Build a DC-AC Inverter into a project

Discussion in 'The Projects Forum' started by pcon2009, Jan 6, 2013.

  1. pcon2009

    Thread Starter New Member

    Aug 5, 2012
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    0
    I am interested in building, as a part of a larger project, a 12v DC to 120v AC inverter. I have been reading around on the internet and on this forum a bit and I think it is possible, but I want some confirmation and a bit of help before I go leaping into it.

    First, the reason I want to build it and not just buy a ready-made one is that it will be a part of a larger project, built into a case with its own batteries. The larger project is actually to be used to briefly open pneumatic solenoid valves. I work in the special effects industry and want to build a controller that can fire our confetti cannons. This is typically is just done with mains power and a switch on a power strip. I want to have a controller that is capable of doing this on battery power so that local power is not needed, and also so that we have more "safety" than if someone bumps a switch and shoots the air cannons.

    What kind of circuitry would I need in order to do something like this? I would like to use this battery, because it is something we stock for use with other equipment:

    PS-1229:
    http://www.power-sonic.com/images/powersonic/sla_batteries/ps_psg_series/12volt/PS-1229_11_Feb_21.pdf

    I have looked around and this circuit seems to be the simplest solution:

    [​IMG]


    C1, C2 68 uf, 25 V Tantalum Capacitor
    R1, R2 10 Ohm, 5 Watt Resistor
    R3, R4 180 Ohm, 1 Watt Resistor
    D1, D2 HEP 154 Silicon Diode
    Q1, Q2 2N3055 NPN Transistor (see "Notes")
    T1 24V, Center Tapped Transformer
    Misc. Wire, Case, Receptacle (for output)
    Fuses, Heatsinks, etc.

    (Found at: http://www.i4at.org/lib2/inverter.htm)


    That seems like something I could easily put together. Would it work? When I go back to work on Monday I will try to find the specs on the solenoid valves (how much they draw, etc). The output from the controller would just be standard AC wall outlet type, switched on or off by the toggle switches on the control surface. I know that the solenoid valves probably draw quite a bit of amps/watts, but it is only for a very brief period of time (around a second) that they would be switched on.


    (For future reference as well, I would eventually like to add in a charger circuit for the battery. This could be achieved either through a "barrel jack" to connect an external transformer and supply the DC charge voltage, or perhaps through an IEC plug with an internal charge circuit. I wanted to include that information in case it throws any kind of wrench into the gears, and also in case it is a simple enough solution that someone could answer it on this post as well.)

    Thanks all!
     
    Last edited: Jan 6, 2013
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,747
    759
    That ckt is OK for light loads.

    For heavy loads u have to have a better one with proper regulation
     
  3. pcon2009

    Thread Starter New Member

    Aug 5, 2012
    18
    0
    In this case, what would determine a "light" or "heavy" load specifically? I was able to look up the specs online of the solenoids this would be activating, and they are rated at 20W draw each. Does that constitute a heavy load? How many could I power at once with this configuration if at all?
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    You can buy a new 100W inverter for under the cost of the parts needed to build the one you are looking at. That inverter would have the benefit of overload protection, low battery shutdown, regulation, and more.
     
  5. pcon2009

    Thread Starter New Member

    Aug 5, 2012
    18
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    The only issue with that, thatoneguy, is that the one I would be building is something of a prototype and we may be building more in the future, so it seemed like it would be a better idea to build from scratch in order to put it into the form-factor (enclosure and control surface) that I want. If I bought one, though, is it something I could take apart, to some extent, and just keep the internals, wiring the input directly to the batteries and the output through the switches, etc, to the 6 outlets on the back?
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    You will end up spending a great deal of money on just the transformer.

    Look at the better schematics for inverters. There are literally dozens of threads here of people trying to make the one you posted work. It has several disadvantages, such as a square wave output going into an inductor, which wastes a lot of power straight off.
     
  7. Tahmid

    Active Member

    Jul 2, 2008
    344
    25
    If you ARE going to design an inverter with a 50Hz transformer, you should be looking into using PWM controllers (that can deal with 50Hz) and incorporating additional circuitry to add additional features such as battery low-cut, overload, etc. Using a PWM controller has several advantages, the largest one being feedback - your output will stay fixed with line and load variations. One popular PWM controller is SG3525. You could start at first by trying to get the SG3525 to work properly with no feedback and then proceed to add feedback and additional features. For a simple basic tutorial on SG3525, check out:

    http://tahmidmc.blogspot.com/2013/01/using-sg3525-pwm-controller-explanation.html

    Keep in mind that this will give you a quasi-sine wave output. If you want a sine wave output, you can use SPWM.

    Before you attempt SPWM (if you want to, that is), get the square wave inverter working.

    Hope this helps.
    Tahmid.
     
  8. toffee_pie

    Active Member

    Oct 31, 2009
    162
    7
    you can get a cheap modified sine wave for the output or a pure sine wave, which is more difficult and all depends on what you need it for.

    i agree than unless absolutely essential you would be better off buying a COTS unit than making your own.
     
  9. pcon2009

    Thread Starter New Member

    Aug 5, 2012
    18
    0
    I think I am just going to go with an off-the-shelf unit and build it into the case. I can, presumably, just use as I mentioned above the internals and wire the input direct from the batteries and the output to the different receptacles.

    There is actually talk now of changing the 120v solenoid valves on our units over to 12v DC solenoids which would mean I don't need an inverter in the end. Just depends on what the company goes with. I would like to have a unit built for both but I worry about someone using the wrong voltage and frying a bunch of equipment. I think I will just make two units, clearly labeled, one for 120vAC output with a store bought (and slightly modified) inverter and a 12vDC version without the inverter.
     
  10. donpetru

    Active Member

    Nov 14, 2008
    186
    25
    First, what is the power you want to own inverter? You said 120VAC but how much amps?

    Second, indeed, the scheme presented above only works at low loads, such as a lamp or something like that.

    SG3525 inverter is suitable in applications that modify a voltage greater than 12VDC to another AC or DC voltage. If the input voltage is 12VDC when an application with the TL494 is more appropriate. So, it would be better to look for a electronic diagram with TL494, like the diagram atach below.

    In the diagram, to obtain 120VAC, the transformer must change. In this case the current bridge rectifier can be small but should not miss to better stabilize the output voltage. You need to add an LC filter on the output transformer to filter higher harmonics and get 50 or 60Hz signal. Rectifier bridge above scheme will link the transformer windings before the filter. These are just some tips.
     
    Last edited: Jan 7, 2013
  11. Tahmid

    Active Member

    Jul 2, 2008
    344
    25
    donpetru,
    Can you please reupload the image as there is some problem with the image link?

    SG3525 is okay for 12V input. The minimum voltage is 8V. So for 12V SG3525 is fine.
     
  12. donpetru

    Active Member

    Nov 14, 2008
    186
    25
    I loaded image again. Now there should be no problem.

    Indeed, both ICs can operate at 12Vdc but from experience, I noticed that TL494 goes something better.
     
  13. Tahmid

    Active Member

    Jul 2, 2008
    344
    25
    Yes, both TL494 and SG3525 can be run from 12V and so it comes down to preference you could say. As for performance and reliability, there certainly is no issue with SG3525 running off 12V. There are literally millions of inverters with 12V input in Bangladesh and India that have SG3525 as the PWM controller and run smoothly for years.
     
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