power supply...

Discussion in 'The Projects Forum' started by SUf1, Oct 3, 2008.

  1. SUf1

    Thread Starter New Member

    Oct 3, 2008
    in know that topics abt power supply's have been answered to in the past, but i can't find what my little project requires....

    i know how to make a +/- 5V power supply(using LM317/LM337 or the LM7805/LM7905);
    i know how to make a +/- 12V power supply (using LM317/LM337 or the LM7812/LM7912)and;
    i know how to make an AC to DC 5V-12V power supply(incorporating the LM317/LM337 with the variable resistor to increase or decrease the output voltage)....

    my power supply is supposed to incorporate all three....

    i will be using a step-down transformer... but how to incorporate it for all three and as well as how to get the....

    yeah i know i am asking u brilliant ppl to do my project for me but am stumped...
    google'ed it but to no avail as the diagrams only show one or the other....
    doesn't explain whether the +/- ??V supply circuit can be used for the 5-12 V variable power supply as well or not..
    if it can how does it work exactly???

    as i got no scanner so can't show u my circuit diagram...
    but the link here is almost exactly the same except the C3,4,5,7 am not using...
    no rush...

    i got 2 weeks to go...
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Just use the same DC supply for all the regulators. It will have to be about 14 volts DC for the 12 volt regulators, so the 5 volts might get a bit hot, but that's waht heat sinks are for.

    With a center tapped transformer and a full wave bridge, you can get symmetrical + & - DC voltages off one transformer.

    Here's a link to out Ebook about that - http://www.allaboutcircuits.com/vol_3/chpt_3/4.html.
    Last edited: Oct 3, 2008
  3. Audioguru


    Dec 20, 2007
    That schematic and many other schematics with the LM317 adjustable regulator shows the wrong values for R2 and R3. 240 ohms is used with the more expensive LM117, the LM317 needs 120 ohms. Then the value of the pots also must be halved.

    The circuit on the first page of the datasheet has 240 ohm resistors with the LM117.
    If 240 ohms is used with an LM317 then the output voltage might rise when the load is removed (as is explained in the datasheet).
  4. SgtWookie


    Jul 17, 2007
    As Audioguru explains, an LM317/LM337 requires 10mA current in order to guarantee voltage regulation.

    Vref (the reference voltage between the Vout and ADJ terminals) is nominally 1.25v, but it may be as low as 1.2v or as high as 1.3v. So, in order to guarantee regulation under no-load conditions, a 120 Ohm resistor is used between the Vout and ADJ terminals.

    Looking at the worst case where Vref = 1.2v; since I = E/R, and E = 1.2v, then I=1.2/120 Ohms = 10mA.

    The nominal case where Vref = 1.25v; I = 10.41666...mA. You can then figure out what you need for R2.

    Vout = Vref x (1+ R2/R1) + Iadj(R2)
    Iadj is typically 50uA, or 0.05mA.

    When Vref=1.25 and R1=120, for every 100 Ohms of resistance in R2, Vout will increase above Vref by 1.05v. So, if Vref=1.25 and R2=0, Vout=1.25; if Vref=1.25 and R2=1000, then Vout=11.75 (approximately).

    Note that the LM317 has a typical dropout voltage of 1.7, and your bridge rectifier will typically have a 1v drop across it. But for good regulation under load, you should allow 3v across the regulator, plus the 1v across the bridge. So, if you need 20v out, you'll need a 24v transformer.
  5. SUf1

    Thread Starter New Member

    Oct 3, 2008
    thnx for the response guys :)

    what u guys want me to do is tinker a lil with R2 and R3 for my desired results and make sure that my transformer gives out a little excess V to compensate for the bridge and regulators... np will make sure of that...

    once again thnx for the advice.... and i'll definantly get back to ya on the outcome of the experimentation :)
  6. SUf1

    Thread Starter New Member

    Oct 3, 2008
    i got a 24V 1.2amp transformer...
    will this change anything???
    do i need to change the capacitors/resistors for the change in output of the transformer or it won't matter....

    i know i just could just make it out on a board and see for myself but u guys know better due to experience...
  7. SgtWookie


    Jul 17, 2007
    Well, you'll still need 120 Ohm resistors from the Vout to the ADJ terminals, and 1k pots to adjust the output voltage. +/-10v is about the most you can expect to get out of your supply.

    I suggest that you keep C5 and C7. The 0.1uF capacitors are for suppression of high frequency transients. Large capacitors have relatively slow response times due to the parasitics involved (inductance, resistance).