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 Post subject: Are APC Battery Backup Transformers Useful?
PostPosted: Oct Tue 22, 2019 3:50 pm 
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I have several of these transformers I took out of small APC UPS backup power supply's for computers.
I tried hooking up the black & white wires to 120 volts and blew the 3 amp fuse in my varac.
I also have a big one that seems to work but draws 700 ma. unloaded, I thought a transformer does not draw currant unless under load, it is not a surge but a steady 700 ma.
However the two small ones blow fuses.
I was going to use them to make a linear 12 volt power supply for my 12 volt CCTV cameras.

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 Post subject: Re: Are APC Battery Backup Transformers Useful?
PostPosted: Oct Tue 22, 2019 7:13 pm 
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I have one of those supplies I no longer use, too, but I haven't torn mine apart yet. My guess would be that those transformers are a ferroresonant type and require a capacitor across on of the windings. Did you see an AC cap connected to any of the windings?
John

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 Post subject: Re: Are APC Battery Backup Transformers Useful?
PostPosted: Oct Tue 22, 2019 7:57 pm 
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OldWireBender wrote:
I have one of those supplies I no longer use, too, but I haven't torn mine apart yet. My guess would be that those transformers are a ferroresonant type and require a capacitor across on of the windings. Did you see an AC cap connected to any of the windings?
John

Hi John,
After I made this post I found out I made a big BOO BOO, I connected the line across the RED & BLACK heavy wires!
What a bone head I am...LOL
That is why I blew the fuse....LOL
I just went by the circuit board, those heavy red & black leads went to terminals marked were "LINE".. :shock:
Later on I used the two THIN red & black wires and everything was fine but the voltage across the heavy BLACK & WHITE wires wires read 15.6 volts AC and 20.1 volts DC rectified with a 2000 mf cap, that is too high for my cameras.

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 Post subject: Re: Are APC Battery Backup Transformers Useful?
PostPosted: Oct Tue 22, 2019 10:45 pm 
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Location: Long Island NY
Transformers are normally rated for output voltage under rated load. With no load the voltages will be higher. And if you rectify AC and connect a capacitor, the capacitor will attempt to charge to the peak voltage present (1.41 times the RMS AC voltage). Again, putting a load on it brings the voltage down.

Sounds like an ideal place for a L7812 voltage regulator to get you down to 12 volts. Something like a 7812 would do the job with nothing more than a couple of small capacitors https://cdn.sparkfun.com/datasheets/Components/General/TO-220.pdf, depending on how much current your camera draws. Providing a heat sink for it would be a good idea though.

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 Post subject: Re: Are APC Battery Backup Transformers Useful?
PostPosted: Oct Wed 23, 2019 2:33 am 
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Chris108 wrote:
Transformers are normally rated for output voltage under rated load. With no load the voltages will be higher. And if you rectify AC and connect a capacitor, the capacitor will attempt to charge to the peak voltage present (1.41 times the RMS AC voltage). Again, putting a load on it brings the voltage down.

Sounds like an ideal place for a L7812 voltage regulator to get you down to 12 volts. Something like a 7812 would do the job with nothing more than a couple of small capacitors https://cdn.sparkfun.com/datasheets/Components/General/TO-220.pdf, depending on how much current your camera draws. Providing a heat sink for it would be a good idea though.


Yes I know that but I want to keep it simple maybe using a fropping resistor in series with the DC output, that I think would be easy.
I have 4 cameras and the switching supply is rated for 6 amps but makes a lot of RFI on my AM radios.

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 Post subject: Re: Are APC Battery Backup Transformers Useful?
PostPosted: Nov Sat 02, 2019 3:16 pm 
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Location: dayton oh usa
some ups transformers are autoformers.
i dont recall apc being that design.
double check to be safe.


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 Post subject: Re: Are APC Battery Backup Transformers Useful?
PostPosted: Nov Sat 02, 2019 8:28 pm 
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Bill Benson wrote:
Yes I know that but I want to keep it simple maybe using a fropping resistor in series with the DC output, that I think would be easy.
I have 4 cameras and the switching supply is rated for 6 amps but makes a lot of RFI on my AM radios.

Unless you want to blowup your cameras you don't power them from 20V DC with a dropping resistor to get 12V DC! You cannot guarantee that all the cameras will always draws the same current. What will happen when all the cameras don't turn On or Off all at the same time or you turn on only one or two cameras or a couple of the cameras fail for some reason, the DC voltage powering the rest of the cameras will be all over the place and could be as high as 20V!

Just because your existing power supply is rated at 6A it doesn't mean your cameras need 6A! Measure exactly how much current all your cameras draw under all conditions. I have no idea what kind of cameras you have but unless they have some scanning or focusing motors, I doubt that each camera draws more than a few hundred of mA. I would try some different switching power supplies or a liner regulated power supply to power these cameras.


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 Post subject: Re: Are APC Battery Backup Transformers Useful?
PostPosted: Nov Sun 03, 2019 8:44 pm 
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simple bridge,big filter cap,and a few 7812 will do the trick.
i suspect 6 cameras are going to want more than 1a.
so you might go with 3 separate 7812 circuits.
apc transformers are usually good quality.
i have used them in the past for replacements in car battery chargers and small power supplies.
i even built a few around them.


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 Post subject: Re: Are APC Battery Backup Transformers Useful?
PostPosted: Nov Mon 04, 2019 9:15 pm 
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Location: Long Island NY
Agree that a big dropping resistor in series with a 20-volt supply to power delicate 12-V electronics is begging for trouble. There are any number of circuits online for adding one or more pass transistors to voltage regulator ICs like the 7812. With the right pass transistor(s) you can handle all the current your transformer is capable of dishing out. The best designs include foldback circuits so the voltage regulator shuts down if the pass transistor fails or if something shorts the power supply output.

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 Post subject: Re: Are APC Battery Backup Transformers Useful?
PostPosted: Nov Tue 05, 2019 8:17 pm 
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Thanks guys.
All my cameras are cheap EBAY cameras, no special features, just a camera with 48 IR LEDs.
They are not that good in darkness and not very clear but better than no cameras at all.
One of the cameras goes out (no video signal) and I thought it was the power supply but I get 12.3 volts at the camera so it is the camera itself.
The camera does come back on by itself, but I can't expect much for a $20 Chinese camera.

Thanks again for the info.

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