The strength of a phaser blast can be adjusted, at least on the Enterprise-D (and probably the other ships with which we're familiar). In "Galaxy's Child," for example, the space entity is shot with a phaser beam that is only at 3% strength. In "The Ultimate Computer," phasers are initially at low intensity during the training exercise.
Can the strength of the photon torpedoes also be adjusted?
Yes. Data does just that while he is in command of the Sutherland in the TNG episode Redemption, Part II. Data is part of a sensor net blockade tasked with preventing cloaked Romulan Warbirds from sneaking supplies to the Duras faction in the Klingon civil war. The Romulans fire a massive tachyon burst to disable the sensor net long enough to slip past the blockade, but Data figures out a way to expose them, using torpedoes set to a low-power yield that will not damage them, but will expose their presence.
DATA: Overlay display with tachyon emissions. Highlight any tachyon signatures which show subspace inertial displacement. Concentrate a sensor sweep in that area.
(four targets emerge)
HOBSON: Sir, this is pointless. Those readings could be anything. There's no way for us to be sure the Romulans are out there.
DATA: Reconfigure photon torpedo warhead yields. Set for high energy burst level six.
HOBSON: Level six? But that won't even
DATA: Do it!
