I'd like to discuss these 2 styles because almost all brake controllers fall into these 2 catagories. i'm purposely skipping the old fashioned hydraulic/electric controllers. They were very good, but dont do well with the newer tow vehicles. So lets break it down:
Timer style: these controls are timer based. you basically set 2 features; overall gain, and delay. When the brake is depressed far enough to activate the stop lights, the controller activates. You set the delay for the controller to ramp up to the gain setting you chose. So it takes 1 to 5 seconds to ramp up to however many amps you set. It does this every time you activate it regardless of how you brake. drawbacks:
1. hard to anticipate. you end up braking and lifting alternately to make a stop.
2. since there is a delay in activation, valuable time may be lost in a panic stop.
3. since it activates the same way every stop, the user generally adjusts the brake controller too low for anything more than a gentle stop wasting a great deal of potential braking force.
4. as a driver negotiates long gentle downhill grades, he eases up on the brake pedal leaving the towed vehicle to do a disproportionately greater amount ofmthe braking causing premature brake wear.
I found dealers love these controllers. The uninitiated can always get a strong sense of braking out of these. Drawtite activator, POD are a couple of brands. They have a gain knob and another knob or slider that will say "sync" or delay.
Inertia style: These activate by the stoplight switch as well. However, they sense how hard the driver is trying to stop and brake proportionately. They kind of have a pulsation that takes a little getting use to. But the benefits over the timer are plentiful. they offer immediate braking that is potentially life saving. properly adjusted they will brake the trailer to near the point of lockup in the hardest braking application. since they are proportional, the trailer will do only its share of braking and no more, saving brake wear. no more pumping the brakes to make a common stop, so the benefits are felt every stop. drawbacks:
1. since they sense the change in inertia, if the antilock brakes on the tow vehicle activate in snow or ice, the controller will send little power to brakes. during these types of conditions, it is preferred to drive with one hand on the controller to manually activate the brakes.
2. These controllers need to be specifically mounted in a ridgid fashion to operate. timers dont care.
3. Some users may think they are not applying properly because the big "tug" is gone. But think for a moment, if the truck stops the truck and the trailer stops the trailer, there is no big drag, even though braking distances havent dramatically increased. these can still be adjusted for a "lead" that most drivers prefer.
These controllers have a gain knob and usually a "level" adjustment on the lower end models. Upper end stuff now levels automatically leaving you with only a gain knob...hooray! Tekonsha is the primary manufacturer of this style of controller, seemingly having the technology sewn up. models include prodigy, primus, voyager.
cequent recently bought tekonsha and has been marketing the latest inertia controllers under the drawtite moniker. tekonsha also made timers...so do your homework and choose wisely.