12VAC to 12VDC Converter

Discussion in 'General Electronics Chat' started by parklol, Oct 12, 2012.

  1. parklol

    parklol Thread Starter New Member

    Joined:
    Aug 17, 2012
    Messages:
    16
    Hi, I am trying to design a 12VAC to 12VDC converter. The simplified circuit is attached below.The problem is that I have to use SMD component. The capacitor I can only find is 16V 220uF, and it is not safe to just put the capacitor right after the rectifier. Are there any way to solve this problem? -Leo

    Attached Files:

  2. bertus

    bertus Administrator Staff Member

    Joined:
    Apr 5, 2008
    Messages:
    12,446
    Location:
    Amsterdam,Holland (GMT + 1)
    Hello,

    The peak voltage of a sinus is sqr(2) * the input voltage , the voltage drop over the diodes is about 0.7 Volts
    So the output voltage will be around 12 * 1.414 - 2 * 0.7 = 15.6 Volts

    Bertus
  3. mcgyvr

    mcgyvr Well-Known Member

    Joined:
    Oct 15, 2009
    Messages:
    3,150
    Location:
    North Carolina, USA
    What Bertus said is obviously correct..

    I would just use the bridge.. and a LM7812 12V regulator or similar with the required input/output caps as per the datasheet.

    You didn't specify a current requirement though..
  4. chrissyp

    chrissyp Active Member

    Joined:
    Aug 25, 2008
    Messages:
    82
    Location:
    guildford,surrey, uk
    If the current is low a zener diode could be used to pull the voltage back down to 12vdc
  5. MrChips

    MrChips Moderator Staff Member

    Joined:
    Oct 2, 2009
    Messages:
    9,442
    Why do you say "it is not safe to just put the capacitor right after the rectifier"?
  6. ScottWang

    ScottWang Well-Known Member

    Joined:
    Aug 23, 2012
    Messages:
    1,713
    Location:
    Taiwan, Asia (GMT +8)
    I' afraid the Cap value is not enough, you may use 4 pieces of 220uf/16V, each two capacitors series and then parallel, the cap will become 440uf/32V.
  7. parklol

    parklol Thread Starter New Member

    Joined:
    Aug 17, 2012
    Messages:
    16
    The capacitor I found is 16V, but the peak voltage of 12VAC is about 17V. Even there is some voltage drop through the rectifier, it is still not safe to put the capacitor right after the rectifier.
  8. takao21203

    takao21203 Well-Known Member

    Joined:
    Apr 28, 2012
    Messages:
    2,477
    Location:
    Ireland
    Use a 25V capacitor. They are as readily available (in SMD) as 16V caps.

    If you load the circuit with current, voltage will drop anyway.

    It takes about 20 or 22V to blow up a 16V eletrolytic cap. At least.
  9. parklol

    parklol Thread Starter New Member

    Joined:
    Aug 17, 2012
    Messages:
    16
    Yes, you are right. this seems work based on the calculation. The power supply I am using is a 120VAC to 12VAC transformer. However, the line voltage varies, so sometimes the output of transformer will give like 12.7VAC.

    I can add another diode to reduce more voltage or add a regulator in between. The maximum current I need is 1.5A, adding these components will produce too much heat.

    Are there any other solutions?
  10. MrChips

    MrChips Moderator Staff Member

    Joined:
    Oct 2, 2009
    Messages:
    9,442
    All linear voltage regulators have to give up excess power as heat.
    You have to switch to a switching regulator if you want to be cool.
  11. takao21203

    takao21203 Well-Known Member

    Joined:
    Apr 28, 2012
    Messages:
    2,477
    Location:
    Ireland
    60 Hz transformer (for 1.5A, so 20W class) and small SMD cap. Why this particular requirement? SMD circuits are often supplied by electronic transformers.
  12. parklol

    parklol Thread Starter New Member

    Joined:
    Aug 17, 2012
    Messages:
    16
    I am trying to design a LED circuit. The LED and other components are on the same PCB, and there will be a lens on top of it. Eletrolytic capacitors are too big. I found tantalum capacitors are smaller, but these capacitors are very expensive when having a high rated voltage.
  13. takao21203

    takao21203 Well-Known Member

    Joined:
    Apr 28, 2012
    Messages:
    2,477
    Location:
    Ireland
    Oh I see. Use a smaller capacitor value maybe. Can you post the circuit? Some circuits can live with relatively high ripple current.
  14. parklol

    parklol Thread Starter New Member

    Joined:
    Aug 17, 2012
    Messages:
    16
    Due to limited PCB space I cannot use electronic transformers. So I have to use a power transformer to supply many single units.
  15. takao21203

    takao21203 Well-Known Member

    Joined:
    Apr 28, 2012
    Messages:
    2,477
    Location:
    Ireland
    If it is a toroidal transformer, you could take off some of the secondary windings. I guess under load, voltage will go down a little as well.
  16. parklol

    parklol Thread Starter New Member

    Joined:
    Aug 17, 2012
    Messages:
    16
    I used a 12V regulator and six 220uF capacitors to regulate the voltage. It works right now, but I am trying to find a way to not use the regulator.

    By the way, do you know how to use a 2 line inductor? I am trying to add an inductor to reduce current ripples. If the inductor is 1.5A, do the both modes in the attached image work for 1.5A?

    Thank you ;)

    Attached Files:

  17. takao21203

    takao21203 Well-Known Member

    Joined:
    Apr 28, 2012
    Messages:
    2,477
    Location:
    Ireland
    Obviously the copper resistance is 4 times as high for the differential mode. I have not used an 1.5A common mode choke in that manner, even if I have a few around here.

    Indeed 220uF Tantalum caps rated at 25V are not common for 60Hz filtering (because of their high price). There are also very small chinese electrolytic caps (220uF) for SMD. They are usually blank, with a black plastic washer at the bottom.

    TRIAC regulators are not so good for transformers however if you only need to reduce a few volts, it is not a big issue (for 20W class). There are very small 400V TRIACS, same size as a 2n3904 (TO 92 I think).

    I'd experiment with that, if the particulary arrangement has to be used. This would be some kind of a fix, not a production level solution. I used a TRIAC regulator one time for a 50Hz transformer experiment and yes they can regulate the primary voltage. Commercially it is not done that way, because it causes an assymetric load, among other things. As I say, for a few precent reduction, and 20W class, neglible.

    I found a photo with one of these electrolytic caps. They are not reasonable larger for 220uF/25V, just a bit.
    Indeed I bought a kit which contained 10V, 16V and 25V caps.

    Attached Files:

    Last edited: Oct 16, 2012
  18. parklol

    parklol Thread Starter New Member

    Joined:
    Aug 17, 2012
    Messages:
    16
    Thank you very much for the information.
Similar Threads
Forum Title Date
General Electronics Chat Change 12VDC to 12VAC, can it be done without transformer? Jun 27, 2014
General Electronics Chat 12vac to 12vdc Dec 25, 2012
General Electronics Chat 12vdc vs. 12vac May 6, 2011
General Electronics Chat faulty 12vac dimmer/switch: revised Apr 30, 2014
General Electronics Chat faulty 12vac dimmer/switch Apr 30, 2014

Share This Page