How to??? Filter AC current out... ?

[MaDHaVoK]

I have a line with 9 volts DC and 18 Volts AC.... how do i get rid of the 18 volts AC?

 

[Colt45]

diode, followed by caps and resistor or choke

edit: err wait.. youll end up with ~20VDC if you do that. hmm.
i dont know

 

[RossGr]

A simple cap to ground should do it, you will need to need to get the right value of capasitance to filter the 60hz but they should be easy to come up with. ( I would think farily large (1000mF is a guess) there are calculations to find the correct value, can't recall the formula off the top of my head, and have not referance at hand. Perhaps others will be able to provid the value.
EDit:

I did a quick web search for you Found this It looks very similar to what you are wanting. They use 5000mF.

 

[Zugzwang152]

doesnt 1000mF = 1 Farad?

 

[RossGr]

Pardon me, I used m = micro (which is the common units of capasitance.) You are correct it is generally reserved for milli, but I can't make a mu here! Sorry

 

[NovaTone]

low pass filter with a pole frequency just above DC

 

[InverseOfNeo]

Originally posted by: NovaTone
low pass filter with a pole frequency just above DC

I would actualy say a high pass filter because the AC (assuming its 60Hz) is a low frequency so a LPF would pass the AC whereas a HPF would block the 60Hz AC signal.

 

[NovaTone]

We want to keep the DC signal, not filter it out, so you use a low pass filter with a pole at say 5 Hz to pass frequencies below 5Hz and to filter out frequencies higher than 5Hz.

 

[RossGr]

Hey! I thought I posted a link to page with a simply 18V PS that used a 5000MicroF cap. Not sure where it went, humm.... you don't suppose that it is over in the .999...=1 thread???

 

[arcas]

You might try using superposition to cause the AC current to cancel itself out by setting up a feedback loop with a 180deg phase shift. If you use a capacitor filter in the loop to block the 9VDC from feeding back, what you should be left with after cancellation is the original 9VDC component.

 

[Analog]

Originally posted by: MaDHaVoK
I have a line with 9 volts DC and 18 Volts AC.... how do i get rid of the 18 volts AC?

what frequency is the AC? How and why is the AC there? This is important to come up with a real solution.