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Transforming 24vac

  1. Jan 23, 2012 #1

    [Newbie question]

    I need to convert 24vac to ~5vac for ultimate rectification/regulation to +5vdc. I'm looking at transformers to step down to 5vac from 24vac. Most of what I'm finding take 115vac input. Would I be foolish to use a transformer rated at 115vac input to 24vac output, but input my 24vac instead of the 115vac and expect ~5vac out (i.e. 24v*(24v/115v) = ~5.00v)?

    My goal is to rectify the ~5vac (resulting in ~7vdc) and then regulate that to +5vdc using an LM7805.


    PS I tried a solid state approach using a TL783 to go from rectified 24vac (34 vdc) directly to 5vdc, but producing 50-70ma the TL783 heated a very hefty heat sink to the point that it was too hot to touch in ~15 minutes.
  2. jcsd
  3. Jan 23, 2012 #2
    If you can find a "regular" transformer that has the step down ratio you need (I think around 120->35v) you should be good to go.

    However, you don't mention how much current you are interested in? Looking at the specs, the 7805 would do ok with circa 34vdc as input if you don't need the full output current. Or, if your load is fairly constant and power efficiency is not an issue you could just put a power resistor and filter cap in line to the regulator to drop the input voltage to something reasonable. Ohms law will help in figuring out the resistance you need -- think of it as a voltage divider that operates at your needed 5v current.
  4. Jan 23, 2012 #3
    Thanks for the reply.

    The 'current budget' for my board is <=250ma. My most recent measurement was ~195ma but I do expect peak current consumption to hit (and possibly on occasionally briefly exceed) 250ma.

    Power efficiency is not an issue for this project as it is supplied by a 24 vac, 1 amp wal-wart.

    However, the LM7805 has a maximum VI of 35v, so I'm worried about 34vdc being too close to the max to allow for spikes, etc.

    One of the questions I have is how to determine the current requirements of the transformer I choose at lower input voltages. For example, given a 120/35v transformer rated at 200ma, how much current can I expect with a 24v input and 7v out? I'm sure there is a simple formula for this, but I can't seem to find it (or figure it out myself).

    I like your "power resistor" suggestion. However, while power efficiency is not an issue, my load does vary (i am driving 8 5vdc/12ma relay coils that come on/off in an unpredictable fashion). Does the variable nature of my load eliminate this as an option?

    Thanks again for your help.
  5. Jan 23, 2012 #4

    jim hardy

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    feeding the transformer with lower voltage will sorta work.
    what you'll find is the transformer's resistance will make the voltage sag .

    it's built to have some nominal voltage drop at full current, let's just say 5%.

    well, 5% of 24 volts is what % of 5 volts?

    when you lower the applied voltage you dont lower the resistance of the windings.
    and it gets worse - since you only draw current in big gulps near the sinewave peaks, the effect is exaggerated.

    you should run that experiment it wont take long and you'll get that gut feel for the problem. someday you'll use it to advantage so get it into your bag of tricks now.

    I'd look in thrift shops. There's always a basket full of "wall warts", those little transformers for video games and cellphones.
    If you can find a charger for 7.2 volt nicad battery packs...now there's a transformer !
  6. Jan 23, 2012 #5

    jim hardy

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    here - these guys literally wrote the book...

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