Why does my electricity tester work on DC Adapter?
That the neon glow discharge lights up when you connect it to either output wire of the DC adapter, is probably the result of the internal circuit of the adapter. If there is even a small capacitive coupling from the 220 V AC input to the DC output, this can be sufficient to produce a high AC voltage (but only very small AC current) on the DC output leads with respect to the ground which causes the neon tester to glow. For this only a very low AC current is necessary.