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Problems with Inverter and transformer

  1. Apr 12, 2009 #1

    i am building an inverter 12VDC- 120VAC, 50 Hz with square wave as gate signal for MOSFET.

    Looks like my circuit works fine because i am getting square out -12 to 12 AC. Now i am using transformer to step up 12 VAC to 120 VAC. But i am experiencing some problems.

    But i am supplying just 12V from the power supply in lab and it automatically jumps to 16V when transformer is connected to the circuit. My square wave osc. freq is 50Hz and the output from transformer jumps to almost 54Hz. When i look at the oscilloscope pk-pk voltage is close to 400v.
    I am not sure if the inductance of the transformer is large and that's why it draws more current. i don't think 50Hz supply is hurting the transformer eventhough it is rated for 60 Hz.
    So could someone please help me find the explanation.

  2. jcsd
  3. Apr 12, 2009 #2
    Did you measure voltage with a separate multimeter, or just read off the power supply?

    It could be that there is some voltage being induced from the primary winding of the transformer if it's not shunted with diodes.

    You are measuring open voltage, see if you can put a resistor across the output just to the get the current going. Then measure it again.
  4. Apr 12, 2009 #3
    I suggest you use a 'True RMS' multimeter. An ordinary multimeter could show incorrect readings as inverters are non-linear loads, i.e. they do not produce a pure sinewave and often introduce harmonics into the system resulting in incorrect readings by ordinary meters
  5. Apr 12, 2009 #4
    Thanks all the suggestions.

    Yes i used separate multimeter to read the voltage as well as i used oscilloscope to see the measurement.
    When i used the regular lab power supply, i was jumping to 17V. But i used 12V battery source and the RMS voltage was close to 12-13V.
    So considering +.- 10%, i was getting sec. out from the transformer to be close to 400V since i am using 1:10 transformer. But actually when i used the car battery my frequency dropped to 46-47Hz even though i designed for 50Hz. Why does a transformer drops the frequency? When i measured, i measured the open voltage, i will try with the load and see some resistance/inductance helps.
    Moreover, i am trying to get the efficiency of the inverter i made. How can i measure the efficiency? I am using a square wave to pulse the gate of the comp. MOSFETs. And i have a full bridge inverter.

    Any help is appreciated. Thanks
  6. Apr 17, 2009 #5


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    Sounds like the inverter is not properly bypassed. AC from the inverter could be getting into the power source via the supply lines.

    Where the power supply leads enter the inverter, there needs to be a LARGE capacitor from the + supply line to ground. If you have one, try 5000 uF rated at 25 volts.

    As you have a CRO, have a look at the voltage across this capacitor after you install it.

    An alternative would be to try running it off a 12 V car battery.

    Your output seems a bit high, but maybe you could put two identical 120 volt lamps in series across it just to give it a bit of a load. One might be overloaded with your present setup.

  7. Apr 21, 2009 #6

    I used big capacitors to bypass the dc high voltage. It got rid of most of the problem. I have voltage swing once in few period.
  8. Apr 22, 2009 #7


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    Electrolytic capacitors do not work very well above audio frequencies, so if there are high frequency transients present, you may also need an additional capacitor.
    Something like 0.47 uF 200 V polyester would be good but anything would be OK as long as it is not an electrolytic or a tantalum.

    You should be able to see on a CRO if there is any voltage remaining across the capacitors.
    If there is, you can add extra capacitors to see what works best.
  9. Apr 22, 2009 #8
    Your frequency source should be a separate circuit like a NE555 timer to reduce the voltage sensitivity. Ideally the NE555 should be running at 100 Hz, and driving a JK flip flop to get 50 Hz. This will give you a 50% duty cycle square wave and ensure that you have no DC in the primary. For driving the transformer you should have a full-bridge (H- bridge) circuit.
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