Connect with us

24 VAC to 12 VDC

Discussion in 'General Electronics Discussion' started by sundy, Dec 16, 2016.

Scroll to continue with content
  1. sundy

    sundy

    64
    5
    Aug 5, 2016
    I am trying to rectify 24 VAC to 12 VDC without a step down transformer.
    1. I used one of my PCB's as an experiment. The circuit works but the 12 V regulator heatsink gets way too hot.
    2. I am also worried about loading the source PS as the unit I am working on are connected in parallel, otherwise I would do something like this http://www.brighthubengineering.com...9-make-a-transformerless-power-supply/#imgn_2
    3. I have searched all over for a SMPS that operates at 24 VAC input, I have not found a off the shelf option yet.
    4. I don't have the time or the skill to design a SMPS.
    5. The unit I am driving draws only 0.080 A at 12 VDC.
    6. I am thinking of using an isolation transformer designed for the audio industry since they are cheap.
    I am looking forward to comments.
     

    Attached Files:

  2. Bluejets

    Bluejets

    4,151
    879
    Oct 5, 2014

    Attached Files:

  3. davenn

    davenn Moderator

    13,619
    1,882
    Sep 5, 2009
  4. AnalogKid

    AnalogKid

    2,375
    661
    Jun 10, 2015
    Am I correct in assuming that the 24 Vac is distributed power in a master clock system, and already is isolated from the AC powerline?

    If your numbers are correct, the max power dissipation in the regulator should be 0.38 W. That's 0.34 W for the output current and 0.04 W for the 7812 operating current. -ish. That is warm enough to notice but not melt fingerprints. "way too hot" is a circuit assembly problem, not a design problem.

    12 Vac > 1 diode (1N4004) > filter cap > 7812 > output cap > output

    Efficiency is around 70%, so I don't think the complexWithout isolation or a bridge rectifier, you will have to pay more attention to the AC connections.

    ak
     
  5. sundy

    sundy

    64
    5
    Aug 5, 2016
    It's 24 VAC, isolated from mains.
     
  6. sundy

    sundy

    64
    5
    Aug 5, 2016
    Yes it is a clock system. I got my thermometer out and the heat sink is up to about 150 F at the hottest spot. I think it's going to be OK. I'm using full wave bridge rectifiers. I wonder what load the they present to the PS?
     
  7. davenn

    davenn Moderator

    13,619
    1,882
    Sep 5, 2009

    BUT that link you provided ( the PDF) ISNT, it's at mains voltage
    hence why I included the link when I quoted you !!
     
    sundy likes this.
  8. sundy

    sundy

    64
    5
    Aug 5, 2016
    Excellent point. The author did make that point in the article but you are correct to point out the safety issue.
     
  9. sundy

    sundy

    64
    5
    Aug 5, 2016
    I am considering this question answered, solved, and put to bed.
     
  10. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    I get a lot more than that.

    24VAC will rectify to 32VDC. You now have 20V to drop in a linear regulator at 80mA which is 1.6W. A smallish heat sink should be enough for that, though. So if it is getting very hot, I agree with your conclusion that something else is wrong.

    Bob
     
  11. AnalogKid

    AnalogKid

    2,375
    661
    Jun 10, 2015
    I magically converted 24 Vac to 12 Vac before starting. oops.

    1.6 W in a bare TO-220 is going to get very hot, but even a small heatsink will bring that down to comfortable.

    ak
     
  12. sundy

    sundy

    64
    5
    Aug 5, 2016
    I am interested in this 'fixed impedance' of a capacitor. How could I overcome the other shortcomings of the circuit to utilize the fixed impedance?

    Transformers are heavy and expensive, I am trying to avoid them.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
    2,779
    Jan 21, 2010

    Rectify, filter, and use a small DC-DC switch mode regulator.
     
  14. hevans1944

    hevans1944 Hop - AC8NS

    4,521
    2,102
    Jun 21, 2012
    I have just this week finally moved to my new home in Venice, Florida, but I haven't forgotten about you. It will take awhile to get setup here for electronics work, but since you said you had already hired someone to handle your electronics work, further examination of your secondary clock circuit is a zero priority for me right now.
     
    Last edited by a moderator: Dec 22, 2016
  15. Bluejets

    Bluejets

    4,151
    879
    Oct 5, 2014
    Just wondering if the Op went for #2 and #21 as suggested.
     
  16. sundy

    sundy

    64
    5
    Aug 5, 2016
    Right now a test clock is running of the schematic I posted in the OP. The heat sink is not as hot as initially thought. If I can run 20 clocks off a 24 VAC 4 A transformer I will call it good.
     
  17. davenn

    davenn Moderator

    13,619
    1,882
    Sep 5, 2009
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-