120VAC Step Downs

Discussion in 'The Projects Forum' started by 40connorsmith40, Jan 11, 2011.

  1. 40connorsmith40

    Thread Starter New Member

    Dec 26, 2010
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    Hey guys,

    I'm planning on building a TDA7294 circuit (http://www.st.com/stonline/books/pdf/docs/1057.pdf)

    I've been playing around with smaller amps such as the tda2050 and tda2030 but so far I have just been using batteries to power them. I recently found an old wall unit that used a transformer, rectifiers, and a capacitor to drop the voltage from 120VAC to about 10VDC and I hooked this up to a TDA2030 and it works fine... But I'm kinda confused about the +40,-40, and ground required to run a TDA7294. Now if this were 12V I'm pretty sure I could get +,-, and ground using a split power supply? but how am I to get 40V. Also, does it matter if this voltage is AC or DC? And if I need to use a transformer, where do I get one that steps down 120 VAC to 40V. Basically what I'm asking is what is the best way to drop 120V to 40V?

    Thanks!

    Connor Smith
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    The datasheet you cited shows the IC will work at a minimum of ±10v, so if you want to use ±12v, it should be OK. And yes, it needs to be DC. If you want higher voltage, you can find a different transformer+rectifier+filter cap. Wall warts aren't usually above 12v.

    But, if you really want good audio power, you'll want to look at better power supplies. Controlling the power is relatively easy - the limiting factor is usually supplying the power.
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,753
    760
    U will need a center tap transformer of around 200VA if you want to drive two channels for stereo operation.
    Voltage rating should be 120VAC primary to 24V-0V-24V center tapped secondary.
    With this secondary voltage you can get approx ±33V for the TDA chip.

    Note that ±40 is the max rating so it is not advisable to operate it at that voltage unless you can provide very good heat sinking as these chips will generate lot of heat.

    on a side note
    I did manage to fry 2 chips just by replacing it without checking the rest of the circuitry. Bear in mind that these chips will blow quite easily if you made a mistake in the circuit.

    With a 200VA transformer, you can get around 3A for a stereo audio reproduction.
    The current demand depends on your speaker load and Transformer secondary voltage ratings.

    So first you need to find a Tx. that has 120VAC primary to secondary voltage anywhere from 20V-0-20V to 28V-0-28V max. The secondary current rating to drive one channel will max at 2A.
    So to avoid tx overheating you can get a 4A secondary current rating Tx to drive a total of two TDA Chips.
     
  4. 40connorsmith40

    Thread Starter New Member

    Dec 26, 2010
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    Okay, thank you both!

    Now, I don't have very much experience with transformers. If I get a 24V-0-24V transformer, then how does that make +-33 volts. Also, for the ground would i just use the "earth" wire from the wall?
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,753
    760
    Nope..

    The ground wire used for the amp GND will be connected to center tap that is 0V of the tx secondary.

    From the 24-0-24 tx you will have to bridge it. Bridge will be connected to the outer 24V connections. This way the Bridge out will be capacitor filtered to 67.872V from negative to positive.

    This 67VDC when referenced to the amp GND you will get ±33.5VDC
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Here's how to connect it up:

    [​IMG]

    Be certain to use a fuse.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    If the OP is fairly new to such things, could he end up fitting a couple of rather big 15Ω resistors?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    That was just representative of a ~2A load per rail, but of course I didn't mean a purely resistive load.

    10mF = 10,000uF. I just used the 10mF for shorthand; it's really outdated though.
     
  9. 40connorsmith40

    Thread Starter New Member

    Dec 26, 2010
    5
    0
    Thanks SgtWookie,

    Ok so if 10mf=10,000uf then I'm guessing the 15 on the resistors is Ωs? Do the resistors need to be a specific wattage (not sure if that's worded right)? And I've never worked with rectifiers, are there certain types I should use?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Ah, no - don't connect in Rload1 nor Rload2. They just represent a load. I didn't happen to have your amplifier in the library.
     
  11. 40connorsmith40

    Thread Starter New Member

    Dec 26, 2010
    5
    0
    Alright, thanks again

    Last question: Are there any links or articles you can post that explain more in depth how bridging a transformer works? And how to know what voltage you are going to get out? Is that based on the cap size? Because I'm still having trouble understanding how you get 33v out of this size transformer.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    A 24v-0v-24v transformer rated at 3A will output (nominally) 24v*1.414 = 33.936v peak output, less the drop in the diodes (about 1.4v).
     
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