I would suggest that these exercises be done in addition to regular dojo training, not instead of it. Next, remember your goal. Try to do exercises which use the same kinds of motions found in Karate techniques. The use of light weight has advantages. Heavy weight has advantages, too.
Low weight allows for better form throughout the exercise, as well as speed.
Heavy weight may force you to push your limits more and is sometimes a more efficient use of time. When in doubt, though, start out light and work your way up gradually. Just like with regular training, don't forget to warm up and stretch out before and after exercise sessions. Finally, please note that this is a list of strengthening exercises. I am intentionally leaving off aerobic, and other types of exercises due to space restraints, not because I do not think they are important.