Phone Line Polarity Checker

Posted on Feb 5, 2014

Do you suspect your broadband speed (DSL) is slower than it should be It could be as simple as incorrect polarity in your phone wall socket. Build this very cheap, very simple device to find out whether you need to change your wiring! It might not seem that telephone line polarity is important, since the ring and voice signals sent over telephone

Phone Line Polarity Checker
Click here to download the full size of the above Circuit.

lines are AC. However the lines are actually biased to 48V DC (less when in use) and so the polarity can matter. The main problem with incorrect polarity is that some DSL (Digital Subscriber Line) modems and routers can perform poorly in this circumstance. Since telephone wires are color-coded, it should be possible to simply check that the sockets are wired correctly. Unfortunately, there are multiple wiring colour schemes and they have changed over time. Believe it or not, the old color scheme (from around 15 years ago) is identical to the current colour scheme except that the polarity of both lines is reversed! This is why so many homes have this problem and yours may well be one of them. The common telephone line is simply a copper pair, ie, two wires. As mentioned, there is usually a 48V DC bias across the pair which drops to around 8V when a telephone is off-hook . The ring voltage (around 90V AC) and the audio signal voltage (also AC) are overlaid on this DC bias. The DC power is rectified  by each telephone on that line to run its own circuitry. Note, though, that this does not include cordless phones which usually use a plugpack, as their power requirements are far in excess of what the telephone line can deliver. (As an aside, that is the reason it is important to keep a line-powered telephone in your home so you can still make and receive calls if the mains power goes out. Telephone exchanges can usually...

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