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Thread: High current voltage doubler with autotransformer

  1. #1

    High current voltage doubler with autotransformer

    Hello everyone!

    What is given:

    1x 24V 2600w car inverter (due to arrive in a week)
    1x 1500VA UPS transformer (primary intact, 9-0-9 VAC secondary intact)
    1x 600VA UPS transformer (primary might be burnt, 9-0-9 secondary intact)
    1x 12V Pb car battery

    I want to use the 1500VA UPS transformer to charge the battery and then power the inverter from the battery. Unfortunately, due to my lack of attention, i bought the 24v version, not the 12v.

    Could I pulse the DC from the battery into the 9-0 half of the 600VA UPS transformer secondary and get the doubled voltage? Like in a autotransformer. To be more specific, the secondary has blue-brown-red wires (corresponding to 9-0-9) and i want to connect the ground to blue, pulse 12v into brown and collect 24v from the red wire (referenced to blue).

    The inverter will be used to power my beast of a computer which pulls around 1200W from the wall (display included) when in full load, essentially making an active UPS from two dead UPS'.

    I have access to high current mosfets, mostfet drivers, 555 timers and atmegas. Schottky diodes and low ESR capacitors are not a problem to come by around here. I personally have a cheap hobby oscilloscope, part analyser, multimeter and signal generator. For anything else, i have access to a laboratory with lots of instruments. I also am familiar with power electronics, logic, microcontrollers and programming for them.
    Last edited by brlmat; 05-09-2018 at 10:09 AM.

  2. #2
    diysmps Senior Member Silvio's Avatar
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    Quote Originally Posted by brlmat View Post
    Hello everyone!

    What is given:

    1x 24V 2600w car inverter (due to arrive in a week)
    1x 1500VA UPS transformer (primary intact, 9-0-9 VAC secondary intact)
    1x 600VA UPS transformer (primary might be burnt, 9-0-9 secondary intact)
    1x 12V Pb car battery

    I want to use the 1500VA UPS transformer to charge the battery and then power the inverter from the battery. Unfortunately, due to my lack of attention, i bought the 24v version, not the 12v.

    Could I pulse the DC from the battery into the 9-0 half of the 600VA UPS transformer secondary and get the doubled voltage? Like in a autotransformer. To be more specific, the secondary has blue-brown-red wires (corresponding to 9-0-9) and i want to connect the ground to blue, pulse 12v into brown and collect 24v from the red wire (referenced to blue).

    The inverter will be used to power my beast of a computer which pulls around 1200W from the wall (display included) when in full load, essentially making an active UPS from two dead UPS'.

    I have access to high current mosfets, mostfet drivers, 555 timers and atmegas. Schottky diodes and low ESR capacitors are not a problem to come by around here. I personally have a cheap hobby oscilloscope, part analyser, multimeter and signal generator. For anything else, i have access to a laboratory with lots of instruments. I also am familiar with power electronics, logic, microcontrollers and programming for them.
    Make a schematic of your idea maybe we can understand better what are your intentions.

    One other question is how are you going to supply 1200W form a 12V battery?
    A small calculation is 1200W / 12v that will be 100amps giving 80% efficiency will be around 120 amps
    Well the load is like cranking on a car engine. I do not know how long it will last at that current.

    One other thing is that your ups is 24v then you need a 24v charger for the battery. You can work on mains till you have a power cut then the battery will give you time to close down. Only a change over relay is needed without a lot of complications
    Last edited by Silvio; 05-09-2018 at 06:40 PM.

  3. #3
    The transformer without the center tap is the bigger one. It will keep the battery charged all the time and provide power to the rest of the circuit. The smaller transformer is the one which has it's primary unused. The mosfet in the middle of the drawing will switch the regulated 12v in the center tap of the secondary and then the outside edge of the secondary is filtered and fed into the inverter.
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    Last edited by brlmat; 05-09-2018 at 07:52 PM. Reason: Wrong photo

  4. #4
    diysmps Senior Member Silvio's Avatar
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    Quote Originally Posted by brlmat View Post
    The transformer without the center tap is the bigger one. It will keep the battery charged all the time and provide power to the rest of the circuit. The smaller transformer is the one which has it's primary unused. The mosfet in the middle of the drawing will switch the regulated 12v in the center tap of the secondary and then the outside edge of the secondary is filtered and fed into the inverter.
    So I am assuming that you are going to make a boost converter in a way and regulating the output voltage to around 24v to the ups. (invertor)

    1) I cannot see the reason for inserting the boost converter in the middle and not taking the DC voltage directly from the battery. This will only bring the need to feed the battery with an adequate charging system not to let it drop with load.

    2) Starting with 24v battery will make life simpler and the current will be half of 12v around 50-60A

    3) Having iron cored transformers switching at 50 to 100 Hz at that current is not an easy task and the system may not be so efficient as a lot of current is involved. Proper devices have to be selected. This is not an easy task but if you have the time and money to experiment well please yourself.

    Regards Silvio

  5. #5
    I have a pretty limited budget and i already have all the parts. The boost converter from 12 to 24 is needed for the inverter. The efficiency is not that big of a concern as it will mostly sit around 600w. My pc reaches 1200 only when gaming. Also I was thinking about switching at a few KHz

  6. #6
    diysmps Senior Member Silvio's Avatar
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    Iron core transformers cannot be switched more than 400Hz as magnetic flux will not transfer energy to the other windings and the core material is not suitable for a higher frequency than that stated.
    If you think that the transformer that you are going to use for the boost converter has its secondary winding burnt (220v) or shorted then this will affect to whole transformer and will not work for you even if it has one shorted turn.

    You can check it out by means of giving it 220v AC and see the output of the secondary 9-0-9 with the volt meter. Check also the current draw and see that it is within a reasonable limit. This should draw milliamps. Put a 100w lamp in series when trying out the trafo so that if there is a short circuit in the winding it will protect from blowing fuses etc.

    Have you found a circuit to start with your project?
    Last edited by Silvio; 05-10-2018 at 11:36 AM.

  7. #7
    The 220v winding tests as open circuit, but the 9-0-9 is intact.
    I'll make the schematic myself some time today. It will be based on an arduino with feedback loop and switching at a frequency of around 200hz or something the arduino can do by using timers. I'll post the schematic and code when ready

  8. #8
    diysmps Senior Member Silvio's Avatar
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    Ok we see how it gets along we try to build it

  9. #9
    I haven't had time to make a schematic, but i have tried the circuit (built off of my thoughts), both with only a high side mosfet and with a high and low side mosfet, but i only got heat and lots of it. I have tried different switching frequencies (50hz, 100 hz, 200 hz, 400 hz and 7khz) at 10% duty cycle and 50% duty cycle. The highest voltage i could get was close, but under the input voltage. on the oscilloscope it looked like the free wire pulsed almost the same way as the input (the drains of the mosfets) at low frequencies and like the inverted input with a peak to peak of 12,2v (same as the input voltage) but centered around 0 (so +-6.1 v), roughly. The only test that didn't set my wires on fire was the 7khz one. I am thinking more and more that i need a primary and a secondary winding, like a classical transformer, not a autotransformer.

    Any thoughts ?

  10. #10
    Your schematic isn't ever going to work. Even with a properly configured schematic, you are going to need quite high power switching devices and a ferrite cored auto-transformer, so it can be used at a high switching frequency, to even have any hope of getting the amperage required to drive that inverter at full power. You could build a conventional high-current boost converter to place between the battery and the inverter to boost the voltage up to 24V, but then you are introducing a lot on inefficiency into a system that needs to be as efficient as possible to give maximum run-time during a power outage. Your best bet would be to buy a second battery, exactly like the first one, and run them in series to give you the 24V, then find a way to keep them charged. That way you can run the inverter directly from the batteries.

    -Brad

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