Resistor Hack for Innotek IUC-4100 & Innotek IUC-5100
The biggest drawback of the Innotek systems (Innotek IUC-4100 and Innotek IUC-5100) is that you cannot adjust the correction level of each collar separately. There is a little workaround that involves running a resistor across the probes of the collar to reduce the correction level on a collar.
First, get the collar and unscrew the two probes to expose the two mounting threads underneath. Now wind one leg of the resistor around each mounting thread. Finally, screw the probes back on to lock the resistor into place. The resistor reduces the amount of correction delivered to the dog on only the collar is affixed to. It does this by creating a parallel path for the current to flow.
Using a 33K ohm resistor, you can reduce the correction level on the collar by 50%. Using a 5.6K ohm resistor, you can reduce the correction level on the collar by 75%.
One thing to remember is that when you recharge the collar, remove the resistors before placing the collars in their charging cradle. If you forget, the collar will not charge properly. This makes the hack a bit of a pain. So consider a system with the independent correction built straight in. If you want something rechargeable, the Dogtra EF-3000 would be a good choice. If you don't mind having a disposable battery, the PetSafe Stubborn, PetSafe Little Dog and PetSafe Deluxe combination is good because it allows you to mix and match collars between systems.