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# how to reduce the ac voltage? from a mix signal? Answered

suppose that there is a mix signal (with 60vdc+110vrms). now i want to completely block the dc component (OBVIOUSLY I NEED A CAPACITOR BUT)what should be the specifications* of the capacitor. and if i want to reduce the the dc component to 15v then what should be the specification of the capacitor.

ALSO
i need the complete calculation/procedure required to find each of the value of the capacitor.

*by specification i mean
1- capacitance
2- voltage rating
3- type (ceramic/electrolytic etc)

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## Discussions

The voltage at the output of the generator will be the AC signal added to the DC signal. It will be a sine wave offset by the DC. The max voltage the cap will ever have across it will be the peak value of the AC signal plus the DC. So, for this case it would be 60V + 1.41*(110 VRMS), or about 215Volts. Your cap will need a rating at least this large, so choose one with at least the next availble rating higher than 215V.

The cap must be a non polarized part, as it will see negative voltages, so most common electrolytics would be unsuitable.

The capacitance you will need will depend on what you want to do with the output. If this is a high pass filter type of application, then your cap value will be chosen based on the desired frequency rolloff. You don't specify the frequency of the 110V RMS. If it is 60Hz, then a larger cap value will be needed if the signal needs to be passed through with very little attenuation.

yes its a simplified version of a one port network
and the frequency of the ac signal is 30hz

and i have seen polarized caps in many ac instruments is this wrong to use ?

now i need the calculations to reduce the dc voltage to 15 voltage (complete calculations)

If the load on the reduced dc output is known, then a voltage divider could be used. The calculations are simple and very widely available.

If the load is variable, a linear regulator can be used. Depending on the loading on the output, the power dissipation in the regulator may need to be considered and mitigated with proper heatsinking.

Overall, the questions you ask a somewhat vague, so I can't really get any more specific in my responses.

So--what's the goal here?

Is this something that can be achieved with an elevated (or negative) DC component? I.E., having either an AC or a DC offset voltage? Or simply having two parallel power supplies (AC and DC) with a common ground?

RE: blocking only part of the DC voltage with caps--nope, it doesn't work like that. You might implement a low-pass filter, but it's a strange thing to first introduce the AC component, and then promptly try to filter it out--after all, you're starting with very stable DC (a battery).

An inductor would be the more appropriate component to block AC and pass DC. But any type of "voltage drop" is going to depend on the current draw, the impedance of the inductor (at the set frequency) and any voltage dropping resistors that are included.

Most of that is also the case with a low-pass filter.

At some point, using a low-pass filter would begin to look suspiciously like a reservoir capacitor...i.e., a filter cap. But there would be cap polarity issues...unless rectification were used.

So again the question must be asked--why combine the two sources, only to promptly separate them?

Also, if this application is being designed, why not make the DC supply 15V to start with?

bro its a one port network shown in the diagram and its not like that that i have attached a batterry and an AC source

That's not really enough information to answer the Q.

--Block the DC?
Use a simple RC high-pass filter. But the cap used depends on the impedance of the components following--and they are not shown on your diagram. So a definitive answer cannot be given.

If the next stage has very high input impedance, just use the standard formula (1/ 2pi R C) for the resistor and cap. Otherwise, more information is needed...

--Block the AC?
Why block it? Just tap the circuit between the battery and the AC source. Done. Electrically that will be no different than running it through various filters.

--Drop the DC voltage?
Any number of ways to do that. One way would be to use a voltage regulator.

If you want to drop voltage in "brute force" fashion with passive components, it cannot be done without knowing the load; I.E., the current draw expected.

What is the frequency of your a/c signal? I could assume 60 hz. but I assumed something yesterday and I was wrong. The capacity should be chosen for the frequency to be passed.

Your cap can be a mica or mylar, anything that is non-polorized. Most anything except an electrolytic. An electrolytic should never be exposed to a/c.

The voltage should be 200 v or more. I rebuild tube radios and amps and usually just use 600 volt caps. They only cost about a dime more and that way I don't have to keep a bunch of different voltages in stock. As long as the voltage is higher it all works out.

Looking at your schematic, I don't know what the 60 volt battery is going to do when the 110 v. a/c hits it. Maybe this is just your very simplified diagram of your project.

yes its a simplified version of a one port network
and the frequency of the ac signal is 30hz