15 V AC backup supply to serve as power backup for modem.

[ericgibbs]

If you are nitpicking then sugest a both polarity from a reference point politicaly CORRECT.

hi neon,

I hate to say this to anyone, but this time I'll make an exception.

You are talking absolute nonsense!

 

[Hero999]

Voltage is not born with polarity WE the designers users assign polarity. Current flow assign polarity not the presence of a voltage which can be +/- depending on current flow. Want to know more?

Either we're misunderstanding you due your poor grammer or you're talking rubbish, charge comes in two forms: positive and negative.

 

[mvs sarma]

Voltage is not born with polarity WE the designers users assign polarity. Current flow assign polarity not the presence of a voltage which can be +/- depending on current flow. Want to know more?

Dear neon, voltage is really born with polarity only. two points of different potentials create a potential difference and voltage is a measure of the same, as i understand it. As the two points are at different potentials the polarity with reference to one point is always there.

I only earnestly hope that my post would not be taken as offense,

 

[rezer]

I believe everyone (including neon) is saying the same thing. No one argues the polarity of a specific voltage with respect to another point, whether it be ground or not (At least I hope not). I think neon is not understanding that you can still hook up a transformer to a modem and it will work.

Neon,
Don't miss the point. The rectification and regulation happen inside the modem giving you your voltage polarity, whether it be pos. or neg (Refer to the schematic Eric posted earlier and it should come to light). Think "DC supply inside" begin fed AC from the transformer.

 

[pkshima]

If I recall correct, I read sometime back that some devices (like clock radio) derive thier clock signals from the mains line since its a perfect 50/60 Hz signal.

I have been believing this for a while now and this I though was the reason why the modem needed AC.

I also beleive this to be the reason for the radio clocks brought from one world (americas) to not work in another(India and rest of asia)

Please correct me if thats not true

 

[Hero999]

That's true but you can normally convert them - sometimes it's as easy as changing a jumper on the board.

 

[rezer]

If I recall correct, I read sometime back that some devices (like clock radio) derive thier clock signals from the mains line since its a perfect 50/60 Hz signal.

I have been believing this for a while now and this I though was the reason why the modem needed AC.

I also beleive this to be the reason for the radio clocks brought from one world (americas) to not work in another(India and rest of asia)

Please correct me if thats not true

I believe you are correct. In fact I service equipment that uses the 60Hz AC line for just that.