Change DC device to AC with battery backup

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  #1  
Old 06-01-07, 10:06 PM
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Change DC device to AC with battery backup

Is there a way/product that will convert a DC device (operates on a 9 volt battery) into an AC device (standard home voltage) with 9 volt battery backup? I would like to make the divice similar to smoke dectectors that run on AC with 9 volt battery backup. Any help on how to do this would be appreciated.
 
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Old 06-02-07, 04:43 AM
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Welcome to the forums! You won't change the device to AC, but use a transformer to change the AC to DC, using line voltage to the transformer. You can still use the battery backup with proper circuitry.
 
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Old 06-02-07, 03:19 PM
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Where can I find a transformer for this application and what is the circuitry to make it a battery backup?
 
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Old 06-02-07, 03:37 PM
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you would need a 9 volt (dc) power supply with a back up battery circuit that is controlled by the presence (or lack of) AC supply voltage to the power supply.

the control could be a simple relay with line voltage energizing the relay coil and the battery and the power supply output passing through the controlled contacts (one through N/O contacts (power supply) and the other through N/C contacts (battery)) I know what some may say, isn;t this backwards but no, it isn;t.

with the relay enerized the N/C contacts are open and the N/O contacts are closed. when the control voltage drops out, they will allow the battery voltage to pass and cut out the power supply voltage circuit (prevents voltage feedback into the supply. Not sure it would make a diff but no reason not to take this simple step to be sure)

I would use a 4 pole relay so you could run the pos an neg of each power source throgh the relay. That would totally isolate each from the other and prevent any problems.

Now if you are really into electical engineering, you could design and toss in a battery charging circuit to maintain the charge in the battery when not being used but that is another question not asked.

btw, you need more than a transformer. a transformer would only reduce the 120 AC to 9 volts AC. you want an unit designed to alter the power to DC as well as reduce the voltage.

what you are looking for very simply put is a 9 volt DC UPS. I don;t know of any or where one would seek one but I would imagine there may be some out there.
 
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Old 06-02-07, 03:43 PM
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those more into electronics may be able to design a simple electronic method of providing the same thing I described in a more compact design.

well lookey here. here is a schematic for exactly that. Notice the ac input is 15 volts so this circuit would need to be fed by a 120-15 volt transformer

http://www.delabs.net/cirdir/power/power-circuits/dact0014.pdf
 
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Old 06-03-07, 07:38 AM
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You would hook the trasnformer supply whivh has built int recrifiers and a filter, to the circuit as if it were the battery. Then hook ther battery to the circuit , -ve to the -ve on the circuit, +ve through a 1N4004 or similar diode, with the band lead on the circuit side.
 
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Old 06-03-07, 10:36 AM
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well, that link is a tough road to hoe. How about getting one of these little doobers of the correct wattage (you have never told us what you are wantung to power) and tie in a battery back-up.

http://www.world-import.com/adaptors.htm

Now I am not an electronics whiz but classicsats method looks like a a solution. Maybe he can come back and even design a charging circuit (obviously self regulating of course) to keep a chargable battery up to full charge while it is waiting in the wings to be used.

I'm just too used to larger electrical systems and overlooked how simply this could be done with low power systems.
 
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