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By abdshebl
Dear All
I have a Central Battery System which outputs 240 AC Voltage during normal operation and in case the Central Battery system loses its AC input voltage it will output 220 VDC, i need a circuit that blocks DC Voltage and only output AC because my Exit Light Fittings doesn't accept DC Voltage and it has its own internal battery to make it run during power outage.

The idea of keeping this Exit light fittings to the central battery system is that there is monitor module that will report if the light fittings is not running during normal operation to change it in case it is defected.

I am thinking of using capacitor between the output of the monitor module (v-cg-se 4-400 w) and the Exit light fittings to block the DC Voltage is this right solution and what should be the capacity of the capacitor.
By theropod
Capacitors have no current blocking ability. You are not going to be able to do this with a single component of any sort. Your battery bank never outputs AC under any circumstances as batteries are not capable of this. I don't think you understand the system with which you are dealing, and the voltages present here are deadly. Consult a professional as advice provided here, if only slightly incorrect, could cost someone their lives.

By jremington
Capacitors have no current blocking ability.
If you are going to answer a question, at least give a technically correct response. Capacitors are very often used to block DC current, and pass AC current.

However, I do not believe that a system designed to output 240 VAC should EVER output 220 VDC, and agree that the OP probably does not understand his/her system.
By abdshebl
Dear Jremington
the Central Battery system outputs 240 VAC in normal operation when it has 240VAC input and during this it charges 18 series-connected 12V Batteries, when the Central Battery System loses its normal 240VAC input it switch its output to 220VDC without any inversion.
here is a link to the system ... ystem-zb-s

I tested the output myself, the problem is the main contractor has provided light fixtures that accept both 240VAC and 220VDC but by mistake he order Exit sign Fixture that only accept 240VAC and has internal battery, so they are looking for a solution to connect this Exit light to the system and when in emergency mode when it switches to DC they want to block it.

also here is a link to the module that monitor and do the switching. ( ... put_12.pdf)

can you help recommending a solution to do that
By mattylad
Just to backup the Op, its been a while but ISTR it is common for central battery system to also output DC when the AC fails for fire alarms and emergency lighting. It is inefficient to provide AC as that would flatten the battery quicker, so DC is used.
I have built systems that do this.

An exit sign MUST be illuminated when in emergency mode, it points people who may be panicking during an emergency to the exit door. That is what its internal battery is for.
If you want to de-power it when it is on DC only, so that the internal battery works then put an AC only relay inside to remove the incoming AC feed until the AC is restored.
This will of course mean that it may not last as long as the CB supply - or it may last longer.
However, BEFORE doing anything, consult your countries fire regulations.

You could just (after checking the regs) order the correct fitting, seeing as lives could be at stake, it might be the better option.
By jremington
That makes sense. Very interesting, thanks! Obviously, such a system should be used as intended, and not subverted.

@OP: you should replace the entire lighting system, rather than attempt to modify AC-only fixtures to ignore DC power. It would be totally impractical (if not impossible) to use capacitors to block DC in this case.
By theropod
jremington wrote:
Capacitors have no current blocking ability.
If you are going to answer a question, at least give a technically correct response. Capacitors are very often used to block DC current, and pass AC current.

You are absolutely right. Having messed about with circuits my entire life I had a misconception of the actual function of a capacitor. After reading your comment I went to my bench for some empirical testing. I connected a small LED in series with a small capacitor and a 5 volt DC power supply. With the capacitor in series the LED will not illuminate, but the instant it is bypassed the LED glows brightly. Indeed the capacitor will, and does, block DC current flow. Thank you for the correction. I was completely wrong.