Connect with us

problems with my power supply.

Discussion in 'Power Electronics' started by evilheart, Sep 3, 2009.

Scroll to continue with content
  1. evilheart

    evilheart

    8
    0
    Sep 3, 2009
    i am working on split power supply (5,12,-5,-12v) , that i am going to use for general purposes


    i used :
    12-0-12 about 1A transformer
    2A bridge
    7805
    7912
    2x2200u 16v

    for testing the circuit before soldering

    i had about 12v on the 7805 o/p and about 0.78v on the 7912
    also the 7912 was over heated , i don't know if it is a latch up , and what is its reason if it was a latch up?

    and i don't know also why there is that short cct on the 7912 specially and not the 7805 also

    that is the second time that problem happens to me and i dont know how to fix it,

    please helllllllllp,
     
  2. amdNRA

    amdNRA

    50
    0
    Sep 3, 2009
    Check your wiring first. Make sure the center tap from the transformer goes to the common of the outputs and those of the regulators.The + from the bridge goes to the 7812 input, the - goes to the 7912 input. By the way, you only mention 2 regulators but you actually need 4, for the 4 outputs that you want. Use the 5 and -5 volt regs after the 12 and -12 volt regs. It is recommended to use a 1uF cap on each output. Verify that the input voltage to the regulators is about equal or greater than the regulating voltage plus 2.5 volts ie. for a 12volt reg INPUT> or = to 14.5 vdc.
    Hope that helps, Gil.
     
  3. evilheart

    evilheart

    8
    0
    Sep 3, 2009
    i think my wiring is right , and by the way the cct i gave here was a testing cct for the components , any way , the problem is that the volt drops on 7912 input and o/p and it becomes over heated which means that there is high current, that happens when i add the regulators , or just one of them , it seems as if there is a short cct formed by the regulator 7912,it may be a latch up but, i don't know how to fix that.

    any way thx Gil
     
  4. amdNRA

    amdNRA

    50
    0
    Sep 3, 2009
    If You say the wiring is correct, then you have bad parts but this is the 2nd time time that this happens! While it is certainly possible to have 2 batches of bad parts it is not likely. Bear in mind that the 7912 voltage reg has a different pinout (available here)http://www.national.com/mpf/LM/LM7912.html than the 7812.
    The 7812 is available here http://www.national.com/ds/LM/LM340.pdf
    Let me know how it goes, Later, Gil. :cool:
     
    Last edited: Sep 4, 2009
  5. evilheart

    evilheart

    8
    0
    Sep 3, 2009
    the 7912 pin config is GND-IN-OUT , right?
    i connect it like that , i suspecting it may be a potential latch up , u know any thing about that Gil?
     
  6. amdNRA

    amdNRA

    50
    0
    Sep 3, 2009
  7. evilheart

    evilheart

    8
    0
    Sep 3, 2009
    thx , Gil , i will see what i can do.
     
  8. davenn

    davenn Moderator

    13,883
    1,966
    Sep 5, 2009
    amdNRA has given you good advice, and once you have the + and - 12V rails working
    you can then use 7805 and 7905 regulators off those 2 x 12V rails to produce the + and -
    rails.
    Just remember tho that the 12V regulators need to be able to supply(carry) the current
    demands of whatever you are supplying of the 12V and the 5 V rails.
    The standard TO 220 case units ~ 1 Amp and the TO3 case ~ 1.5 A, there are adjustable
    regulators like the LM 338 that can handle 5-6 amps !! :D

    cheers
    Dave
     
  9. evilheart

    evilheart

    8
    0
    Sep 3, 2009
    it seems that this project is cursed!!

    i put the diodes and the project test was good on the breadboard and i was sure that the regulators r working well , then after i soldered the components on the PCB, the 7905 ,7912 was ruined the 7905 gives about 10v , 7912 now doesn't regulate at all , is regulators that fragile ?

    any way thx Gil , dave.
     
  10. amdNRA

    amdNRA

    50
    0
    Sep 3, 2009
    These linear regulators are actually quite reliable...But it seems to me that something got lost in the translation :confused: between the breadboard and the soldered pcb. Anyway, don't give up, you'll figure it out. These are actually the times when we learn the most.
    Later, Gil. :)
     
  11. Peter Mathew

    Peter Mathew

    2
    0
    Sep 7, 2009
    power expert

    the pin out of 79** is different from that of 78**. so please reffer the pin out from a data sheet. secondly these regulators manufactured by some manufactures, gives only proper op if at least 470 ohm for 5V and 1K for 12v is loaded at the output (both 1/4 watts).
    one of the possible way of getting 12 V at 7805 op is the gnd pin of 7805 not connected to ground (center tap of transformer )
     
  12. Peter Mathew

    Peter Mathew

    2
    0
    Sep 7, 2009
    the pin out of 79** is different from that of 78**. so please refer the pin out from a data sheet. secondly these regulators manufactured by some manufactures, gives only proper op if at least 470 ohm for 5V and 1K for 12v is loaded at the output (both 1/4 watts).
    one of the possible way of getting 12 V at 7805 op is the gnd pin of 7805 not connected to ground (center tap of transformer ).

    the -ve arm of the bridge shoud be connected to ip of 7912 and center tap to ground
     
  13. neon

    neon

    1,325
    0
    Oct 21, 2006
    12v ac means 12 x 1.41v dc=to over 16vdc unloaded. try a capacitor of at least 25vdc rating. Did you forget to protect the device by adding a diode from out to in. if you put 2200 mfd on the output without the diodes those devices are history. The mechanics are like this shutoff the power and leave the load off there is a lot of voltage on the output reversing the input on the device from the capacitor. look into this.
     
  14. amdNRA

    amdNRA

    50
    0
    Sep 3, 2009
  15. evilheart

    evilheart

    8
    0
    Sep 3, 2009
    i hope that i will really learn from that.
     
  16. neon

    neon

    1,325
    0
    Oct 21, 2006
    how do you intend to get 12v out from 12v in. Read the specs the answer is there.
     
  17. tron-ee

    tron-ee

    12
    0
    Sep 11, 2009
    Hello evilheart and All,

    tron here....

    You have a number of issues at work here, one is the size of your ripple capacitor, its initial ramp-up time could be causing a damaging spike which in theory could cause a latchup you are worried about. Do you NEED that size capacitor?

    Another question, is this power-supply to be used principally with digital or analog circuits? If digital, I would recommend using some small adjustable buck switchers, that would eliminate much of the heat generation, the big capacitors and you can use one bridge for all the voltages. If you are going to be using this power-supply for sensitive analog circuits then I would suggest that you buy a good quality adjustable current/voltage power-supply on eBay, I see them go all the time for around $50 to $150....

    If you are set on making your general purpose power-supply....

    From the center tapped transformer, the center tap goes to 0 ref (do not put this to chassis ground). You wanna keep your DC isolated from AC ground for both noise and safety.

    The bridge for +- 12 Volts should be at least 50 VAC rated, personally I would select something 3 x the RMS to handle power-line spikes, although I have gotten away with much less.

    The +- sides of the bridge I would move to 220 uF caps, rated at 50 Vdc (min) and low ESR, (besure to move the + side of the ripple cap to 0 reference for the minus supply, otherwise there will eventually be a big bang!).

    Ok, other issues, the +- 12 Vdc regulator series (LM78xx) typically need a small amount of power to operate, and as result there is a input to output voltage differential of something like 2.5 Vdc as I recall, meaning that to get 12.0 Volts regulated output, the regulator will require 14.5 Vdc minimum input, ok? That means your transformer needs to output appx = 14.5 x 2 or 29 VAC center tapped to enable the regulator you spec'd to function properly.

    Looking at the spec sheet from DataSheet Server (great databook server)

    www . datasheetcatalog . com (remove spaces) & search on LM7805, LM7812, LM7905 and LM7912 data sheets.

    Regarding Your +-5 Vdc regulators, your design, is differencing 12Vdc down to 5 Vdc, which means you are dropping the difference between the 12 Vdc and 5 Vdc = 7 Vdc as heat. First that simply wastes a lot of power, 2nd, it pulls much power from your transformer which would normally be used by the other parts of the power-supply system, 3rd, it generates a lot of internal heat, cooking all the components and will require some honkin big heat-sinks I would do something different here, like use a 2nd transformer, rated at around 7.5 Vdc (min...you wanna handle mains-primary line voltage sags and still be within the regulation point of the regulator).

    With these above two issues in mind, the transformers should both be center-tapped,
    the 12 Vdc power-supply transformer s/b around 29 VAC (14.5 VAC-CT) not too high to drop a lot of power, yet high enough to handle mains-primary line sags, and the +-5 Vdc power-supply transformer s/b 18 VAC (9 VAC-CT), again not too high but low enough to keep from cooking everything inside.

    Another matter is regulator package configuration, whether to use LM78/79xxT or LM78/79xxK packages. The 'T' package is the familiar TI-220 plastic power-pkg, while the 'K' is the old style TO-3 power-pkg, well beleive it or not, if you use only one transformer and you still wanna get 1.5 Amps out of both +-12 & +-5, you will need the K package for the +-5 and two big finned heat sinks, else they will go into thermal runaway and shutdown. My recommendation is to use the 'T' pkgs & two transformers.

    Regarding transformer current rating, +- 12 @ 1.5 Amps will require 2 x 1.5A (+ the efficiency of the transformer loss), I would rule of thumb select a 29 VAC @ 3.75A or 4 Amp transformer, and for the +-5 transformer, I would rule of thumb select a 18 VAC @ 3.75A.

    Also, according to my old National Linear Handbook section on LM linear regulators, you should to add some small decoupling caps on both input/output to and from the linear regulators (later tune up any ringing with toroidal beads).

    So, here's a quick BOM:
    1 x LM7805T - 5Vdc @ 1.5A positive regulator
    1 x LM7812T - 12Vdc @ 1.5A pos reg
    1 x LM7905T - -5Vdc @ 1.5A negative regulator
    1 x LM7912T - -12Vdc @ 1.5A neg reg
    2 x 40 VAC @ 6 Amp bridge rectifiers (or bigger)
    1 x 29VAC @ 3.75A, center tapped transformer
    1 x 18VAC @ 3.75A, center tapped transformer,
    2 x 220 uFarad, 50 Vdc electrolytic ripple caps (low ESR rating important for P/Ss)
    4 x .1 uF, 75 Vdc tantalum decoupling caps, (orange or blue-drops are nice)
    4 x different coloured LED voltage 'on' status indicators & appropriate resistors

    Its really neat & useful to add a polarity reversal (DPDT) switch on these type power-supplies.

    One last thing, there are many modern linear regulators these days which are known as LDOs or 'low-drop-out', meaning that instead of needing 2.5Vdc to operate, they work on a few miliVolts, so a 13.2VAC transformer can adequately supply and still be regulated +- 12Vdc. Its a matter of looking around and finding the parts, DigiKey or Mouser is premium, but finding these type parts is much harder at places like All Electronics or Electronics GoldMine etc.

    Hope this helps...

    tron
     
    Last edited: Sep 11, 2009
  18. neon

    neon

    1,325
    0
    Oct 21, 2006
    you got 12v input and expect 12v out out of a regulator is thatn right? good luck.
     
  19. evilheart

    evilheart

    8
    0
    Sep 3, 2009
    thx Gil,
    i used a diode on the regulator , any way it finally worked , although it seems somehow unstable (sometimes the voltage of the 7812 change to about 0.78v but it return back with a small shake) it may be just soldering problem,

    the problem is that,i can't get until now what made it that unstable,

    it may because i used ratsnest on the pcb (it is useful as a spare track but it seems to cause alot of problems)

    for the previous project the regulators was really damaged i think , it may be a latch up , but i don't know what caused it.

    finally thx again Gil , Dave for ur help.
     
    Last edited: Sep 19, 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.
Similar Threads
There are no similar threads yet.
Loading...
Electronics Point Logo
Continue to site
Quote of the day

-