I'm a sentinel, and one of my primary reasons for picking the class was I liked the idea of having an ability that improved my shields, since I'm not so great at avoiding being hit. But I can't seem to figure out how to use this ability. Whenever I activate it, it always seems to trigger the "detonate" part of the ability. How do I activate the armor boost effect, and how can I tell whether it is on so that I don't accidentally detonate it by trying to enable it again?
Tech Armor is a two-stage power, of which there are several in this game. (Fortification and Defense Matrix come to mind as being very similar) It's a little confusing though since I think in previous iterations there weren't any powers that worked this way.
The first time you hit the button to activate the power, damage taken will be decreased but your power cooldowns will also lengthen. The second time you push it, it detonates, doing damage to enemies around you.
This class of power is really intended so that you can soak up damage while you get up close and personal, and then get a quick damaging attack when you go in for the kill.
In my experience, there's kind of a faint glow that surrounds you when you've got the armor activated. It's kind of easy to miss, so I've at times set it off when I was trying to activate it. If you're playing on single player, you might be able to go by the powers that are active on the power wheel. The power cooldown penalty is also noticeable enough that it can be an indicator.
Setinals are boss. If you want to be good with a sentinal, then DETONATE TECH ARMOR!!! Tech armor does a decent amount of damage. But the benefits of Tech armor is that it staggers ALL ENEMIES EVERY TIME with the exception of the banshee, atlas, prime, which it staggers about half the time. Also, unlike a lot of area of effect powers, it will affect ALL enemies within range, it does not have a max (compared to a power like shockwave that can only affect 3 enemies at most). The stagger coupled with the very fast cooldown of Tech Armor means that it will actually be safer for you to detonate tech armor when the enemy is near than to keep tech armor active for the damage reduction. If you are set up for some melee damage then a heavy melee follow up will finish off any basic enemy even on platinum/insanity. Also, the heavy melee also staggers, which means that if you detonate tech armor then go into heavy melee on say a phantom you are still safe because they will still be staggered. Plus the heavy melee bonus, giving 20% damage reduction after a heavy melee kill/increasing melee damage by 75%, is very easy to acquire when in game and is useful. Just get a nice shotgun like the wraith to safely get all those pesky bosses and you are good. Works good on gold, but on platinum it is not a good idea to be detonating tech armor constantly because it often fails to stagger the hardest enemies.