A light source unit, comprising:

• a light source circuit including N pairs of a light source group including one or more solid light emitting elements and a constant current circuit connected in series with the light source group to keep, constant, current flowing through the light source group, where N is an integer equal to or greater than 3;
• a full-wave rectification circuit which includes a first output end and a second output end and is configured to perform full-wave rectification on AC voltage to cause DC voltage between the first output end and the second output end;
• and
• a control circuit including a detector configured to detect light or signal from an external source,
• a first pair of the N pairs being connected between the first output end and the second output end of the full-wave rectification circuit,
• a k-th pair of the N pairs being connected in parallel with the constant current circuit of a (k−1)-th pair of the N pairs so that the light source group of the k-th pair and the light source group of the (k−1)-th pair are in series, where k is an integer equal to or greater than 2 and equal to or smaller than N, and
• the control circuit being configured to, when the detector detects the light or signal, terminate operations of constant current circuits of the N pairs or limit current flowing through light source groups of the N pairs.