Yeah I've been reading a lot of post about this subject too, if he's yelping it's probably set too high. Most collars come with the option to set it high, medium, low or something similar... i'd check that too. Although if it worked the first time I guess no point now that he's not barking anymore, but just incase!
I'm more interested in the Cintronella or sonic ones. Mainly just because Tikka thinks she's dying when she's in her kennel, she usually quiets down and falls asleep but that's after about 20 minutes of yelping and barking. So if she associates barking in her kennel with an unpleasant smell or sound I'm hoping that works. I always give her little pieces of chicken to reward her when she's been quiet for 5 min. or so.
it's working... slowly.
I also don't leave my collars on, I put Tikka in a harness when she's on her yard line so there's no pressure on her throat
And pooter never wears his collar unless we're on a walk... it mattes his poofy hair! and his neck is much smaller than tikkas.