Easy answer: It’s the opposite of a power passing splitter. We told you all about power passing splitters in an earlier article. Power passing splitters make sure that your equipment gets the power it needs. DC blocks, also called voltage blocking couplers, make sure that your equipment doesn’t get cooked.
Most devices aren’t designed to take DC power in from their coaxial lines. In fact, if you put a powered connection into an input that isn’t designed to take it, you’ll probably fry that device. (It’s happened to people I know.) When you use something like the DIRECTV SWM system, the ports that carry voltage are very well labeled with red centers, and the splitters used in that system are designed not to let DC power get to those other ports. That’s a good thing.
What if you’re putting together something custom? You might not be able to rely on a splitter to block that DC voltage, and the equipment you’re using might be too expensive to risk. If that sounds like you, you’ll like the cheap insurance offered by voltage blocking couplers. These friendly little devices don’t cost a lot and they’ll block DC power coming into your devices. They don’t contribute to any measurable signal loss, either.
Of course, not every installation will need these couplers. However, if you’re working with power inserters in ways that the manufacturer didn’t count on, you’ll be glad you had a couple of these in your toolbox.