Convert 12v ac into 12v dc

Discussion in 'Electronics & Electrics' started by echelon6, Feb 13, 2020.

  1. echelon6

    echelon6 Member

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    Hi all

    What's the way to convert 12v ac into 12v dc?

    application is rangehood light power source is the old halogen type, which produces 12v ac. I want to retrofit LED that uses 12v dc. Cannot locate power supply to replace this, it's integrated into rangehood. Need to buy a circuit from ebay to turn 12v ac into 12v dc. But worried about buying the wrong one, some circuits need input v > output v by a minimum of 2v or similar. Would something like this do? https://www.ebay.com.au/itm/323367530492
     
  2. dakiller

    dakiller (Oscillating & Impeding)

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    Yes.

    When AC gets turned into DC, that is call rectification, and when you do that the DC you get out is around 1.4x of the AC input, so you have plenty of headroom.
     
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  3. OP
    OP
    echelon6

    echelon6 Member

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    Thanks mate

    Why 1.4x the AC input? I assume 12v ac refers to the rms so at 100% efficient rectification and ripple removal, should it not just be 12v dc?
     
  4. th3_hawk

    th3_hawk Member

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    Any reason you're not just throwing a 240v LED bulb into it? Mines been in there for years without issue.
     
    Last edited: Feb 15, 2020
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  5. dakiller

    dakiller (Oscillating & Impeding)

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    12v AC is the RMS value of the sinewave, such that if you were to feed that into a load resistor it would generate the exact same heat energy output as a constant 12v DC would, ie it does the same work, so that's why we use that measurement. But the peak value of the sinewave is sqrt(2) * the RMS value (the 1.4 approximate)

    Rectifying the AC in DC wouldn't change the RMS value at all (ignoring the diode losses), but you have all the ripple there still. Putting in a smoothing capacitor will reduce the ripple, but it will still have the 1.4x peaks there, and the more ripple you remove, the closer to this 1.4x your DC gets, so the peak AC value is what you end up with as DC.
     
  6. OP
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    echelon6

    echelon6 Member

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    I thought as you add capacitance to reduce ripple, the DC v will approach RMS, i.e. it will go down.
     
  7. dakiller

    dakiller (Oscillating & Impeding)

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  8. ShadowBurger

    ShadowBurger Member

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    rectify your AC voltage and add a capacitor to smooth. it will come out at something like 17v dc. Then use a run of the mill 12v regulator. If that suits your needs speak up and I can give you more info, links to parts etc

    Otherwise that one you linked will do great as an off the shelf solution.
     
    Last edited: Feb 13, 2020
  9. OP
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    echelon6

    echelon6 Member

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    Thanks very much, learned something today. Always did thought a smoothing cap will bring Vdc to rms due to conservation laws but it makes sense - voltage is not a conserved quantity
     
  10. ArmoureD

    ArmoureD Member

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    The output on your rangehood elv electronic transformer may be a high frequency type outputting an ugly ac waveform. It may also have a minium load rating. Not too sure if the circuit you mentioned will work in the scenario.
     
  11. oculi

    oculi Member

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    are you sure they are 12 volt? that seems crazy, I just replaced the 240 volt stupidly expensive bulbs in my rangehood and they were 240 volt.
     
  12. mtma

    mtma Member

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    Old halogen mr16's and stuff were commonly 12vac. For this reason quite often MR16 led bulbs will take 12vac as well.
     
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  13. mjunek

    mjunek Member

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    My place is full of the 12V MR16 halogen fittings. All AC driven, as there is no point in rectifying it for a filament lamp.
    LEDs are supposed to be plug and play in this scenario - the biggest issue you have is the minimum load on some of the electronic transformers. These typically say 10W-60W or something like that, which means they need a minimum load of 10W to operate. With a LED globe only drawing say 6W, sometimes they wont work at all, and sometimes you'll get a disco party with crazy blinking lamps.
    You'll find out within a second if this is the case.
     
  14. OP
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    echelon6

    echelon6 Member

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    I can't just put a led globe in since the housing is all broken and gross. May as well replace with a full LED kit
     
  15. aXis

    aXis Member

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    The RMS definition really only applies to a simple linear load.

    When you add on diode rectification and smoothing capacitors, it draws the current from the AC in a non-continuous way. In fact, it only draws the current from a short interval around the peak of the AC waveform, giving a DC voltage that approaches sqrt(2) x the AC voltage.
     
  16. aXis

    aXis Member

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    Yep, I just remembered this too. In the past I have tried to use an electronic downlight transformer as a cheap DC PSU and it didnt work. The output of the electronic transformer was nasty, full of electrical noise, and sensitive on the type of load it was driving. As I added more filtering to improve the noise, caused the electronic transformer to shutdown.
     
  17. dufflover

    dufflover Member

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    So what's your original plan then? What housing/connector is it now? (can't be that bad can it?)

    I've found Philips brand MR16 LED globes to be the most tolerant (i.e. no flickering) with older 12VAC transformers.
     
  18. ArmoureD

    ArmoureD Member

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    If you don't want to replace the entire hood with a new one that has bright proper led lighting, just install a plug in DC led driver inside the cabinet above the rangehood or inside the flue where the hood plugs into the socket, run a short piece of fig8 flex down and install an led strip on the underside. It'll look fkn shit but it'll work. Use one of those remote controlled rf plug in relay adapters to turn it on and off.
     
  19. OP
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    echelon6

    echelon6 Member

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    Solved thanks all. A simple 4 diode full bridge rectifier with a capacitor on the DC end was enough to turn 12vac into 12.5vdc which is good enough for the LEDs
     

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