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Is 12v DC from a battery same as 12v DC from a transformer?

  1. Mar 1, 2008 #1
    can anyone please help me, i want to know that whether 12v DC from a battery is same as 12v DC from a transformer ( after a 4 diode bridge has been fixed to it ).... please help, because i want to expeiment something, i removed the 12v battery of my UPS and put in its place a 12v transformer, but the ups signals it as weak current and the inverter doesnt turn on, even though the battery was 22Ah while the transformer is 30Ah , please tell me why doesnt the ups detect the transformer DC current.
  2. jcsd
  3. Mar 1, 2008 #2
    Transformers don't have Ah ratings. Can you give a few more details about the whole circuit?
  4. Mar 1, 2008 #3
    my transformer is a 12V AC ,and gives about 30 amperes... i put a 4 diode bridge to make it 12v dc like tht of my ups battery... but the ups doesnt detect the transformer current and flashes the low battery LED, my friend said i need to add a capacitor to the transformer... wht do u think ?
  5. Mar 1, 2008 #4


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    Your friend is correct. Effectively, you're still generating an ac current with your setup. You need at the minimum a capacitor and resistor to complete your project.
  6. Mar 1, 2008 #5
    wher do i place the capacitor and the resistor, and tel me kindly, wht does a resistor do
  7. Mar 1, 2008 #6


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    Well, it's been 28 years since I built mine, but according to my radio amateur handbook, they were placed in parallel across the + and - output terminals. The resistor's purpose was for discharging the energy from the capacitor when you turn off the power supply.
  8. Mar 1, 2008 #7
    okay, so the capicator wud b on the positive wire, and then there wud b a resistor on the other terminal of the capacitor and one resisotr on the negative terminal of the transformer,, right ??
  9. Mar 1, 2008 #8
    In the simple case, your circuit, without a filter cap and at 30A, is likely putting out a dc voltage in the neighborhood of 10V. The inverter thinks that's a badly discharged battery. Can you measure it?
  10. Mar 1, 2008 #9


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    The capacitor is connected to the positive and negative terminals.
    As is the resistor.

    Well this doesn't make sense. (I just took mine apart)
    The ac output of the transformer reads 13.4 volts.
    The dc output of the circuit is 16.6 volts under no load.
    That's weird.

    might explain the odd voltages.

    You really should use an oscilloscope to know what you are really measuring when dealing with ac circuits.

    Anyways, I see that I used a 15,000MFD capacitor and a 1/2 watt 4600 ohm resistor.
  11. Mar 1, 2008 #10
    Your voltages aren't that odd. With no load, the cap is trying to come up to the peak voltage of 18.9 (how far it goes depends on the cap and I am beyond calculating that today) and then you take off two diode drops and you're down to about what you see.
  12. Mar 1, 2008 #11


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    I don't see where yet another transformer in a UPS is going to do anything. It already uses transformers to convert AC to DC and back. Transformers don't store energy, batteries and capacitors do.

    The UPS is expecting a battery like device that it's constantly charging. The only device similar to a battery in this case is a very large capacitor, around 14 to 15 v DC to handle the charging current used in a UPS. Another option would be a DC motor/generator combination, that stores the energy as kinetic energy of the spinning mass.

    Why would you want to eliminate the battery?
  13. Mar 2, 2008 #12
    okay look, the function of the ups is that when the electricity goes out it wud keep my pc turned on for 10 minutes , that is the battery standby time, i hav a single phase power generator, when i turn it on and connect it to my house grid, everything works normal as the generator has AC current too, but the ups doesnt detect the generator current and keeps working on battery, what i did was got a 30 ampere transforemer that gave 12v ac coneverting the 220v ac of the generator... then i added a 4 diode bridge on the trnasformer to convert the volts into 12v dc, took the battery out of the ups, and conected the transformer to the ups, i thought the ups wud work as it did on battery, but it blinks the low battery led and doesnt start at all... thats my plan, and if anyone knows how to fix it then plz help cus iv bought an ultra expensive transformer and i cant return it back :( , also tell me whtever thing i hav to attact to the transformer besides the diode, how it has to b done and on which wires please..
  14. Mar 2, 2008 #13
    If indeed you have a 12 VC transformer voltage, you're not gonna get it to work properly. Your 12 VDC battery likely has a terminal voltage somewhere between 12.6 and 13.5 Volts when it has a reasonable charge. The 12 VAC transformer (assuming it really puts out 12.0 VAC) will give you between 10 and 11 Volts after rectification without a filter cap. If you have the battery in the circuit, that will act somewhat as a filter cap, but it will still usually have a higher voltage than the power supply, so the battery gets used.

    For a fix, you might see if you can add a few turns onto the secondary of the transformer. Or, you can buy one huge honking filter cap.
  15. Mar 4, 2008 #14
    what is a filter cap and what is it function, andhow will it help me replace the battery or ups wit the transformer, and my transformer has an option of making 14v dc too, so shud i use that option? but am scared if i conected the ups then to the transformer wile its giving 14 volts, something might happen to the ups... plz reply
  16. Mar 4, 2008 #15
    Do you have anyone, who has a multimeter and knows about power supplies, who could help you? There are some basics that you need to know and an internet forum is not the best way to learn them.
  17. Mar 4, 2008 #16
    u mean a clamp meter ?
  18. Mar 4, 2008 #17
    Normally a clamp meter is an ammeter, although some of them have the ability to be converted to a voltmeter which is what you really need.

    What you really need to know is this: the input specs for the UPS; the type of battery you have and all its characteristics; the exact voltages put out by the transformer (not by the rectifiers).
  19. Mar 9, 2008 #18
    Single phase at 220 V? is that the output of the generator? Is your house voltage normally 120V?

    If the ratio of the transformer you bought steps 220V down to 12 Volts, then if it was connected to 120V would output only 7 volts or so. (Also note: look at the bottom link here, the output of the rectifier will depend on whether you are dealing with peak voltage AC or RMS)

    make sure the voltages are right to start.

    Couldn't you just buy one of those things that plugs into the wall with a switch that outputs the desired DC voltage level, assuming it had sufficiant VA rating? like one of these things:

    Also, do you have any idea why the battery charger doesn't recognise your generator AC power?

    Heres a link showing how to set up your rectifier bridge, with smoothing capacitor:
    Last edited: Mar 9, 2008
  20. Jun 14, 2008 #19
    how to conect a capicator to a generator

    can anyone please tell me how do i connect an AC capacitor to my standby power generator( 2500 watts 20amps 230 volts ), i mean , should i connect it parallel to the load, or both the cap wires would b connected to the live wire coming from the generator ? please help.. thx, and tell me would it harm the generator's alternator if a capacitor is connected to it ?.. thx, also tell me the rating of the capacitor that i shuld conect to it.
  21. Jun 14, 2008 #20


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    If the generator is connected to the house wiring when power goes out, you should be able to just keep the UPS plugged in and have it run normally, without having to modify it.

    If you're not getting voltage at the UPS from the generator, there is a problem in how the generator is connected to the house wiring.

    This scheme of using a transformer and bridge rectifier just sounds all wrong to me. At the very least, you need somebody who is familiar with how they work there in person, to look at how things are wired up.

    p.s. By the way, with a capacitor properly connected across the rectifier bridge output, you should get about 15 to 16 Vdc open circuit. It will be something less than that when under load.
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