What the 500W rating means at the repair bench
The SA15A-T is a 500W Zener TVS diode in a DO-15 axial package, rated for a 15V reverse standoff voltage. When a transient exceeds 16.7V (the minimum breakdown voltage), the diode clamps it at 24.4V maximum, shunting up to 20.6A of peak pulse current (10/1000µs waveform) to ground. The 500W peak pulse power rating is the product of the clamp voltage and the peak current — it tells you how much energy the part can absorb in a single surge without failing short. On a 15V rail, this part catches transients that would otherwise punch through downstream ICs.
The DO-204AC (DO-15) axial package is a through-hole form factor with 0.032-inch diameter leads. It mounts into a 0.040-inch plated through-hole on a standard PCB — the body is 7.6 mm long and the lead spacing is 5.1 mm nominal. This is a unidirectional device (one channel) with no power-line protection — it is designed for signal or low-voltage DC rail clamping, not AC mains. The cathode band on the body marks the ground side; reverse the polarity and it conducts like a forward-biased diode.
Diodes Incorporated lists the SA15A-T as obsolete.