Connect with us

How to choose appropriate transformer

Discussion in 'General Electronics Discussion' started by vinod chandran, Aug 14, 2011.

Scroll to continue with content
  1. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi,
    i am in the middle of a sub-woofer amp project. the components of this project are, a fm radio board, sub-woofer filter board, 20w stereo amp board for satellite speakers, an led vu meter board. My problem is how to choose an appropriate transformer for this project. all are12 volt boards My doubt is how much voltage and how much amperage
    help me plz.
    -vinod
     
  2. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010
    The boards require a regulated 12VDC? Or do they have their own power supplies?

    Anyway,

    A 12V transformer would be adequate. I'd say a 2-3A transformer would also probably be decent. Not sure of the power consumption on the other boards, but you should have a little headroom on the transformer regardless.

    The transformer will produce 12VAC. You'll need to convert that to DC by using a bridge rectifier. When AC is converted to DC, you multiply 1.41 x the AC voltage. So a 12VAC transformer would produce roughly 16.9VDC after rectification.

    If the boards require a regulated 12V on the spot, you'll need to a pretty high power regulator. I searched Digikey and found the LM1085IT-12-ND...which is a 3A 12V regulator. You'll need a heatsink on that, I'm sure.

    I don't want to explain too much because I might be leading you on the wrong path if this is not how your boards are set up.
     
  3. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Power reqirments

    I would go with calculating total peak and possible surge current, you have already said the voltage, i might go + 5 % up on the power due to line and load regulation, a second easy rgulator is the LM338K TO3 type steel case, and only needs a couple or so external components for its function, its good for 5 amps if your in put is not more than a few volts higher. Dave.
     
  4. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi TheLaw,
    Thanks for the information. i'm going to build this with a 2A transformer. and of course there will be 12v regulator also. once again thanks to share the knowledge about rectifier output(v in*1.41). because i don't know anything abt rectifier's output. So thanks.
     
  5. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Thanks Dave,
    5A transformer is not affordable for me to this project. i'm planning to buy a 2 or 3A transformer
     
  6. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi TheLaw,
    Thanks for the information. i'm going to build this with a 2A transformer. and of course there will be 12v regulator also. once again thanks to share the knowledge about rectifier output(v in*1.41). because i don't know anything abt rectifier's output. So thanks.
     
  7. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    RE Tranformer

    You dont need a 5 amp transformer to use the LM338K 5 amp regulator, 5 amps is the maximum power that regulator will provide, anything under is fine, including the transformer you use, rectified smoothed then regulated, some amps will run on rectified dc smoothed only, depends on the amps requirments. Dave.
     
  8. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    thanks dave. i am going to google for a circuit with lm338k
     
  9. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
  10. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi,
    Thanks Dave. Last day i've got a 12-0-12 transformer (5A). Now i want a bridge rectifier for that transformer. I mean what type of diodes and capacitors should i use.
    -vinod
     
  11. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010
    I'm going to simplify it a little bit, so anyone who knows better...I might not be politcally correct.

    I don't know the availability in India, but something like a GBU7 (7A) or GBU8 (8A) bridge rectifier will do. You don't really wan to have anything lower for reliability.

    The 0V is a shared ground type of thing. It doesn't get hooked up to the rectifier.

    Then take each of the 12V wires and hook it up to each of the AC pins on the rectifier. On the DC output of the rectifier, the positive pin will be +17V and the negative pin -17V. The -17V is not the return/ground of the +17V however. The ground is...well..the ground that you already hooked up the 0V of the center tap to.

    Capacitors wise, you'll need 2 capacitors, one for each rail. I'm no expert on the matter, but I'd say anywhere from 4700-6300uF would be good. You should use 25V rated capacitors.
     
  12. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Thanks TheLow,
    But the bridge rectifier is not available in my area. so plz tell me the value of 4 diodes. is a 2 diode rectifier suit for this transformer. could you pls send me an image of your idea?.
     
  13. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010
    Okay 6A10 diodes, but that's pretty much a minimum. If you only use 2 diodes, you only get one output. It's full wave, but if you want 2 outputs (~+/-17V), you need 4 diodes.

    I'm about to leave for vacation so maybe someone else can help.
     
  14. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi TheLaw, enjoy your vacation. i jast need an ordinary single output power supply. I connect my 12-0-12 5A transformer in my sub-woofer project with two 6A Diode and one 2200uF 25V capacitor. but my stereo amp produced a grrrr noise in every second. is that a problem with 5A current or 2200uF capacitor ?. anyhow this power supply giving me an output of 17V. So do i need a 12V regulator?.
     
    Last edited: Aug 17, 2011
  15. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010
    What are you using for your amplifier? A lot of amp chips do require +/- voltages.
     
  16. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi,
    My baas filter board is dameged. today i just turn a preset on that board. after that the board is not working.
    -vinod
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,264
    2,717
    Jan 21, 2010
    Any reason you don't want to try to fix this one?

    My guess is that you changed the biasing in some way that was destructive. It may just need (as a guess) some new transistors and to correctly set up the biasing again (that may not be trivial).

    Did smoke come out of anywhere?
     
  18. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi steve,
    No, that was my mistake . i just turned a preset on that subwoofer filter board. after that board is not working. i thought that the preset is to adjust the bass. anyhow my board was damaged. so i need a subwoofer filter circuit. but the next problem is does the new filter circuit suit with my 12v 5A power supply?.By the way i am sorry to say that i post this subject as a new thread in this forum. if that is a mistake, plz ignore or delete that thread. i attached an image of my new idea.
     
    Last edited: Aug 19, 2011
  19. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Rectification

    Hi your transformer has a center tap, middle wire on the secondary AC output. there is a link below for a full wave center tapped rectifier. Dave. PS diodes need to be larger on there current and voltage ratings than the transformers current and voltage output. :)

    http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectct.html

    Sorry for the late reply, hope you got it soughted.
     
    Last edited: Aug 19, 2011
  20. vinod chandran

    vinod chandran

    192
    2
    Jun 21, 2011
    Hi Dave,
    Thanks for the link. according to that page a 10k resistor will work(My capacitor is 2200uF). And then the rippiling problem is over. But what about the subwoofer filter board?.is there any way to repair it . if no i need a filter circuit..
     
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

-