Connect with us

My Power Supply Saga

Discussion in 'Power Electronics' started by alfa88, Jun 10, 2012.

Scroll to continue with content
  1. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    Am I unclear on a concept or what? I'm trying to put together an adjustable 1-15V power supply. I tried a lm317 with a Voltage follower. Didn't work out so I'm going with a LM338. The most I can get out of it is 2A. I made sure the bottleneck wasn't the transformer to bridge section by loading just that part down. My current meter was able to exceed 6A and the transformer was still cool. The bridge got a bit warm. As a load I'm using 1 Ohm, 10W resistors connected in series and I'm picking off taps as I test. I realize that the resistors are under rated but I'm only leaving them connected long enough to get readings. I have the regulator on a nice big heat sink and it has never gotten beyond warm. What gives? This regulator is supposed to give me 5A.
     

    Attached Files:

    Last edited: Jun 10, 2012
  2. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    How many?

    Looking at your datasheet, it states that
    Icl = Vin - Vout < or = to 10V 5A min
    Icl = Vin - Vout = 30V 1A
     
    Last edited: Jun 10, 2012
  3. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    I have a string of 10 resistors for a total of 10 Ohms. 3 Ohms is as low as I go.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    Read your input voltage and output voltage across a known resistance (tell us that too) and let's see what's going on.

    Also the input voltage when there is no load.
     
  5. gorgon

    gorgon

    603
    24
    Jun 6, 2011
    What's your input voltage with and without load?
    What's your output voltage with and without load? (over the load)
    How do you measure the current in the load?
    Where do the voltage drop?

    TOK ;)
     
  6. alfa88

    alfa88

    344
    5
    Dec 1, 2010

    Attached Files:

    Last edited: Jun 12, 2012
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    It would probably help if you wired up the LM338 correctly.

    Looks like a simple mistake :)
     
  8. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    Well I finally broke the 4Amp barrier. and no it wasn't a miswire. I had opted for the TO-3 package and went bargain hunting on EBay. It seems that I got a bad batch. 4 baddies in a row. Fortunately I bought some TO-220 types at the same time. They're not giving me the full 5A but at least now I have a bit of dignity intact.:D
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    So you're using a different circuit than the one posted above?
     
  10. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    Uh?! The bad part was a LM338K(TO-3), the good part was a LM338T(TO220). The application is the same. I like the heat dissipation of the TO-3 and besides, my heat sink is drilled for a TO-3.. The seller refunded me so I'll shop around again.
     
    Last edited: Jun 12, 2012
  11. davenn

    davenn Moderator

    13,877
    1,964
    Sep 5, 2009
    Alfa !

    compare your circuit to the one steve linked to

    you will see your circuit is incorrect!! :)
    you need to sort out the problems, the reason you have killed LM338's probably has much more to do with your incorrect cct than with faulty reg chips

    Dave
     
  12. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    I think his circuit would try to make ground = 1.25V, which means it would just open up the pass transistor fully.

    Bob
     
  13. alfa88

    alfa88

    344
    5
    Dec 1, 2010
    Are you referring to R1? In all cases I'm using a 150Ohm from Output to Adjust. I corrected my simplified drawing to reflect...
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    OK, that clears things up considerably :)

    edit: My guess is that you have some LM317's (or similar) rebadged as LM338's

    Having said that, I've not used LM338's and I don't know if they vary their maximum current based on input/output voltage differential. I've also not deliberately overloaded a 317 to see what it does in high current regimes, so take my comment with a grain of salt.
     
    Last edited: Jun 13, 2012
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

-