varying transformer output with ballast

Discussion in 'General Electronics Chat' started by lokeycmos, Aug 30, 2011.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    i have a transformer that puts out a few volts more than i need. im thinking that i could use an inductor on the primary side to create a voltage drop kinda like a ballast. is there any truth to my idea or am i totally wrong? is there a formula for this application? please let me know if there is a better way. thank you!!
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    An inductor in an AC circuit will cause a voltage drop, that's right. However, the transformer output voltage would become more susceptible to load changes too.

    Post your circuit, in-output voltages and output current. Are you going to rectify/regulate this voltage later? There might be better ways to do this.
     
  3. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    im building a power amp based on the lm3886. it has a maximum voltage rating of about +/- 80 volts (40+ and 40-). the transformer i have puts out +/- 110 (55+ and 55-) volts after rectifying and smoothing. im using the circuit in the attachment as my supply.
     
  4. #12

    Expert

    Nov 30, 2010
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    I suffer from high line voltage here...125RMS and it does nasty things like burn out the nichrome in my clothes dryer or make the high pressure sodium bulbs fail in half their predicted life. The clothes dryer received a Calrod heater element which is absolutely not recommended by the manufacturer and the HPS light fixtures got old 24 volt air conditioner transformers wired with 120V on the 208 winding so the secondary would buck the line voltage by about 15 volts. Works like a charm. Does not vary by load current (except for IR losses in the copper). In other words, it's stable, dependable, and safe.
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Your load will vary considerably, so will the voltage drop on the inductor and therefore on the transformer output, i.e. using an inductor in series with the transformer primary is not a good idea.

    If you consider buying an inductor, why not use a different transformer?

    If you want to use THIS transformer you will need to step down the output voltage, if you use any linear regulator you would waste quite some energy, if you use any switching method, the circuit will become much more complex.
    Best option is to use a different transformer.
     
  6. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    i was kinda afraid of that. i try to build things with what i have in my large parts bins without having to buy stuff. this is a nice transformer(for free). how does the "buck transformer" work that #12 is talking about?
     
  7. #12

    Expert

    Nov 30, 2010
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    Not knowing what country you are in is a pain, so I'll just pretend you are in USA, like me, and convert to RMS to keep it simple.

    Your transformer puts out about 40Vrms per half winding of the secondary, or 80Vrms center-tapped. You need to lose about 10.6 Vrms per half-winding.

    If you connect 120 Vac RMS power to the primary of the 24 volt transformer using the "common" terminal and the 240 volt primary tap. The transformer is labeled 120/208/240 so those are the proportions. 24 x 120/240 = 12 Vrms. You will need 2 of these transformers, one for each half of your secondary winding.

    Then connect the secondary of the 24 volt transformer in series with the secondary of the transformer that has too much voltage. In one direction, the volts will add and you will get 52Vrms per half-winding. Reverse the connection of the secondary of the 24 V transformer, and you will get 28Vrms per half.

    28 x radical2 = 39.6 Vpeak
    subtract 1.3 volts for the rectifiers and the result is 38.3 volts peak on the first filter capacitor.

    The other math is the current capacity of the 24V transformer. Suppose it is labeled, "40VA". Divide the 40VA by the rated voltage and you get the current the secondary is rated for...40/24 = 1.666...amps.

    Got it?
     
  8. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    could you do a simple schem in paint? yes USA. 122.5vac in. 80vac out. Thank you much!!
     
  9. #12

    Expert

    Nov 30, 2010
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    It's a PIB, but I'll do it. Takes time. BRB
     
  10. #12

    Expert

    Nov 30, 2010
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    I didn't connect the remaining 2 transformer commons to the power line because I don't know how to do that in paint.
     
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