Change 12VDC to 12VAC, can it be done without transformer?

  1. I've 3 batteries ( each 3kw X3= 9kw ) and If Inverter for 9kw is too expensive, I just want the 12V AC, so it can be done without transformer? So, in this diagram: Can I just remove the transformer and put powerful transistors to withstand 9kw input VDC?
    It can be done and how can I edit the diagram without transformer?
  2. jcsd
  3. it can be done without a transformer. I'm not sure about that circuit. Usually, converting DC to AC without magnetics is done using something called PWM, pulse-width modulation. The idea is to pulse the 12V in such a way that the average power is a sine wave.
  4. berkeman

    Staff: Mentor

    Welcome to the PF. Google "motor-generator" :smile:

    BTW -- Those sound like some *big* batteries!
  5. Baluncore

    Baluncore 3,305
    Science Advisor

    @ mosta. The circuit you linked requires the centre tapped transformer primary to operate.
    You may as well keep the secondary to increase the voltage and so reduce the output current.

    I would expect the heavy batteries from a telephone exchange to have a 3000 amp hour capacity.
    Are you sure your batteries are rated at 3 kW ? Is that kilowatt or amp hours?
  6. NascentOxygen

    Staff: Mentor

    Best advice will be forthcoming if you explain what you want to use the 12V AC for. Don't skimp on the detail. :approve:
  7. 3 batteries, 12V 250AH.. indeed, I've got those batteries as gift long time and I want to use it but inverters for that capacity is kinda expensive so I tried to think in a way to make it AC and 220 with cheap methods.

    And I've a lot of small rechargeable batteries ( 18650, 3.7v 3000mah ) so I thought to wire 50 of them with my main batteries in series to increase voltage to 220VDC. then just with circuit that change it to sinewave, and make it more simple (especially because I am just amateur in electricity)

    So, What I want 3 main batteries + wired with small ones in series to increase voltage to 220VDC and change it to AC easily by simple powerful circuit. And it'd be much cheaper I think

    I hope these things I said is enough

  8. NascentOxygen

    Staff: Mentor

    So you're really wanting 220VAC?

    Do you have in mind powering something in particular with that 220VAC? There are different types of "AC" from inverters, that's why your end goal is vitally relevant here.
  9. Baluncore

    Baluncore 3,305
    Science Advisor

    You may have a problem there. Lead acid batteries that are left standing for some time without having charge maintained by trickle charging will develop a high resistance and fail to meet their specifications. Your batteries may have died, you will need to test the voltage while under a high current load.

    If the batteries are OK there is a possible low cost solution to getting HV AC. Get a 1 kW UPS that uses either 12V or 24V batteries. Get it cheap, second hand because it's batteries are dead. Replace the dead batteries with yours, maybe fitted on longer leads in an external battery box. You then have high voltage AC.
  10. 9 kW is really a lot of power. There is a good reason it costs that much. Not the least of which is that very few people need anywhere near that much power. Average home power consumption is usually between 1 and 2 kW. So clearly your 9kW is an industrial amount of power with serious safety concerns.

    Making realistic restrictions based on what you actually plan to use the power for is by far the easiest step for reducing cost.

    p.s. Creating a circuit for 50 small (and expensive) rechargables to use in conjuction with 3 large batteries is a not a good idea. Understand that if the 50 batteries are in parallel, small difference between each battery mean the highest voltage battery drains first and by a different amount than the other batteries. And if they are all connected in series, the normally ignorable internal resistance becomes significant, and the first battery in the chain potentially drains at a much rate than the last battery. And since they are lithium you have to be careful about overcharging and overdraining, or you have a significant fire hazard. Doing this system WELL is pretty difficult. Not doing this system well is dangerous and waste of the batteries.
  11. Baluncore

    Baluncore 3,305
    Science Advisor

    The 9 kW estimate was reached by assuming a discharge time of only one hour for the three fully charged good batteries.

    When when a 12 volt battery, rated at 250 amp hour, is discharged in one hour, the load needs to be rated at 3 kW.
    But if the battery is part of a daily cycle power system, to be discharged over 8 hours, the load will be only 375 watt.

    If the three 12V batteries are wired in parallel, it will give 12V, with 750 amp hour capacity.
    12V, 750 amp hours, divided by 8 hours = 1125 watt.

    If the three 12V batteries are wired in series, it will give 36V, with 250 amp hour capacity.
    36V, 250 amp hours, divided by 8 hours = 1125 watt.

    So we are not necessarily discussing a 9 kW inverter here, we are probably discussing a 1 kW inverter powered from a low voltage. An uninterruptible power supply, (UPS), will usually convert a low voltage, (12V, 24V, 36V or 48V), battery supply to 115VAC or 230VAC.
    Unless there is a good reason for discharging the batteries in 1 hour, a UPS is the way to go here.
  12. Seems like 9 kw is scary number for you :D, anyway. If we said it is 1 kw, I think it can be done and worth?
  13. Baluncore

    Baluncore 3,305
    Science Advisor

    1 kW can be done for less than $100. Search eBay for something like this …
    Cobra CPI 1000 - DC to AC power inverter - 1 kW CPI-1000
  14. NascentOxygen

    Staff: Mentor

    Not really. But the cost of the high power transistors needed? Now that is scary.
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